Which of the following have high electron affinity?
Here, one of the most important things to keep in our mind is that Group 17 elements have maximum electron gain enthalpy because if we add one electron to group 17 elements, they achieve noble gas configuration.
Therefore, F & Cl will have high electron affinity than N and O.
Now, when we have to compare between F & Cl.
As we know,
Atomic size: The smaller the atomic size, the greater will be the electron affinity because the effective attractive force between the nucleus and the valence electrons is greater in smaller atoms, and so the electrons are held firmly.
Nuclear charge: The greater the nuclear charge , greater is the electron affinity because increase in nuclear charge increases the effective attractive force on the valence electrons.
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.