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Per NCERT §3.7.1(c), why is the first ionisation enthalpy of OXYGEN (Z=8) LESS than that of NITROGEN (Z=7)?
AN has higher Z_eff
BN has a larger radius
CN is half-filled 2p³ (extra stability + no pairing); O's 4th 2p electron must pair, causing repulsion — easier to remove
DO has fewer electrons
Answer & Solution
Correct answer: C. N is half-filled 2p³ (extra stability + no pairing); O's 4th 2p electron must pair, causing repulsion — easier to remove
Per NCERT §3.7.1(c) end, N's 2p³ is half-filled (Hund stable); O's 2p⁴ forces pairing → e–e repulsion → easier to ionise. So IE(N) > IE(O).
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