The d- and f-block in the periodic table

The d-block is the middle of the periodic table, groups 3 to 12. Here electrons go into the (n–1)d subshell, the d subshell of the shell below the outermost one. There are four rows: the 3d, 4d, 5d and 6d series.

The f-block sits in two separate rows at the bottom of the table. In the lanthanoids the 4f subshell fills; in the actinoids, the 5f.

Almost every question in this chapter comes back to the 3d series, the ten elements from scandium (21) to zinc (30): Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn. Many students remember them with a Hindi line, 'Scooty Van Car Mat Phenko, Nikuji'.

Writing a configuration is simple: write [Ar], then 3d, then 4s². The 4s takes two electrons, and the d gets the rest (atomic number minus 20). Manganese, 25, is [Ar]3d⁵4s²; cobalt, 27, is [Ar]3d⁷4s². The general pattern is (n–1)d¹⁻¹⁰ns¹⁻².

Two exceptions to learn:

• Chromium (24) is [Ar]3d⁵4s¹, not 3d⁴4s².
• Copper (29) is [Ar]3d¹⁰4s¹, not 3d⁹4s².

In each, one 4s electron moves into the 3d, because a half-filled (d⁵) or completely filled (d¹⁰) d subshell is extra stable.

A transition element has a partly filled d subshell, either in the atom or in one of its common ions. Zinc, cadmium and mercury have a full d¹⁰ subshell in the atom and in their ions, so they are not counted as transition elements. Silver is counted, even though the atom is 4d¹⁰, because its Ag²⁺ ion is 4d⁹.

When a transition atom forms a positive ion, it loses its 4s electrons first, before any 3d electrons. So iron, [Ar]3d⁶4s², becomes Fe²⁺ = [Ar]3d⁶.