alleryWhy is the first ionisation energy of nitrogen HIGHER than that of oxygen?
ANitrogen has a half-filled stable 2p subshell
BNitrogen has a larger atomic radius
COxygen has fewer protons
DNitrogen has more shells than oxygen
Answer & Solution
Correct answer: A. Nitrogen has a half-filled stable 2p subshell
1. Across a period, ionisation energy generally rises with nuclear charge.
2. Nitrogen has the configuration 2, 5 (2s²2p³).
3. The three 2p electrons each sit in a separate orbital, giving a half-filled subshell.
4. A half-filled subshell has extra stability due to exchange energy.
5. Removing an electron from this stable arrangement needs extra energy.
6. Oxygen has 2s²2p⁴; one orbital is paired and that paired electron is easier to remove.
7. So nitrogen's first ionisation energy exceeds oxygen's despite oxygen having more protons.
_Source: Selina Concise Chemistry ICSE Class 10, Ch 1 'Periodic Table' (aplustopper.com extract)_
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