The absolute value of the electron gain enthalpy of halogens satisfies :

Solution

Chemistry ma’am · 3 steps

Electron gain enthalpy falls down Group 17, but fluorine breaks the pattern because it is too small.

1. General trend. Going down the group, the atom gets bigger. The incoming electron sits farther from the nucleus and is held less strongly, so less energy is released. From Cl to Br to I, the magnitude of Δ_(eg)H falls.
2. Why fluorine is odd. Fluorine is a very small atom with a crowded 2p subshell. The added electron is pushed back hard by the electrons already there. So fluorine releases less energy than chlorine, though still a little more than bromine.

   Halogen	Δ_(eg)H (kJ mol⁻¹)	Absolute value
   F	-328	328, second (small-size repulsion)
   Cl	-349	349, the largest
   Br	-325	325, third
   I	-295	295, the smallest

3. So. The order is Cl > F > Br > I, which is option (b).

The idea to keep. In Group 17, chlorine has the most negative electron gain enthalpy, not fluorine. Fluorine's small size causes electron repulsion, but it still stays just ahead of bromine.

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