Rubidium

Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. The atomic weight is 85.4678. Elemental rubidium is very soft and highly reactive, with properties similar to other elements in group 1, such as very rapid oxidation in air. Rubidium has one stable isotope,85Rb. The isotope 87Rb which composes almost 28% of naturally occurring rubidium is slightly radioactive, with a half-life of 49 billion years—more than three times longer than the estimated age of the universe.

Two German chemists, Robert Bunsen and Gustav Kirchhoff, discovered rubidium in 1861 by the newly developed method of flame spectroscopy. Its compounds have chemical and electronic applications. Rubidium metal is easily vaporized and has a convenient spectral absorption range, making it a frequent target for laser manipulation of atoms.

Rubidium is not known to be necessary for any living organisms. However, like caesium, rubidium ions are handled by living organisms in a manner similar to potassium ions: it is actively taken up by plants and by living animal cells.

Rubidium is the second most electropositive of the non-radioactive alkali elements and melts at a temperature of 39.3 °C (102.7 °F). Like other group 1 elements, this metal reacts violently with water. As with potassium (which is slightly less reactive) and caesium (which is slightly more reactive), this reaction is usually vigorous enough to ignite the hydrogen gas it liberates. Rubidium has also been reported to ignite spontaneously in air. Like other alkali metals, it forms amalgams with mercury and it can form alloys with gold, caesium, sodium, and potassium. The element and its ions give a reddish-violet color to a flame. It was named after two strong emission lines in the dark red area of the spectrum.

As a symmetrical effect of rubidium metal's high reactivity toward oxidation and tendency to subsequent formation of the rubidium cation Rb+, this cation, once formed, is very stable, and is normally unreactive toward further oxidative or reductive chemical reactions.

Krypton

Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other rare gases in fluorescent lamps. Krypton is inert for most practical purposes. Krypton can also form clathrates with water when atoms of it are trapped in a lattice of the water molecules.

Krypton, like the other noble gases, can be used in lighting and photography. Krypton light has a large number of spectral lines, and krypton's high light output in plasmas allows it to play an important role in many high-powered gas lasers, which pick out one of the many spectral lines to amplify. There is also a specific krypton fluoride laser. The high power and relative ease of operation of krypton discharge tubes caused (from 1960 to 1983) the official meter to be defined in terms of the orange spectral line of krypton-86.

Krypton was discovered in Britain in 1898 by Sir William Ramsay, a Scottish chemist, and Morris Travers, an English chemist, in residue left from evaporating nearly all components of liquid air. Neon was discovered by a similar procedure by the same workers just a few weeks later. William Ramsay was awarded the 1904 Nobel Prize in Chemistry for discovery of a series of noble gases, including krypton.

In 1960, an international agreement defined the meter in terms of wavelength of light emitted by the krypton-86 isotope (wavelength of 605.78 nanometers). This agreement replaced the longstanding standard meter located in Paris, which was a metal bar made of a platinum-iridium alloy (the bar was originally estimated to be one ten-millionth of a quadrant of the earth's polar circumference), and was itself replaced by a definition based on the speed of light — a fundamental physical constant. In October 1983, the Bureau International des Poids et Mesures (International Bureau of Weights and Measures) defined the meter as the distance that light travels in a vacuum during 1/299,792,458 s.

Bromine

Bromine is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists in 1825-26. Elemental bromine is a fuming red-brown liquid at room temperature, corrosive and toxic, with properties between those of chlorine and iodine. Free bromine does not occur in nature, but occurs as colorless soluble crystalline mineral halide salts, analogous to table salt.

Bromine is rarer than about three-quarters of elements in the Earth's crust, however the high solubility of bromide ion has caused its accumulation in the oceans, and commercially the element is easily extracted from brine pools, mostly in the United States, Israel, and China. About 556,000 metric tons were produced in 2007, an amount similar to the far more abundant element magnesium.

At high temperatures, organobromine compounds are easily converted to free bromine atoms, a process which acts to terminate free radical chemical chain reactions. This makes such compounds useful fire retardants and this is bromine's primary industrial use, consuming more than half of world production of the element. The same property allows volatile organobromine compounds, under the action of sunlight, to form free bromine atoms in the atmosphere which are highly effective in ozone depletion. This unwanted side-effect has caused many common volatile brominated organics like methyl bromide, a pesticide that was formerly a large industrial bromine consumer, to be abandoned. Remaining uses of bromine compounds are in well-drilling fluids, as an intermediate in manufacture of organic chemicals, and in film photography.

Bromine has no essential function in mammals, though it is preferentially used over chloride by one antiparasitic enzyme in the human immune system. Organobromides are needed and produced enzymatically from bromide by some lower life forms in the sea, particularly algae. As a pharmaceutical, simple bromide ion, Br-, has inhibitory effects on the central nervous system, and bromide salts were once a major medical sedative, before being replaced by shorter-acting drugs. They retain niche uses as antiepileptics.

Bromine Liquid

Selenium

Selenium is a chemical element with the atomic number 34, represented by the chemical symbol Se, an atomic mass of 78.96. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature.

Isolated selenium occurs in several different forms, the most stable of which is a dense purplish-gray semi-metal (semiconductor) form that is structurally a trigonal polymer chain. It conducts electricity better in the light than in the dark, and is used in photocells (see section Allotropes below). Selenium also exists in many non-conductive forms: a black glass-like allotrope, as well as several red crystalline forms built of eight-membered ring molecules, like its lighter cousin sulfur.

Selenium is found in economic quantities in sulfide ores such as pyrite, partially replacing the sulfur in the ore matrix. Minerals that are selenide or selenate compounds are also known, but are rare. The chief commercial uses for selenium today are in glassmaking and in chemicals and pigments. Uses in electronics, once important, have been supplanted by silicon semiconductor devices.

Selenium salts are toxic in large amounts, but trace amounts of the element are necessary for cellular function in most, if not all, animals, forming the active center of the enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) and three known deiodinase enzymes (which convert one thyroid hormone to another). Selenium requirements in plants differ by species, with some plants, it seems, requiring none.

Arsenic

Arsenic  is the chemical element that has the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, mainly combined with sulfur and metals, and also naturally in the native (elemental) state. It was first documented by Albertus Magnus in 1250.

Arsenic is a metalloid. It can exist in various allotropes, although only the grey form is industrially important. The main use of metallic arsenic is for strengthening alloys of copper and especially lead (for example, in automotive batteries). Arsenic is a common n-type dopant in semiconductor electronic devices, and the optoelectronic compound gallium arsenide is the most common semiconductor in use after doped silicon.

A few species of bacteria are able to use arsenic compounds as respiratory metabolites, and are arsenic-tolerant. However, arsenic is notoriously poisonous to multicellular life, due to the interaction of arsenic ions with protein thiols. Arsenic and its compounds, especially the trioxide, are used in the production of pesticides (treated wood products), herbicides and insecticides. These applications are declining, however, as many of these compounds are in the process of being banned. Meanwhile, arsenic poisoning as a result of the natural occurrence of arsenic compounds in drinking water remains a problem for many parts of the world including the United States.

Poisoning due to Arsenic

Germanium

Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. Germanium has five naturally occurring isotopes ranging in atomic mass number from 70 to 76. It forms a large number of organometallic compounds, including tetraethylgermane and isobutylgermane.

Germanium was discovered comparatively late because very few minerals contain it in high concentration. Germanium ranks near fiftieth in relative abundance of the elements in the Earth's crust. In 1869, Dmitri Mendeleev predicted its existence and some of its properties based on its position on his periodic table and called the element eka-silicon. Nearly two decades later, in 1886, Clemens Winkler found it in the mineral argyrodite. Winkler found that experimental observations agreed with Mendeleev's predictions and named the element after his country, Germany.

