Fluorine has the highest electronegativity among the ns2 np5 group on the Pauling scale, but the electron affinity of fluorine is less than that of chlorine because-
Small size, high electron density and an increased electron repulsion makes addition of an electron to fluorine less favourable than that in the case of chlorine.
We know that atomic size of Flourine is smaller than chlorine.
⟶ Flourine already has 7 electrons in valence shell. Therefore if we will add one more electron, electrons present in the valence shell will repel that incoming electron because of the smaller size and high electron density.
Chlorine is larger in size and have 7 electrons in valence shell. Therefore it has less electron density and there will be less electron repulsions.
Therefore, because of small size, high electron density and increased electron repulsion, addition of an electron is less favourable in case of Flourine than Chlorine.
⇒ Cl > F [EA]