Why does chlorine have a higher electron affinity than fluorine?

Given that the second row atoms have an increased nuclear charge and should be easier to add electrons to, why should the first row atoms have a reduced (magnitude) electron affinity? Two important factors are (i) nuclear charge and (ii) shielding by other electrons.

Additionally, in the case of the second row atoms, electrons are entering larger and more diffuse orbitals (building on an inner electron shell), which reduces electron-electron repulsion.

Together, the two elements produce the peculiar Periodic trend.

The energy released when one mole of an element in its gaseous state gains an electron under standard conditions is known as the element's electron affinity.

Since the atomic radius increases down a group, the electron gained ends up in the outermost shell, where it experiences a weaker attraction from the nucleus and releases less energy. As a result, electron affinity generally decreases down a group.

But there are some exceptions: fluorine, which is higher on the group than chlorine, has a lower electron affinity than chlorine because the electrons in a fluorine atom's outermost shell are closer together, and because the gained electron experiences strong repulsion from the original electrons in the outermost shell, requiring energy to maintain the gained electron in the shell.

Chlorine has a higher electron affinity than fluorine because chlorine has a larger atomic size and a higher effective nuclear charge, making it easier for chlorine to accept an additional electron.