Germanium is an important semiconductor material used in transistors and various other electronic devices. Its major end uses are fiber-optic systems and infrared optics, but it is also used for polymerization catalysts, and in electronics and solar cell applications. It is finding a new use in nanowires.

Germanium is mined primarily from sphalerite, though it is also recovered from silver, lead, and copper ores. Some germanium compounds, such as germanium chloride and germane, can irritate the eyes, skin, lungs, and throat.

Gallium

Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium(III) salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies slightly above room temperature, it will melt in the hand. Its melting point is used as a temperature reference point, and from its discovery in 1875 to the semiconductor era, its primary uses were in high-temperature thermometric applications and in preparation of metal alloys with unusual properties of stability, or ease of melting; some being liquid at room temperature or below. The alloy Galinstan (68.5% Ga, 21.5% In, 10% Sn) has a melting point of about −19 °C (−2.2 °F).

In semiconductors, the major-use compound is gallium arsenide used in microwave circuitry and infrared applications. Gallium nitride and indium gallium nitride, minority semiconductor uses, produce blue and violet light-emitting diodes (LEDs) and diode lasers. Semiconductor use is now almost the entire (> 95%) world market for gallium, but new uses in alloys and fuel cells continue to be discovered.

Gallium is not known to be essential in biology, but because of the biological handling of gallium's primary ionic salt gallium(III) as though it were iron(III), the gallium ion localizes to and interacts with many processes in the body in which iron(III) is manipulated. As these processes include inflammation, which is a marker for many disease states, several gallium salts are used, or are in development, as both pharmaceuticals and radiopharmaceuticals in medicine.

Zinc

Zinc, also known as spelter, is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2. Zinc is the 24th most abundant element in the Earth's crust and has five stable isotopes. The most exploited zinc ore is sphalerite, a zinc sulfide. The largest exploitable deposits are found in Australia, Asia, and the United States. Zinc production includes froth flotation of the ore, roasting, and final extraction using electricity (electrowinning).

Brass, which is an alloy of copper and zinc, has been used since at least the 10th century BC. Impure zinc metal was not produced in large scale until the 13th century in India, while the metal was unknown to Europe until the end of the 16th century. Alchemists burned zinc in air to form what they called "philosopher's wool" or "white snow".

The element was probably named by the alchemist Paracelsus after the German word Zinke. German chemist Andreas Sigismund Marggraf is normally given credit for discovering pure metallic zinc in 1746. Work by Luigi Galvani and Alessandro Volta uncovered the electrochemical properties of zinc by 1800. Corrosion-resistant zinc plating of steel (hot-dip galvanizing) is the major application for zinc. Other applications are in batteries and alloys, such as brass. A variety of zinc compounds are commonly used, such as zinc carbonate and zinc gluconate (as dietary supplements), zinc chloride (in deodorants), zinc pyrithione (anti-dandruff shampoos), zinc sulfide (in luminescent paints), and zinc methyl or zinc diethyl in the organic laboratory.

Zinc is an essential mineral of "exceptional biologic and public health importance". Zinc deficiency affects about two billion people in the developing world and is associated with many diseases. In children it causes growth retardation, delayed sexual maturation, infection susceptibility, and diarrhea, contributing to the death of about 800,000 children worldwide per year. Enzymes with a zinc atom in the reactive center are widespread in biochemistry, such as alcohol dehydrogenase in humans. Consumption of excess zinc can cause ataxia, lethargy and copper deficiency.

Copper

Copper is a chemical element with the symbol Cu (Latin: cuprum) and atomic number 29. It is a ductile metal, with very high thermal and electrical conductivity. Pure copper is rather soft and malleable, and a freshly exposed surface has a reddish-orange color. It is used as a thermal conductor, an electrical conductor, a building material, and a constituent of various metal alloys. Copper metal and alloys have been used for thousands of years. In the Roman era, copper was principally mined on Cyprus, hence the origin of the name of the metal as Cyprium, "metal of Cyprus", later shortened to Cuprum.

There may be insufficient reserves to sustain current high rates of copper consumption. Some countries, such as Chile and the United States, still have sizeable reserves of the metal which are extracted through large open pit mines.

Copper compounds are commonly encountered as salts of Cu2+, which often impart blue or green colors to minerals such as turquoise and have been used historically widely as pigments. Copper metal architectural structures and statuary eventually corrode to acquire a characteristic green patina. Copper as both metal and pigmented salt, has a significant presence in decorative art.

Copper(II) ions (Cu2+) are soluble in water, where they function at low concentration as bacteriostatic substances, fungicides, and wood preservatives. In sufficient amounts, copper salts can be poisonous to higher organisms as well. However, despite universal toxicity at high concentrations, the Cu2+ ion at lower concentrations is an essential trace nutrient to all higher plant and animal life. In animals, including humans, it is found widely in tissues, with concentration in liver, muscle, and bone. It functions as a co-factor in various enzymes and in copper-based pigments.

Nickel

Nickel (Ni) is the only element named after the devil. The name comes from the German word Kupfernickel, meaning "Old Nick's copper," a term used by German miners.

Nickel was discovered by the Swedish chemist Axel Fredrik Cronstedt in the mineral niccolite (NiAs) in 1751. Today, most nickel is obtained from the mineral pentlandite (NiS•2FeS). Most of the world's supply of nickel is mined in the Sudbury region of Ontario, Canada. It is believed that this large deposit of nickel ore is a result of an ancient meteor impact.

Nickel is a hard, corrosion resistant metal. It can be electroplated onto other metals to form a protective coating. Finely divided nickel is used as a catalyst for the hydrogenation of vegetable oils. Adding nickel to glass gives it a green color. A single kilogram of nickel can be drawn into 300 kilometers of wire. Nickel is also used to manufacture some types of coins and batteries.

Nickel is alloyed with other metals to improve their strength and resistance to corrosion. Nickel is alloyed with steel to make armor plate, vaults and machine parts. It is alloyed with copper to make pipes that are used in desalination plants. Very powerful permanent magnets, known as Alnico magnets, can be made from an alloy of aluminum, nickel, cobalt and iron.

Cobalt

Cobalt  is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal.

Cobalt-based blue pigments have been used since ancient times for jewelry and paints, and to impart a distinctive blue tint to glass, but the color was later thought by alchemists to be due to the known metal bismuth. Miners had long used the name Kobold ore (German for goblin ore) for some of the blue-pigment producing minerals; they were named because they were poor in known metals, and gave poisonous arsenic-containing fumes upon smelting. In 1735, such ores were found to be reducible to a new metal (the first discovered since ancient times), and this was ultimately named for the Kobold.

Today, some cobalt is produced specifically from various metallic-lustered ores, for example cobaltite (CoAsS), but the main source of the element is as a by-product of copper and nickel mining. The copper belt in the Democratic Republic of the Congo and Zambia yields most of the cobalt metal mined worldwide.

Cobalt is used in the preparation of magnetic, wear-resistant, and high-strength alloys. Smalt (cobalt silicate glass) and cobalt blue (cobalt(II) aluminate, CoAl2O4) gives a distinctive deep blue color to glass, ceramics, inks, paints, and varnishes. Cobalt occurs naturally as only one stable isotope, cobalt-59. Cobalt-60 is a commercially important radioisotope, used as a tracer and in the production of gamma rays for industrial use.

