The d block and the first series in use

Look at the long form of the periodic table. On the left is the s block (groups 1 and 2) and on the right the p block (groups 13 to 18). Between them, from period 4 downwards, sits the d block: the transition elements (counting the inner transition elements too, more than 60 elements, over half of all the elements known). In each of them the electrons being added go into a d sublevel. A d sublevel has five orbitals and holds at most 10 electrons, so each series has ten elements and the block has ten columns, groups 3 to 12 (in the older numbering IIIB, IVB, VB, VIB, VIIB, then three columns together called VIII, then IB and IIB). The three VIII columns (8, 9, 10) resemble one another along a row more than they resemble the elements below them. There are four horizontal series: the 3d series, Sc to Zn, in period 4; the 4d series, Y to Cd, in period 5; the 5d series, from La to Hg, in period 6; and the 6d series in period 7, which is incomplete. The f block (inner transition elements) is only mentioned here. In this course we study the first series.