JEE Main 2024 · 4 April · morning · Single correct · +4 / −1

The correct order of first ionization enthalpy values of the following elements is :
(A) O
(B) N
(C) Be
(D) F
(E) B
Choose the correct answer from the options given below :

  1. (a) A<B<D<C<E
  2. (b) C<E<A<B<D
  3. (c) E<C<A<B<D
  4. (d) B<D<C<E<A
Answer and solution

Answer (c) E<C<A<B<D

Solution

Chemistry ma’am · 3 steps

The trend rises across period 2, but watch the two dips: B falls below Be, and O falls below N.

  1. General trend. Across period 2 the nuclear charge increases while the electrons stay in the same shell. They are held more tightly, so first ionisation enthalpy generally rises from Be to F.
  2. Two exceptions. B loses a 2p electron, which is shielded by the filled 2s² pair and is easier to remove than Be's 2s electron. O loses a paired 2p electron, and the repulsion within that pair makes it easier to remove than an electron from N's stable half-filled 2p³.
    ElementConfigurationFirst IE (kJ mol⁻¹)
    B (E)2s² 2p¹801
    Be (C)2s², fully filled899
    O (A)2s² 2p⁴1314
    N (B)2s² 2p³, half-filled1402
    F (D)2s² 2p⁵1681
  3. So. The order is B < Be < O < N < F, that is E < C < A < B < D, which is option (c).

The idea to keep. In periods 2 and 3, ionisation enthalpy rises across the period with two dips: group 13 < group 2 and group 16 < group 15. Filled and half-filled subshells are extra stable.

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

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Asked about 120 times in JEE Main since 2020. This question is from 2024.