Cobalt is an essential trace element for all animals, as the active center of coenzymes called cobalamins. These include vitamin B12 which is essential for mammals. Cobalt is also an active nutrient for bacteria, algae, and fungi.

Manganese

Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature (often in combination with iron), and in many minerals. As a free element, manganese is a metal with important industrial metal alloy uses, particularly in stainless steels.

Manganese phosphating is used as a treatment for rust and corrosion prevention on steel. Depending on their oxidation state, manganese ions have various colors and are used industrially as pigments. The permanganates of alkali and alkaline earth metals are powerful oxidizers. Manganese dioxide is used as the cathode (electron acceptor) material in standard and alkaline disposable dry cells and batteries.

Manganese(II) ions function as cofactors for a number of enzymes in higher organisms, where they are essential in detoxification of superoxide free radicals. The element is a required trace mineral for all known living organisms. In larger amounts, and apparently with far greater activity by inhalation, manganese can cause a poisoning syndrome in mammals, with neurological damage which is sometimes irreversible.

The most common oxidation states of manganese are +2, +3, +4, +6 and +7, though oxidation states from −3 to +7 are observed. Mn2+ often competes with Mg2+ in biological systems. Manganese compounds where manganese is in oxidation state +7, which are restricted to the unstable oxide Mn2O7 and compounds of the intensely purple permanganate anion MnO4−, are powerful oxidizing agents. Compounds with oxidation states +5 (blue) and +6 (green) are strong oxidizing agents and are vulnerable to disproportionation.

The most stable oxidation state for manganese is +2, which has a pale pink color, and many manganese(II) compounds are known, such as manganese(II) sulfate (MnSO4) and manganese(II) chloride (MnCl2). This oxidation state is also seen in the mineral rhodochrosite, (manganese(II) carbonate). The +2 oxidation state is the state used in living organisms for essential functions; other states are toxic for the human body. The +2 oxidation of Mn results from removal of the two 4s electrons, leaving a "high spin" ion in which all five of the 3d orbitals contain a single electron. Absorption of visible light by this ion is accomplished only by a spin-forbidden transition in which one of the d electrons must pair with another, to give the atom a change in spin of two units. The unlikeliness of such a transition is seen in the uniformly pale and almost colorless nature of Mn (II) compounds relative to other oxidation states of manganese.

Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable. The name of the element is derived from the Greek word "chrōma" (χρώμα), meaning colour, because many of its compounds are intensely coloured. It was discovered by Louis Nicolas Vauquelin in the mineral crocoite (lead chromate) in 1797. Crocoite was used as a pigment, and after the discovery that the mineral chromite also contains chromium this latter mineral was used to produce pigments as well.

Chromium was regarded with great interest because of its high corrosion resistance and hardness. A major development was the discovery that steel could be made highly resistant to corrosion and discoloration by adding chromium to form stainless steel. This application, along with chrome plating (electroplating with chromium) are currently the highest-volume uses of the metal. Chromium and ferrochromium are produced from the single commercially viable ore, chromite, by silicothermic or aluminothermic reaction or by roasting and leaching processes. Although trivalent chromium (Cr(III)) is required in trace amounts for sugar and lipid metabolism, few cases have been reported where its complete removal from the diet has caused chromium deficiency. In larger amounts and different forms chromium can be toxic and carcinogenic. The most prominent example of toxic chromium is hexavalent chromium (Cr(VI)). Abandoned chromium production sites often require environmental cleanup.

Vanadium

Vanadium  is a chemical element with the symbol V and atomic number 23. It is a soft, silvery gray, ductile transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature. Andrés Manuel del Río discovered vanadium in 1801 by analyzing a new lead-bearing mineral he called "brown lead," and named the new element erythronium (Greek for "red") since, upon heating, most of its salts turned from their initial color to red. Four years later, however, he was convinced by other scientists that erythronium was identical to chromium. The element was rediscovered in 1831 by Nils Gabriel Sefström, who named it vanadium after the Scandinavian goddess of beauty and fertility, Vanadis (Freya). Both names were attributed to the wide range of colors found in vanadium compounds. Del Rio's lead mineral was later renamed vanadinite for its vanadium content.

The element occurs naturally in about 65 different minerals and in fossil fuel deposits. It is produced in China and Russia from steel smelter slag; other countries produce it either from the flue dust of heavy oil, or as a byproduct of uranium mining. It is mainly used to produce specialty steel alloys such as high speed tool steels. The most important industrial vanadium compound, vanadium pentoxide, is used as a catalyst for the production of sulfuric acid.

Large amounts of vanadium ions are found in a few organisms, possibly as a toxin. The oxide and some other salts of vanadium have moderate toxicity. Particularly in the ocean, vanadium is used by some life forms as an active center of enzymes, such as the vanadium bromoperoxidase of some ocean algae. Vanadium is probably a micronutrient in mammals, including humans, but its precise role in this regard is unknown.

Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the "space age metal", it has a low density and is a strong, lustrous, corrosion-resistant (including sea water, aqua regia and chlorine) transition metal with a silver color.

Titanium was discovered in Cornwall, England, by William Gregor in 1791 and named by Martin Heinrich Klaproth for the Titans of Greek mythology.

The element occurs within a number of mineral deposits, principally rutile and ilmenite, which are widely distributed in the Earth's crust and lithosphere, and it is found in almost all living things, rocks, water bodies, and soils. The metal is extracted from its principal mineral ores via the Kroll process or the Hunter process. Its most common compound, titanium dioxide, is a popular photocatalyst and is used in the manufacture of white pigments. Other compounds include titanium tetrachloride (TiCl4), a component of smoke screens and catalysts; and titanium trichloride (TiCl3), which is used as a catalyst in the production of polypropylene).

Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other elements, to produce strong lightweight alloys for aerospace (jet engines, missiles, and spacecraft), military, industrial process (chemicals and petro-chemicals, desalination plants, pulp, and paper), automotive, agri-food, medical prostheses, orthopedic implants, dental and endodontic instruments and files, dental implants, sporting goods, jewelry, mobile phones, and other applications.

The two most useful properties of the metal form are corrosion resistance and the highest strength-to-weight ratio of any metal. In its unalloyed condition, titanium is as strong as some steels, but 45% lighter. There are two allotropic forms and five naturally occurring isotopes of this element, 46Ti through 50Ti, with 48Ti being the most abundant (73.8%). Titanium's properties are chemically and physically similar to zirconium, because both of them have the same number of valence electrons and are in the same group in the periodic table.

Scandium

Scandium is a chemical element with symbol Sc and atomic number 21. A silvery-white metallic transition metal, it has historically been sometimes classified as a rare earth element, together with yttrium and the lanthanoids. In 1879, Lars Fredrik Nilson and his team, found a new element with spectral analysis, in the minerals euxenite and gadolinite from Scandinavia.

Scandium is present in most of the rare earth element and uranium deposits, but it is extracted from these ores in only a few mines worldwide. Due to the low availability and the difficulties in the preparation of metallic scandium, which was first done in 1937, it took until the 1970s before applications for scandium were developed. The positive effects of scandium on aluminium alloys were discovered in the 1970s, and its use in such alloys remains its only major application.

The properties of Sc compounds are intermediate between the properties of Al and Y, and there is a diagonal relationship between the behavior of Mg and Sc, just as there is between Be and Al. There has been controversy as to whether yttrium is in the same group as lanthanum or as lutetium. In the chemical compounds of the elements shown as group 3, above, the predominant oxidation state is +3. The ions M3+ will all have the electronic configuration of a noble gas, so it is reasonable that they should be in the same group of the periodic table. Most modern text-books place Sc, Y, La and Ac in the same periodic group. Scandium metal is hard and has a silvery appearance. It develops a slightly yellowish or pinkish cast when exposed to air. It is not resistant to weathering and dissolves slowly in most dilute acids. It does not react with a 1:1 mixture of nitric acid (HNO3) and hydrofluoric acid, HF, presumably due to the formation of an impermeable passive layer on the surface of the metal.

