While first IE generally increases across Period 2, there are two anomalous dips that examiners love to test:
1st Ionisation Energy across Period 2
Anomaly 1: Be → B (Group 2 → 13)
Beryllium to Boron (Be → B): 2s vs 2p Subshell
Be (\(1\text{s}^2\,2\text{s}^2\)): Electron removed from the \(2\text{s}\) subshell.
B (\(1\text{s}^2\,2\text{s}^2\,2\text{p}^1\)): Electron removed from the \(2\text{p}\) subshell.
The \(2\text{p}\) subshell is higher in energy and less penetrating than \(2\text{s}\) (shielded by \(2\text{s}^2\)). Therefore, the single \(2\text{p}\) electron in Boron requires less energy to remove despite higher nuclear charge.
Anomaly 2: N → O (Group 15 → 16)
Nitrogen to Oxygen (N → O): Spin-Pairing Repulsion
N (\(2\text{p}^3\)): Three unpaired electrons in separate p-orbitals (\(\uparrow\, \uparrow\, \uparrow\)) with zero pairing repulsion.
O (\(2\text{p}^4\)): One doubly occupied p-orbital (\(\uparrow\!\downarrow\, \uparrow\, \uparrow\)).
The paired electron in Oxygen experiences inter-electron spin-pairing repulsion within the same orbital. This raises its potential energy and makes it easier to remove, resulting in a lower \(\text{IE}_1\) than Nitrogen.
Period 2 Anomalies Mirror Period 3
The exact same two anomalies repeat across Period 3: \(\text{Mg} \rightarrow \text{Al}\) (3s vs 3p subshell) and \(\text{P} \rightarrow \text{S}\) (3p spin-pairing repulsion). You must be prepared to write electron configurations and orbital box diagrams for both periods.
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