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.
