The correct order of electron gain enthalpy (- ve value) is :

Solution

Chemistry ma’am · 3 steps

Oxygen is the odd one out here: it is too small, so sulphur, not oxygen, tops this group.

1. Usual trend. Going down a group, atoms get bigger and the added electron sits farther from the nucleus. So the negative electron gain enthalpy normally becomes smaller down the group.
2. Oxygen exception. Oxygen is very small, and its compact 2p subshell is already crowded. The incoming electron feels strong repulsion from the electrons already there, so less energy is released than for sulphur, where the larger 3p subshell has more room.

   Element	Electron gain enthalpy (kJ mol⁻¹)	Why
   O	-141	small size, strong electron repulsion
   S	-200	largest: more room, nucleus still close
   Se	-195	normal trend: size increases
   Te	-190	normal trend: size increases further

3. So. By magnitude, O is below S, and after S the values fall steadily: O < S > Se > Te. That is option (c).

The idea to keep. In groups 16 and 17, the period 3 element (S, Cl) has the most negative electron gain enthalpy. The period 2 element (O, F) loses out because its small size causes strong electron repulsion.

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