Question

Why is chlorine the element with highest electron affinity if fluorine is at the top of the halogen group because electron affinity decreases down a group.

Answer 1

Fluorine, though higher than chlorine in the periodic table, has a very small atomic size. This makes the fluoride anion so formed unstable (highly reactive) due to a very high charge/mass ratio. Also, fluorine has no d-orbitals, which limits its atomic size. As a result, fluorine has an electron affinity less than that of chlorine.