Question

Which of the following has the lowest electron affinity?

• A. $O$
• B. $S$
• C. $F$
• D. $Cl$

Answer 1

The correct option is A $O$

As we move from left to right in a period, electron affinity increases due to the increase in the effective nuclear charge.

However, it will not be easier to add an electron to a smaller atom.
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.