A-level Chemistry/OCR (Salters)/Electronic configurations

Period 3 transition metals
All these elements have electron configurations that begin 1s2 2s2 2p6 3s2 3p6 but they have different 3d and 4s subshells. 1s2 2s2 2p6 3s2 3p6 is the electronic configuration of argon, so instead of writing this out over and over again, it's neater to write [Ar], which means the same thing.

All these d-block metals follow the same pattern, with the 4s subshell filled and the 3d subshell filling, except Cr and Cu which are 4s1 in order to allow a half-full and full 3d subshell, respectively. The exception for chromium and copper occurs because a half-full or full 3d subshell is lower in energy than expected, probably due to a quantum mechanical effect.

Transition metal ions
The electronic configurations of transition metal ions can seem counter-intuitive. For neutral transition metal atoms, the 4s subshell fills first, followed by the 3d subshell, which we assume is because 3d is slightly higher in energy than 4s.

However, in transition metal ions, the 4s empties first, which implies that it is higher in energy than the 3d. So which is higher in energy, 3d or 4s? The answer is they are very close in energy, and changing from a neutral atom to an ion causes the 3d to decrease in energy relative to the 4s, meaning that in ions, the 4s empties first, whereas in neutral atoms, the 4s fills first.