[potions09]

I know that electron affinity increases as you move across the periodic table because the non-metals at the right are more likely to gain electrons to become stable, so they release more energy after gaining an electron. But why does electron affinity decrease down a group?

[Fluorine]

Take fluorine and iodine for example. They are both in the same group but iodine is much larger so the electrons will be farther away from nucleus. Fluorine on the other hand is fairly small, it can keep due to being so compact. Electron affinity will decrease as you go down due to size of the atom.

[AMEDIO]

Down a group, the electron affinity decreases because of a large increase in the atomic radius, electron-electron repulsion and the shielding effect of inner electrons against the valence electrons of the atom.