Calcium

 Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust. Calcium is also the fifth most abundant dissolved ion in seawater by both molarity and mass, after sodium, chloride, magnesium, and sulfate.
Calcium is essential for living organisms, particularly in cell physiology, where movement of the calcium ion Ca2+ into and out of the cytoplasm functions as a signal for many cellular processes. As a major material used in mineralization of bones and shells, calcium is the most abundant metal by mass in many animals. Chemically calcium is reactive and soft for a metal (though harder than lead, it can be cut with a knife with difficulty). It is a silvery metallic element that must be extracted by electrolysis from a fused salt like calcium chloride. Once produced, it rapidly forms a gray-white oxide and nitride coating when exposed to air. In bulk-form (typically as chips or "turnings") the metal is somewhat difficult to ignite, more so even than magnesium chips; but when lit, the metal burns in air with a brilliant high-intensity red light. Calcium metal reacts with water, evolving hydrogen gas at a rate rapid enough to be noticeable, but not fast enough at room temperature to generate much heat. In powdered form, however, the reaction with water is extremely rapid, as the increased surface area of the powder accelerates the reaction with the water. Part of the slowness of the calcium-water reaction results from the metal being partly protected by insoluble white calcium hydroxide. In water solutions of acids, where this salt is soluble, calcium reacts vigorously.

Calcium, with a density of 1.55 g/cm3, is the lightest of the alkaline earth metals; magnesium (specific gravity 1.74) and beryllium (1.84) are more dense, although lighter in atomic mass. From strontium onward, the alkali earth metals become more dense with increasing atomic mass.
It has two allotropes.

Calcium has a higher electrical resistivity than copper or aluminium, yet weight-for-weight, due to its much lower density, it is a rather better conductor than either. However, its use in terrestrial applications is usually limited by its high reactivity with air.

Calcium salts are colorless from any contribution of the calcium, and ionic solutions of calcium (Ca2+) are colorless as well. Many calcium salts are not soluble in water. When in solution, the calcium ion to the human taste varies remarkably, being reported as mildly salty, sour, "mineral like" or even "soothing." It is apparent that many animals can taste, or develop a taste, for calcium, and use this sense to detect the mineral in salt licks or other sources. In human nutrition, soluble calcium salts may be added to tart juices without much effect to the average palate.

Calcium is the fifth most abundant element by mass in the human body, where it is a common cellular ionic messenger with many functions, and serves also as a structural element in bone. It is the relatively high atomic-numbered calcium in the skeleton which causes bone to be radio-opaque. Of the human body's solid components after drying and burning of organics (as for example, after cremation), about a third of the total "mineral" mass remaining, is the approximately one kilogram of calcium which composes the average skeleton (the remainder being mostly phosphorus and oxygen).

Potassium

Potassium is the chemical element with the symbol K (Latin: kalium), atomic number 19, and atomic mass 39.0983. Elemental potassium is a soft silvery-white metallic alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.

Potassium and sodium are alkali metals and are chemically very similar. For this reason, historically their salts were not differentiated. They were finally realized to be different elements when the metals were isolated by electrolysis in the early 19th century. Potassium in nature occurs only as ionic salt. As such, it is found dissolved in seawater, and as part of many minerals. Industrical chemical applications of potassium tend to employ potassium ion's extreme water-solubility as part of chemicals which depend for activity on their non-potassium components. Potassium metal has only a few specialty applications, being replaced in most chemical reactions with sodium metal.

Potassium ion is necessary for the function of all living cells, and is thus present in all plant and animal tissues. It is found in especially high concentrations within plant cells, and in a mixed diet, it is most highly concentrated in fruits. The high concentration of potassium in plants, associated with comparatively very low amounts of sodium there, historically resulted in potassium first being isolated from the ashes of plants (potash), which in turn gave the element its modern name. Heavy crop production rapidly depletes soils of potassium, and agricultural fertilizers consume 93% of the potassium chemical production of the modern world economy.

The functions of potassium and sodium in living organisms are quite different. Animals, in particular, employ sodium and potassium differentially to generate electrical potentials in animal cells, especially in nervous tissue. Potassium depletion in animals, including humans, results in various neurological dysfunctions.

Argon

Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table (noble gases). Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide. Nearly all of this argon is radiogenic argon-40 derived from the decay of potassium-40 in the Earth's crust. In the universe, argon-36 is by far the most common argon isotope, being the preferred argon isotope produced by stellar nucleosynthesis in supernovas.

The name "argon" is derived from the Greek word meaning "the inactive one", a reference to the fact that the element undergoes almost no chemical reactions. The complete octet (eight electrons) in the outer atomic shell makes argon stable and resistant to bonding with other elements. Its triple point temperature of 83.8058 K is a defining fixed point in the International Temperature Scale of 1990.

Argon is produced industrially by the fractional distillation of liquid air. Argon is mostly used as an inert shielding gas in welding and other high-temperature industrial processes where ordinarily non-reactive substances become reactive; for example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning. Argon gas also has uses in incandescent and fluorescent lighting, and other types of gas discharge tubes. Argon makes a distinctive blue-green gas laser. Argon has approximately the same solubility in water as oxygen gas and is 2.5 times more soluble in water than nitrogen gas. Argon is colorless, odorless, and nontoxic as a solid, liquid, and gas. Argon is inert under most conditions and forms no confirmed stable compounds at room temperature.

Although argon is a noble gas, it has been found to have the capability of forming some compounds. For example, the creation of argon fluorohydride (HArF), a marginally stable compound of argon with fluorine and hydrogen, was reported by researchers at the University of Helsinki in 2000. Although the neutral ground-state chemical compounds of argon are presently limited to HArF, argon can form clathrates with water when atoms of it are trapped in a lattice of the water molecules. Argon-containing ions and excited state complexes, such as ArH+ and ArF, respectively, are known to exist. Theoretical calculations have predicted several argon compounds that should be stable, but for which no synthesis routes are currently known.

Chlorine

Chlorine is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17. As the chloride ion, which is part of common salt and other compounds, it is abundant in nature and necessary to most forms of life, including humans. In its elemental form (Cl2 or "dichlorine") under standard conditions, chlorine is a powerful oxidant and is used in bleaching and disinfectants, as well as an essential reagent in the chemical industry. As a common disinfectant, chlorine compounds are used in swimming pools to keep them clean and sanitary. In the upper atmosphere, chlorine-containing molecules such as chlorofluorocarbons have been implicated in ozone depletion. At standard temperature and pressure, two chlorine atoms form the diatomic molecule Cl2. This is a yellow-green gas that has its distinctive strong smell, the smell of bleach. The bonding between the two atoms is relatively weak (only 242.580 ±0.004 kJ/mol), which makes the Cl2 molecule highly reactive. The boiling point at regular atmosphere is around −34 ˚C, but it can be liquefied at room temperature with pressures above 8 atmospheres.

Sulfur (sulphur )

Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a bright yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals. It is an essential element for life and is found in two amino acids: cysteine and methionine. Its commercial uses are primarily in fertilizers, but it is also widely used in black gunpowder, matches, insecticides and fungicides. Elemental sulfur crystals are commonly sought after by mineral collectors for their brightly colored polyhedron shapes. In nonscientific contexts, it can also be referred to as brimstone.

