reasons for unregularity of the spdf of early and late transition metals of group 13?

Answer 1

nd and (n+1)s are actually very close in energy. So the textbooks tell you that (n+1)s is filled first, but things like the tendency towards filled shell and half filled shell stability can affect this. Something very similar happens with 6s, 5d, and 4f.

In cations, nd is always below (n+1)s.

Examples - a chromium atom is 3d54s1. Fe2+ is 3d6. An Fe atom is 3d64s2. Ni2+ is 3d8.

Actually, the configuration of transition metal atoms does not really matter all that much in chemistry. In the solid metals, the level spread out into overlapping bands. In compounds, the order is always nd below (n+1)s and 4f below 5d. The reasons are quite detailed, and involve the concept of incomplete shielding .