Question

Though flourine occupies upper position than chlorine in the periodic table, why is its electron gain enthalpy more than the latter?

Answer 1

The electron gain enthalpies of halogens become less negative on moving down the group due to increase in the size of halogens.But chlorine has a more negative electron gain enthalpy than fluorine.This is due to small size of fluorine. There is strong electron-electron repulsion in compact 2p-orbital of fluorine.

Therefore, Chlorine has the most negative electron gain enthalpy and Iodine has the least negative electron gain enthalpy.