Which element has the lowest electron affinity?
As we move in a group from top to bottom, electron affinity decreases but fluorine and oxygen have the smallest size in their respective groups. So, on adding another electron externally to the valence shell causes severe electron-electron repulsion. That makes the system comparatively unstable. So, the actual order of electron affinity is Cl>F>S>O . Thus, oxygen has the lowest electron affinity.