At room temperature, sulfur is a soft, bright-yellow solid with only a faint odor, similar to that of matches (the strong "smell of sulfur" usually refers to the odor of hydrogen sulfide (H2S) or organosulfur compounds). Sulfur is an electrical insulator. It melts slightly above 100 °C and easily sublimes. Sulfur burns with a blue flame that emits sulfur dioxide, notable for its peculiar suffocating odor (this is the odor of burnt matches). Sulfur is insoluble in water, but soluble in carbon disulfide — and to a lesser extent in other non-polar organic solvents such as benzene and toluene. Sulfur in the solid state ordinarily exists as cyclic crown-shaped S8 molecules. The crystallography of sulfur is complex. Depending on the specific conditions, the sulfur allotropes form several crystal structures, with rhombic and monoclinic S8 best known.

Unlike most other liquids, molten sulfur increases in viscosity with temperatures of 200 °C (392 °F) due to the formation of polymers. The molten sulfur assumes a dark red color above this temperature. At still higher temperatures, however, the viscosity is decreased as depolymerization occurs.

Amorphous or "plastic" sulfur can be produced through the rapid cooling of molten sulfur. X-ray crystallography studies show that the amorphous form may have a helical structure with eight atoms per turn. This form is metastable at room temperature and gradually reverts to crystalline form. This process happens within a matter of hours to days but can be rapidly catalyzed.

Phosphorous

Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks. Elemental phosphorus exists in two major forms – white phosphorus and red phosphorus, but due to its high reactivity, phosphorus is never found as a free element on Earth.

The first form of elemental phosphorus to be produced (white phosphorus, in 1669) emits a faint glow upon exposure to oxygen – hence its name given from Greek mythology, meaning "light-bearer" (Latin Lucifer), referring to the "Morning Star", the planet Venus. Although the term "phosphorescence", meaning glow after illumination, derives from this property of phosphorus, the glow of phosphorus originates from oxidation of the white (but not red) phosphorus and should be called chemiluminescence.

Phosphorus compounds are used in explosives, nerve agents, friction matches, fireworks, pesticides, toothpastes, and detergents.

Phosphorus is a component of DNA, RNA, ATP, and also the phospholipids that form all cell membranes. It is thus an essential element for all living cells, and organisms tend to accumulate and concentrate it. For example, elemental phosphorus was historically first isolated from the sedement in human urine, and bone ash was an important early phosphate source. Low phosphate levels are an important limit to growth in some aquatic systems. Today, the most important commercial use of phosphorus-based chemicals is the production of fertilizers, to replace the phosphorus that plants remove from the soil.

Slilicon

Silicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon.

Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure free element in nature. It is more widely distributed in dusts, sands, planetoids and planets as various forms of silicon dioxide (silica) or silicates. In Earth's crust, silicon is the second most abundant element after oxygen, making up 27.7% of the crust by mass.

Silicon has many industrial uses. It is the principal component of most semiconductor devices, most importantly integrated circuits or microchips. Silicon is widely used in semiconductors because it remains a semiconductor at higher temperatures than the semiconductor germanium and because its native oxide is easily grown in a furnace and forms a better semiconductor/dielectric interface than any other material.
In the form of silica and silicates, silicon forms useful glasses, cements, and ceramics. It is also a constituent of silicones, a class-name for various synthetic plastic substances made of silicon, oxygen, carbon and hydrogen, often confused with silicon itself.

Silicon is an essential element in biology, although only tiny traces of it appear to be required by animals. It is much more important to the metabolism of plants, particularly many grasses, and silicic acid (a type of silica) forms the basis of the striking array of protective shells of the microscopic diatoms.

Aluminium

Aluminium  is a silvery white member of the boron group of chemical elements. It has the symbol Al and its atomic number is 13. It is not soluble in water under normal circumstances. Aluminium is the most abundant metal in the Earth's crust, and the third most abundant element, after oxygen and silicon. It makes up about 8% by weight of the Earth's solid surface. Aluminium is too reactive chemically to occur in nature as a free metal. Instead, it is found combined in over 270 different minerals. The chief source of aluminium is bauxite ore.

Aluminium is remarkable for the metal's low density and for its ability to resist corrosion due to the phenomenon of passivation. Structural components made from aluminium and its alloys are vital to the aerospace industry and are very important in other areas of transportation and building. Its reactive nature makes it useful as a catalyst or additive in chemical mixtures, including ammonium nitrate explosives, to enhance blast power.

Despite its prevalence in the environment, aluminium salts are not known to be used by any form of life. Also in keeping with the element's abundance, it is well tolerated by plants in soils (in which it is a major component), and to a lesser extent, by animals as a component of plant materials in the diet (which often contain traces of dust and soil). Soluble aluminium salts have some demonstrated toxicity to animals if delivered in quantity by unnatural routes, such as injection. Controversy still exists about aluminium's possible long-term toxicity to humans from larger ingested amounts.

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust, where it constitutes about 2% by mass, and ninth in the known Universe as a whole. This preponderance of magnesium is related to the fact that it is easily built up in supernova stars from a sequential addition of three helium nuclei to carbon (which in turn is made from three helium nuclei). Magnesium ion's high solubility in water helps ensure that it is the third most abundant element dissolved in seawater.

Magnesium is the 11th most abundant element by mass in the human body; its ions are essential to all living cells, where they play a major role in manipulating important biological polyphosphate compounds like ATP, DNA, and RNA. Hundreds of enzymes thus require magnesium ions to function. Magnesium is also the metallic ion at the center of chlorophyll, and is thus a common additive to fertilizers. Magnesium compounds are used medicinally as common laxatives, antacids (i.e., milk of magnesia), and in a number of situations where stabilization of abnormal nerve excitation and blood vessel spasm is required (i.e., to treat eclampsia). Magnesium ions are sour to the taste, and in low concentrations help to impart a natural tartness to fresh mineral waters.

The free element (metal) is not found naturally on Earth, as it is highly reactive (though once produced, is coated in a thin layer of oxide (see passivation), which partly masks this reactivity). The free metal burns with a characteristic brilliant white light, making it a useful ingredient in flares. The metal is now mainly obtained by electrolysis of magnesium salts obtained from brine. Commercially, the chief use for the metal is as an alloying agent to make aluminium-magnesium alloys, sometimes called "magnalium" or "magnelium". Since magnesium is less dense than aluminium, these alloys are prized for their relative lightness and strength.

Sodium

Sodium is a metallic element with a symbol Na (from Latin natrium or Arabic natrun; perhaps ultimately from Egyptian netjerj) and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1" (formerly known as ‘group IA’). It has only one stable isotope, 23Na.

Elemental sodium was first isolated by Humphry Davy in 1807 by passing an electric current through molten sodium hydroxide. Elemental sodium does not occur naturally on Earth, because it quickly oxidizes in air and is violently reactive with water, so it must be stored in an non-oxidizing medium, such as a liquid hydrocarbon. The free metal is used for some chemical synthesis, analysis, and heat transfer applications.

Sodium ion is soluble in water, and is thus present in great quantities in the Earth's oceans and other stagnant bodies of water. In these bodies it is mostly counterbalanced by the chloride ion, causing evaporated ocean water solids to consist mostly of sodium chloride, or common table salt. Sodium ion is also a component of many minerals.

