The electron affinity value are negative for
A. Be → Be⁻
B. N → N⁻
C. O → O²⁻
D. Na → Na⁻
E. Al → Al⁻
Choose the most appropriate answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Negative electron affinity means the atom resists the extra electron, so look for stable shells or repulsion.

1. Sign convention. Electron affinity is the energy released when an electron is added, so it is the negative of Δ_(eg)H. A negative electron affinity means energy has to be supplied, that is, Δ_(eg)H is positive.
2. When energy is needed. Energy has to be put in when the new electron must enter a higher-energy subshell of an already stable configuration, or when it is pushed onto an ion that is already negative.

   Process	Where the electron goes	Energy change	Electron affinity
   A. Be → Be⁻	Into 2p, after the fully filled 2s²	absorbed	negative
   B. N → N⁻	Pairs up in the half-filled 2p³	absorbed (slightly)	negative
   C. O → O²⁻	Second electron pushed onto O⁻	-141 + 780 = +639 kJ mol⁻¹	negative
   D. Na → Na⁻	Into 3s, completing 3s²	released, about 53 kJ mol⁻¹	positive
   E. Al → Al⁻	Into 3p, which is only partly filled	released, about 42 kJ mol⁻¹	positive

3. So. Energy is absorbed only in A, B and C, so these three have negative electron affinity. The answer is (d).

The idea to keep. Electron affinity and electron gain enthalpy have opposite signs. Expect energy to be absorbed for filled or half-filled shells (Be, N, noble gases) and for any second electron added to an anion.

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