Classification of Elements Part 1

(c) Order of electron gain enthalpy (absolute value) Cl>F>Br>1

Element	F	Cl	Br	I
∆He(kJ/ Mole )	−328	−349	−325	−295

Absolute value of electron gain enthalpy decreases down the group in a periodic table due to increase in size.
But magnitude of electron gain enthalpy of second period element F Due to small size of ' F ' there is high electronic repulsion between the existing and approaching electron.
Hence, magnitude of electron gain enthalpy decreases.