Position in the Periodic Table

The d-block of the periodic table lies between the s-block (Groups 1 and 2) and the p-block (Groups 13-18), occupying Groups 3 to 12. It is organized into four horizontal series, one for each principal quantum shell in which the $d$ orbitals are being progressively filled:

• The 3d series (first transition series): scandium (Sc, $Z=21$) to zinc (Zn, $Z=30$).
• The 4d series (second transition series): yttrium (Y, $Z=39$) to cadmium (Cd, $Z=48$).
• The 5d series (third transition series): lanthanum (La, $Z=57$) or hafnium (Hf, $Z=72$) to mercury (Hg, $Z=80$), depending on where the single f-block-adjacent element La is placed.
• The 6d series (fourth transition series): actinium (Ac, $Z=89$) onward, comprising largely short-lived, synthetically produced elements.

This WBCHSE unit restricts its detailed treatment to the first-row (3d) transition elements, Sc through Zn, which are by far the most chemically and industrially important series, and to the two families of the f-block: the lanthanoids (also called lanthanides), the fourteen elements cerium (Ce, $Z=58$) to lutetium (Lu, $Z=71$) that follow lanthanum, in which the 4f subshell is progressively filled; and the actinoids (actinides), the fourteen elements thorium (Th, $Z=90$) to lawrencium (Lr, $Z=103$) that follow actinium, in which the 5f subshell is progressively filled.

The f-block elements are conventionally detached from the main body of the periodic table and printed as two separate rows beneath it, purely as a matter of page layout -- chemically, each row genuinely belongs between Group 3 and Group 4, immediately after lanthanum and actinium respectively. Because lanthanum itself has the configuration $[\text{Xe}]5d^16s^2$ (no 4f electron) and lutetium has the configuration $[\text{Xe}]4f^{14}5d^16s^2$ (a completely filled 4f subshell), chemists differ on whether La or Lu is the "true" first member of the d-block's third row versus the first or last member of the f-block; this chapter follows the common convention of treating La as the first 5d element and Ce-Lu as the fourteen lanthanoids proper. …