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)
