The first ionization enthalpies of Be, B, N and O follow the order

Solution

Chemistry ma’am · 4 steps

IE rises across period 2, but the full 2s in Be and the half-filled 2p in N each beat the next element.

1. General trend. Across period 2, nuclear charge increases and atomic size shrinks, so first ionisation enthalpy generally rises from Be to O. Two well-known breaks in this trend decide the answer.
2. Be beats B. Be is 1s² 2s², so its outer electron comes from a stable, filled 2s subshell. B is 1s² 2s² 2p¹, and its lone 2p electron is higher in energy and shielded by the 2s pair, so it leaves more easily.
3. N beats O. N is 2p³, a stable half-filled subshell with one electron in each 2p orbital. In O (2p⁴), two electrons share one orbital, and their repulsion makes one of them easier to remove.

   Element	Configuration	First IE (kJ mol⁻¹)
   B	2s² 2p¹	801
   Be	2s², filled	899
   O	2s² 2p⁴, one paired 2p	1314
   N	2s² 2p³, half-filled	1402

4. So. The order is B < Be < O < N, which is option (d).

The idea to keep. In any period, group 2 beats group 13 (filled s against a lone p) and group 15 beats group 16 (half-filled p against a paired p). Check these two pairs before applying the general trend.

Answer from the NTA final answer key. Solution by PrepWiser.