The correct order of electron gain enthalpy is :

Solution

Chemistry ma’am · 4 steps

Down group 16 the value drops, except oxygen: it is too small and crowded to welcome an extra electron.

1. What we compare. All four atoms release energy when they gain an electron, so every Δ_(eg)H is negative. The order compares how much energy is released, the size of that negative value.
2. S to Te. Going down the group from S to Se to Te, the atom gets bigger. The added electron sits farther from the nucleus and is held less strongly, so less energy is released.
3. The oxygen exception. You would expect O, at the top, to release the most energy. But its compact 2p subshell is already crowded with electrons, and they repel the incoming one strongly. So O releases the least energy of the four.

   Element	Shell receiving the electron	Δ_(eg)H (kJ mol⁻¹)
   O	2p, small and crowded	-141, least energy released
   S	3p, roomier	-200, most energy released
   Se	4p	-195
   Te	5p	-190

4. So. Ranked by energy released, the order is S > Se > Te > O, which is option (b).

The idea to keep. In groups 16 and 17, the period 3 element (S, Cl) releases the most energy on gaining an electron. The period 2 element (O, F) loses out because its small, crowded 2p shell repels the newcomer.

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