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

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Period 3 transition metals[edit]

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

Transition metal ions[edit]

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 Diagram
Ti 22 [Ar] 3d2 4s2 Electron-config-d2-s2.png
Ti+ 22 [Ar] 3d2 4s1 Electron-config-d2-s1.png
Ti2+ 22 [Ar] 3d2 Electron-config-d2-s0.png
Ti3+ 22 [Ar] 3d1 Electron-config-d1-s0.png
Ti4+ 22 [Ar] Electron-config-d0-s0.png