🟣 This is Higher Level (HL) content.
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)
Be: 1s² 2s². Electron removed from 2s subshell
B: 1s² 2s² 2p¹. Electron removed from 2p subshell
The 2p subshell is higher in energy than 2s (further from nucleus, less penetrating). Therefore the 2p electron in B is easier to remove despite higher Zeff.
Anomaly 2: N → O (Group 15 → 16)
N: 2p³. Three electrons each in separate p-orbitals (↑ ↑ ↑). No pairing
O: 2p⁴. One orbital now has a paired electron (↑↓ ↑ ↑)
The paired electron in O experiences spin-pairing repulsion within the same orbital. This raises its energy and makes it easier to remove, so O has a lower IE than expected.
⚠️ Examiner Trap
The same anomalies repeat in Period 3: Mg→Al (s→p subshell) and P→S (spin-pairing). You must be able to explain both using orbital diagrams.
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