Sodium is an essential element for all animal life (including human) and for some plant species. In animals, sodium ions are used in opposition to potassium ions, to allow the organism to build up an electrostatic charge on cell membranes, and thus allow transmission of nerve impulses when the charge is allowed to dissipate by a moving wave of voltage change. Sodium is thus classified as a “dietary inorganic macro-mineral” for animals. Sodium's relative rarity on land is due to its solubility in water, thus causing it to be leached into bodies of long-standing water by rainfall. Such is its relatively large requirement in animals, in contrast to its relative scarcity in many inland soils, that herbivorous land animals have developed a special taste receptor for the sodium ion.

Neon

Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in discharge tubes and neon lamps and advertising signs. It is commercially extracted from air, in which it is found in trace amounts. Neon is actually abundant on a universal scale; it is the fifth most abundant chemical element in the universe by mass, after hydrogen, helium, oxygen, and carbon (see chemical element). Its relative rarity on Earth, like that of helium, is due to its relative lightness, high vapor pressure at very low temperatures, and chemical inertness, all properties which tend to keep it from being trapped in the condensing gas and dust clouds which resulted in the formation of smaller and warmer solid planets like Earth.

Neon is monatomic, making it lighter than the molecules of diatomic nitrogen and oxygen which form the bulk of Earth's atmosphere; a balloon filled with neon will rise in air, albeit more slowly than a helium balloon.

Mass abundance in the universe is about 1 part in 750 and in the Sun and presumably in the proto-solar system nebula, about 1 part in 600. The Galileo spacecraft atmospheric entry probe found that even in the upper atmosphere of Jupiter, the abundance of neon is reduced (depleted) by about a factor of 10, to a level of 1 part in 6,000 by mass. This may indicate that even the ice-planetesimals which brought neon into Jupiter from the outer solar system, formed in a region which was too warm for them to have kept their neon (abundances of heavier inert gases on Jupiter are several times that found in the Sun).

Neon is a monatomic gas at standard conditions. Neon is rare on Earth, found in the Earth's atmosphere at 1 part in 65,000 (by volume) or 1 part in 83,000 by mass. It is industrially produced by cryogenic fractional distillation of liquefied air.

Fluorine

Fluorine is the chemical element with atomic number 9, represented by the symbol F. Fluorine forms a single bond with itself in elemental form, resulting in the diatomic F2 molecule. F2 (fluorine) is a supremely reactive, poisonous, pale, yellowish brown gas. Elemental fluorine is the most chemically reactive and electronegative of all the elements. For example, it will readily "burn" hydrocarbons at room temperature, in contrast to the combustion of hydrocarbons by oxygen, which requires an input of energy with a spark. Therefore, molecular fluorine is highly dangerous, more so than other halogens such as the poisonous chlorine gas.

Fluorine's highest electronegativity and small atomic radius give unique properties to many of its compounds. For example, the enrichment of 235U, the principal nuclear fuel, relies on the volatility of UF6. Also, the carbon–fluorine bond is one of the strongest bonds in organic chemistry. This contributes to the stability and persistence of fluoroalkane based organofluorine compounds, such as PTFE/(Teflon) and PFOS. The carbon–fluorine bond's inductive effects result in the strength of many fluorinated acids, such as triflic acid and trifluoroacetic acid. Drugs are often fluorinated at biologically reactive positions, to prevent their metabolism and prolong their half-lives.

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots  (oxys) (acid, literally "sharp", referring to the sour taste of acids) and -γενής (-genēs) (producer, literally begetter), because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition.

Oxygen is a member of the chalcogen group on the periodic table, and is a highly reactive nonmetallic period 2 element that readily forms compounds (notably oxides) with almost all other elements. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless, odorless, tasteless diatomic gas with the formula O2. By mass, oxygen is the third most abundant element in the universe after hydrogen and helium and the most abundant element by mass in the Earth's crust. Diatomic oxygen gas constitutes 20.8% of the volume of air.

All major classes of structural molecules in living organisms, such as proteins, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that comprise animal shells, teeth, and bone. Oxygen in the form of O2 is produced from water by cyanobacteria, algae and plants during photosynthesis and is used in cellular respiration for all complex life. Oxygen is toxic to obligately anaerobic organisms. Anaerobes were the dominant form of early life on Earth until O2 began to accumulate in the atmosphere roughly 2.5 billion years ago. Another form (allotrope) of oxygen, ozone (O3), helps protect the biosphere from ultraviolet radiation with the high-altitude ozone layer, but is a pollutant near the surface where it is a by-product of smog. At even higher low earth orbit altitudes atomic oxygen is a significant presence and a cause of erosion for spacecraft.

Oxygen was independently discovered by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley in Wiltshire, in 1774, but Priestley is often given priority because his publication came out in print first. The name oxygen was coined in 1777 by Antoine Lavoisier, whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. Oxygen is produced industrially by fractional distillation of liquefied air, use of zeolites to remove carbon dioxide and nitrogen from air, electrolysis of water and other means. Uses of oxygen include the production of steel, plastics and textiles; rocket propellant; oxygen therapy; and life support in aircraft, submarines, spaceflight and diving.

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere. The element nitrogen was discovered as a separable component of air, by Scottish physician Daniel Rutherford, in 1772.

Many industrially important compounds, such as ammonia, nitric acid, organic nitrates (propellants and explosives), and cyanides, contain nitrogen. The extremely strong bond in elemental nitrogen dominates nitrogen chemistry, causing difficulty for both organisms and industry in breaking the bond to convert the N2 into useful compounds, but at the same time causing release of large amounts of often useful energy when the compounds burn, explode, or decay back into nitrogen gas.

Nitrogen occurs in all living organisms, and the nitrogen cycle describes movement of the element from air into the biosphere and organic compounds, then back into the atmosphere. Synthetically-produced nitrates are key ingredients of industrial fertilizers, and also key pollutants in causing the eutrophication of water systems. Nitrogen is a constituent element of amino acids and thus of proteins, and of nucleic acids (DNA and RNA). It resides in the chemical structure of almost all neurotransmitters, and is a defining component of alkaloids, biological molecules produced by many organisms.

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. There are three naturally occurring isotopes, with 12C and 13C being stable, while 14C is radioactive, decaying with a half-life of about 5730 years. Carbon is one of the few elements known since antiquity. The name "carbon" comes from Latin language carbo, coal.

There are several allotropes of carbon of which the best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form. For example, diamond is highly transparent, while graphite is opaque and black. Diamond is among the hardest materials known, while graphite is soft enough to form a streak on paper (hence its name, from the Greek word "to write"). Diamond has a very low electrical conductivity, while graphite is a very good conductor. Under normal conditions, diamond has the highest thermal conductivity of all known materials. All the allotropic forms are solids under normal conditions but graphite is the most thermodynamically stable.

All forms of carbon are highly stable, requiring high temperature to react even with oxygen. The most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and other transition metal carbonyl complexes. The largest sources of inorganic carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil and methane clathrates. Carbon forms more compounds than any other element, with almost ten million pure organic compounds described to date, which in turn are a tiny fraction of such compounds that are theoretically possible under standard conditions.

Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is present in all known lifeforms, and in the human body carbon is the second most abundant element by mass (about 18.5%) after oxygen. This abundance, together with the unique diversity of organic compounds and their unusual polymer-forming ability at the temperatures commonly encountered on Earth, make this element the chemical basis of all known life.

Boron

Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. A low-abundance element in both the solar system and the Earth's crust, boron is concentrated on Earth by the water-solubility of its more common naturally-occurring compounds, the borate minerals. These are mined industrially as evaporate ores, such as borax and kernite.

Elemental boron is not found naturally. Industrially, very pure isolated boron is produced with difficulty, as boron tends to form refractory materials containing small amounts of carbon or other elements. Several allotropes of boron exist: amorphous boron is a brown powder and crystalline boron is black, extremely hard (about 9.5 on Mohs' scale), and a poor conductor at room temperature. Elemental boron is used as a dopant in the semiconductor industry.

