(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.