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JEE Main 2020 · Chemistry · Periodic Properties · easy

The correct order of first ionisation enthalpy among B, Be, N and O is

Single correct option

The correct order of first ionisation enthalpy among B, Be, N and O is
  1. A. N>O>Be>B\mathrm{N>O>Be>B}
  2. B. N>O>B>Be\mathrm{N>O>B>Be}
  3. C. O>N>Be>B\mathrm{O>N>Be>B}
  4. D. Be>B>N>O\mathrm{Be>B>N>O}
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Answer: (A) N>O>Be>B\mathrm{N>O>Be>B}

Across period 2 IE increases overall, with two dips: Be→B\mathrm{Be\to B} (the 2p2p electron is easier to remove than the filled 2s2s) and N→O\mathrm{N\to O} (pair repulsion in 2p42p^4 versus half-filled 2p32p^3). Values (kJ mol−1^{-1}): N 1402 > O 1314 > Be 900 > B 801.
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