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Per NCERT §3.7.1(c), why is the FIRST ionisation enthalpy of BORON (Z=5) LESS than that of BERYLLIUM (Z=4) despite B having greater nuclear charge?
AThe electron removed from B is a 2p electron (less penetrating, more shielded by the 2s² core) than the 2s of Be — easier to remove
BB has fewer protons than Be
CB is a gas at room T
DBe has a smaller nucleus
Answer & Solution
Correct answer: A. The electron removed from B is a 2p electron (less penetrating, more shielded by the 2s² core) than the 2s of Be — easier to remove
Per NCERT §3.7.1(c) (after Fig 3.6a), in B the 2p electron is shielded by 2s² and penetrates the nucleus less than the 2s electron in Be ⇒ B's IE is anomalously low.
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