The major uses of boron compounds are in sodium perborate bleaches, and the borax component of fiberglass insulation. Boron compounds play specialized roles as high-strength lightweight structural and refractory materials. They are used in glasses and ceramics to give them resistance to thermal shock. Boron-containing reagents are used for the synthesis of organic compounds, and as an intermediate in the synthesis of pharmaceuticals that do not contain boron.

In biology, borates have low toxicity in mammals (similar to table salt), but much more so to many arthropods. A boron-containing natural antibiotic is known. Small amounts of boron compounds play a strengthening role in the cell walls of all plants, making boron a necessary element in soils. Experiments indicate a role for boron as an ultratrace element in animals, but the nature of its role in animal physiology is unknown.

Beryllium

A divalent element, beryllium is found naturally only combined with other elements in minerals. Notable gemstones which contain beryllium include beryl (aquamarine, emerald) and chrysoberyl. The free element is a steel-gray, strong, lightweight brittle alkaline earth metal. It is primarily used as a hardening agent in alloys, notably beryllium copper. Structurally, beryllium's very low density (1.85 times that of water), high melting point (1287 °C), high temperature stability and low coefficient of thermal expansion, make it in many ways an ideal aerospace material, and it has been used in rocket nozzles and is a significant component of planned space telescopes. Because of its relatively high transparency to X-rays and other ionizing radiation types, beryllium also has a number of uses as filters and windows for radiation and particle physics experiments.

Commercial use of beryllium metal presents technical challenges due to the toxicity (especially by inhalation) of beryllium-containing dusts. Beryllium produces a direct corrosive effect to tissue, and can cause a chronic life-threatening allergic disease called berylliosis in susceptible persons. Because it is not synthesized in stars, beryllium is a relatively rare element in both the Earth and the universe. The element is not known to be necessary or useful for either plant or animal life.

Lithium

Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly reactive and flammable. For this reason, it is typically stored in mineral oil. When cut open, lithium exhibits a metallic luster, but contact with moist air corrodes the surface quickly to a dull silvery gray, then black, tarnish. Because of its high reactivity, lithium never occurs free in nature, and instead, only appears in compounds, usually ionic ones. Lithium occurs in a number of pegmatitic minerals, but is also commonly obtained from brines and clays. On a commercial scale, lithium is isolated electrolytically from a mixture of lithium chloride and potassium chloride.

The nuclei of lithium are not far from being unstable, since the two stable lithium isotopes found in nature have among the lowest binding energies per nucleon of all stable nuclides. As a result, they can be used in fission reactions as well as fusion reactions of nuclear devices. Due to its near instability, lithium is less common in the solar system than 25 of the first 32 chemical elements even though the nuclei are very light in atomic weight. For related reasons, lithium has important links to nuclear physics. The transmutation of lithium atoms to tritium was the first man-made form of a nuclear fusion reaction, and lithium deuteride serves as a fusion fuel in staged thermonuclear weapons.

Trace amounts of lithium are present in the oceans and in all organisms. The element serves no apparent vital biological function, since animal and plants survive in good health without it. Nonvital functions have not been ruled out. The lithium ion Li+ administered as any of several lithium salts has proved to be useful as a mood-stabilizing drug due to neurological effects of the ion in the human body. Lithium and its compounds have several industrial applications, including heat-resistant glass and ceramics, high strength-to-weight alloys used in aircraft, lithium batteries and lithium-ion batteries. These uses consume more than half of lithium production.

According to modern cosmological theory, lithium—as both of its stable isotopes lithium-6 and lithium-7—was among the 3 elements synthesized in the Big Bang. Though the amount of lithium generated in Big Bang nucleosynthesis is dependent upon the number of photons per baryon, for accepted values the lithium abundance can be calculated, and there is a "cosmological lithium discrepancy" in the Universe: older stars seem to have less lithium than they should, and some younger stars have far more. The lack of lithium in older stars is apparently caused by the "mixing" of lithium into the interior of stars, where it is destroyed. Furthermore, lithium is produced in younger stars. Though it transmutes into two atoms of helium due to collision with a proton at temperatures above 2.4 million degrees Celsius (most stars easily attain this temperature in their interiors), lithium is more abundant than predicted in later-generation stars, for causes not yet completely understood.

Though it was one of the three first elements (together with helium and hydrogen) to be synthesized in the Big Bang, lithium, together with beryllium and boron are markedly less abundant than other nearby elements. This is a result to the low temperature necessary to destroy lithium, and a lack of common processes to produce it.
Lithium is also found in brown dwarf stars and certain anomalous orange stars. Because lithium is present in cooler, less-massive brown dwarf stars, but is destroyed in hotter red dwarf stars, its presence in the stars' spectra can be used in the "lithium test" to differentiate the two, as both are smaller than the Sun. Certain orange stars can also contain a high concentration of lithium. Those orange stars found to have a higher than usual concentration of lithium (such as Centaurus X-4) orbit massive objects—neutron stars or black holes—whose gravity evidently pulls heavier lithium to the surface of a hydrogen-helium star, causing more lithium to be observed

Helium

Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Next to hydrogen, it is the second most abundant element in the universe, and accounts for 24% of the elemental mass of our galaxy.

An unknown yellow spectral line signature in sunlight was first observed from a solar eclipse in 1868 by French astronomer Pierre Janssen. Janssen is jointly credited with the discovery of the element with Norman Lockyer, who observed the same eclipse and was the first to propose that the line was due to a new element which he named helium. In 1903, large reserves of helium were found in the natural gas fields in parts of the United States, which is by far the largest supplier of the gas.

Helium is used in cryogenics (its largest single use, accounting for about a quarter of production), and the cooling of superconducting magnets, particularly the main commercial application in MRI scanners. Helium's other industrial uses as a pressurizing and purge gas, and a protective atmosphere for arc welding and processes (such as growing crystals to make silicon wafers), account for half of its use. Economically minor uses, such as lifting gas in balloons and airships are popularly known.  As with any gas with differing density from air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice. In scientific research, the behavior of two fluid phases of helium-4, helium I and helium II, is important to researchers studying quantum mechanics (in particular the phenomenon of superfluidity) and to those looking at the effects that temperatures near absolute zero have on matter (such as superconductivity).

Helium is the second lightest element and is the second most abundant in the observable universe, being present in the universe in masses more than 12 times those of all the heavier elements combined. Its abundance is also similar to this in our own Sun and Jupiter. This is due to the very high binding energy (per nucleon) of helium-4 with respect to the next three elements after helium (lithium, beryllium, and boron). This helium-4 binding energy also accounts for its commonality as a product in both nuclear fusion and radioactive decay. Most helium in the universe is helium-4, and is believed to have been formed during the Big Bang. Some new helium is being created currently as a result of the nuclear fusion of hydrogen in stars greater than 0.5 solar masses.

On Earth, the lightness of helium has caused its evaporation from the gas and dust cloud from which the planet condensed, and it is thus relatively rare—0.00052% by volume in the atmosphere. What helium is present today has been mostly created by the natural radioactive decay of heavy radioactive elements (thorium and uranium), as the alpha particles that are emitted by such decays consist of helium-4 nuclei. This radiogenic helium is trapped with natural gas in concentrations up to 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation.

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of 1.00794 u (1.007825 u for Hydrogen-1), hydrogen is the lightest and most abundant chemical element, constituting roughly 75 % of the Universe's chemical elemental mass. Stars in the main sequence are mainly composed of hydrogen in its plasma state. Naturally occurring elemental hydrogen is relatively rare on Earth.

