An electron affinity is the energy change (in kJ/mole), in a neutral atom’s gaseous phase, when an electron is added. This is called a negative ion. This is the probability of a neutral atom gaining an electron.
An electronegativity measure measures the tendency of an atom attract a bonding pair or electrons. The most common scale used is the Pauling scale. Fluorine, the most electronegative element, is given a value of 4.0. Values range from caesium to francium, which are the least electronegative elements at 0.7.
The first electron affinities for the group 7 elements
As you can see, the most electronegative element is Fluorine but its first electron affinity is less than chlorine.
Because of the small size of Fluorine’s atom, this is possible.
An electron is attracted to neutral fluorine by the nucleus, but it is also attracted by other electrons.
Fluorine’s high repulsion and extra stability in atoms results in the release of energy. There is less energy released than with chlorine.