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JEE Main 2025 · Chemistry · Periodic Properties · easy

Sequences correctly represents increasing first ionisation enthalpy?

Single correct option

Which of the following sequences correctly represents increasing first ionisation enthalpy?
  1. A. Be<B<O<N\mathrm{Be < B < O < N}
  2. B. B<Be<N<O\mathrm{B < Be < N < O}
  3. C. B<Be<O<N\mathrm{B < Be < O < N}
  4. D. Be<B<N<O\mathrm{Be < B < N < O}
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Answer: (C) B<Be<O<N\mathrm{B < Be < O < N}

Boron (2p12p^1) loses an electron more easily than beryllium (filled 2s2s), so B<Be\mathrm{B<Be} (801 vs 900 kJ mol−1\mathrm{kJ\,mol^{-1}}). Nitrogen has a half-filled stable 2p32p^3 configuration, so its enthalpy (1402) exceeds that of oxygen (1314), where pairing repulsion in 2p42p^4 lowers it. Order: B<Be<O<N\mathrm{B<Be<O<N}.
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