The most common isotope of hydrogen is protium (name rarely used, symbol 1H) with a single proton and no neutrons. In ionic compounds it can take a negative charge (an anion known as a hydride and written as H−), or as a positively charged species H+. The latter cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds always occur as more complex species. Hydrogen forms compounds with most elements and is present in water and most organic compounds. It plays a particularly important role in acid-base chemistry with many reactions exchanging protons between soluble molecules. As the simplest atom known, the hydrogen atom has been of theoretical use. For example, as the only neutral atom with an analytic solution to the Schrödinger equation, the study of the energetics and bonding of the hydrogen atom played a key role in the development of quantum mechanics.

Hydrogen gas (now known to be H2) was first artificially produced in the early 16th century, via the mixing of metals with strong acids. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, and that it produces water when burned, a property which later gave it its name, which in Greek means "water-former." At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustible diatomic gas with the molecular formula H2.

Industrial production is mainly from the steam reforming of natural gas, and less often from more energy-intensive hydrogen production methods like the electrolysis of water. Most hydrogen is employed near its production site, with the two largest uses being fossil fuel processing (e.g., hydrocracking) and ammonia production, mostly for the fertilizer market.

Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks.

Transition metals (transition element)

The term transition metal (sometimes also called a transition element) has two possible meanings:

• In the past it referred to any element in the d-block of the periodic table, which includes groups 3 to 12 on the periodic table. All elements in the d-block are metals (In actuality, the f-block is also included in the form of the lanthanide and actinide series).
• The modern, IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.

Jensen has reviewed the historical usage of the terms transition element (or metal) and d-block. The word transition was first used to describe the elements now known as the d-block by the English chemist Charles Bury in 1921, who referred to a transition series of elements during the change of an inner layer of electrons (for example n=3 in the 4th row of the periodic table) from a stable group of 8 to one of 18, or from 18 to 32.

There are a number of properties shared by the transition elements that are not found in other elements, which results from the partially filled d shell. These include

• the formation of compounds whose colour is due to d - d electronic transitions
• the formation of compounds in many oxidation states, due to the relatively low reactivity of unpaired d electrons.
• the formation of many paramagnetic compounds due to the presence of unpaired d electrons. A few compounds of main group elements are also paramagnetic (e.g. nitric oxide, oxygen)

The f block (inner transition elements)

The f-block of the periodic table of the elements consists of those elements whose atoms or ions have valence electrons in f-orbitals. Actual electronic configurations may be slightly different from what is predicted by the aufbau principle. The elements are also known as inner transition elements. There are two series. Elements of the series in which the electrons are in 4f orbitals belong to the lanthanoid series. Elements of the series in which the electrons are in 5f orbitals belong to the actinoid series. There is a long-standing controversy as to whether La and Ac or Lu and Lr should belong to the f-block. IUPAC has now compromised by putting all four elements into the block, but this is contested, because there can only be 14 elements in f orbitals, so the block cannot be 15 elements wide.

All elements in the lanthanide series form M3+ ions. In aqueous solution the early lanthanides are surrounded by nine water molecules while the later lanthanides have a coordination number of 8. Cerium also forms compounds with the +4 oxidation state; Ce4+ has the very stable electronic configuration of the noble gas Xenon. Ce(IV) is a strong oxidising agent. Eu2+ has the configuration [Xe]4f7 and is a strong reducing agent. The existence of Eu(II) is attributed to the stability of the half-filled f-shell.
The lighter actinides (uranium to americium) show oxidation states of +3, +4, +5 and +6. The later actinides resemble the lanthanides in that the +3 oxidation state is favored.

The d block

The d-block is a the portion of the periodic table which contains the element groups 3-12 These groups correspond to the filling of the atomic d-orbital subshell, with electron configurations ranging from s2d1 (Group 3) to s2d10 (Group 12). There are however some irregularities in the sequence; for example Cr is s1d5 (not s2d4) and the Group 11 metals are s1d10 (not s2d9), so that the d-subshell is actually complete at Group 11.

The d-block elements are often also known as transition metals or transition elements. However the exact limits of the transition metal region are usually not considered to be identical to the d-block. Although some authors do identify the entire d-block as transition metals, most define transition metals as elements with partly filled d subshells either in the neutral atom or in ions in common oxidation states. This definition has now been adopted by IUPAC and corresponds to including only Groups 3-11 as transition metals. Group 12 metals lack the characteristic chemical and physical properties associated with incomplete d subshells and are considered post-transition metals. Jensen has reviewed the historical usage of the terms transition element (or metal) and d-block.

In the s-block and p-block of the periodic table, similar properties across the periods are generally not observed: the most important similarities tend to be vertical, down groups. However the d-block is notable in that horizontal similarities across the periods do become important.

Although Lutetium and Lawrencium are in the d-block, they are not considered transition metals but a lanthanide and an actinide, respectively, according to IUPAC. Group 12 elements are also in the d-block but are considered post-transition metals as their d-subshell is completely filled.

The S block

The s-block of the periodic table of elements consists of the first two groups: the alkali metals and alkaline earth metals, plus hydrogen and helium.

These elements are distinguished by the property that in the atomic ground state, the highest-energy electron is in an s-orbital. Except in hydrogen and helium, these electrons are very easily lost to form positive ions. The helium configuration is chemically exceedingly stable and thus helium has no known stable compounds; thus it is generally grouped with the noble gases.

The other elements of the s-block are all extremely powerful reducing agents, so much so that they never occur naturally in the free state. The metallic forms of these elements can only be extracted by electrolysis of a molten salt, since water is much more easily reduced to hydrogen than the ions of these metals. Sir Humphry Davy, in 1807 and 1808, was the first to isolate all of these metals except lithium, beryllium, rubidium and caesium. Beryllium was isolated independently by F. Wooler and A.A. Bussy in 1828, while lithium was isolated by Robert Bunsen in 1854, who isolated rubidium nine years later after having observed it and caesium spectroscopically. Caesium was not isolated until 1881 when Carl Setterberg electrolysed the molten cyanide.

The s-block metals vary from extremely soft (all the alkali metals) to quite hard (beryllium). With the exception of beryllium and magnesium, the metals are too reactive for any structural use except as very minor components (<2%) of alloys with lead. Beryllium and magnesium, though very expensive, are valuable for uses that require strength and lightness. They are extremely valuable as reducing agents to extract titanium, zirconium, thorium and tantalum from their ores, and have other uses as reducing agents in organic chemistry.

All the s-block metals are dangerous fire hazards which require special extinguishants to extinguish, except for beryllium and magnesium, storage must be under either argon or an inert liquid hydrocarbon. They react vigorously with water to liberate hydrogen, except for magnesium, which reacts slowly, and beryllium, which reacts only when amalgamated with mercury to destroy the oxide film. Lithium has similar properties to magnesium due to the diagonal relationship with magnesium in the periodic table.

The p-block

The p-block of the periodic table of the elements consists of the last six groups minus helium (which is located in the s-block). In the elemental form of the p-block elements, the highest energy electron occupies a p-orbital. The p-block contains all of the nonmetals (except for Hydrogen and Helium which are in the s-block) and semimetals, as well as some of the metals.

The groups of the p-block are:
•    13 (IIIB,IIIA): Boron Group
•    14 (IVB,IVA): Carbon Group
•    15 (VB,VA): Nitrogen Group
•    16 (VIB,VIA): Chalcogens
•    17 (VIIB,VIIA): Halogens
•    18 (Group 0): Noble gases (excluding Helium)