JEE Main 2023 · 11 April · morning · Single correct · +4 / −1
For elements B, C, N, Li, Be, O and F, the correct order of first ionization enthalpy is
- (a) Li<Be<B<C<O<N<F
- (b) B>Li>Be>C>N>O>F
- (c) Li<Be<B<C<N<O<F
- (d) Li<B<Be<C<O<N<F
Answer and solution
Answer (d) Li<B<Be<C<O<N<F
Solution
Chemistry ma’am · 4 steps
Draw the rising line from Li to F, then fix the two dips: B below Be and O below N.
- General trend. Across period 2, nuclear charge rises while the electrons go into the same shell. The outer electron is held more tightly, so first ionisation enthalpy broadly increases from Li to F.
- Be above B. Be is 1s² 2s² and B is 1s² 2s² 2p¹. The 2p electron of B sits a little farther out and is shielded by the 2s pair, so it is easier to remove than a 2s electron of Be.
- N above 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 Valence configuration First IE (kJ mol⁻¹) Li 2s¹ 520 Be 2s² 899 B 2s² 2p¹ 801, below Be C 2s² 2p² 1086 N 2s² 2p³ 1402 O 2s² 2p⁴ 1314, below N F 2s² 2p⁵ 1681 - So. With both dips in place the order is Li<B<Be<C<O<N<F, which is option (d).
The idea to keep. In any period, ionisation enthalpy rises left to right with two dips: group 13 falls below group 2 (a new p subshell starts), and group 16 falls below group 15 (paired p electrons repel).
Answer from the NTA final answer key. Solution by PrepWiser.