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.

Element	Proton number	Electron configuration
Sc	21	[Ar] 3d¹ 4s²
Ti	22	[Ar] 3d² 4s²
V	23	[Ar] 3d³ 4s²
Cr	24	[Ar] 3d⁵ 4s¹
Mn	25	[Ar] 3d⁵ 4s²
Fe	26	[Ar] 3d⁶ 4s²
Co	27	[Ar] 3d⁷ 4s²
Ni	28	[Ar] 3d⁸ 4s²
Cu	29	[Ar] 3d¹⁰ 4s¹
Zn	30	[Ar] 3d¹⁰ 4s²

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.

Species	Proton number	Electron configuration
Ti	22	[Ar] 3d² 4s²
Ti⁺	22	[Ar] 3d² 4s¹
Ti²⁺	22	[Ar] 3d²
Ti³⁺	22	[Ar] 3d¹
Ti⁴⁺	22	[Ar]