why is fluorine more electronegative than chlorine even though its electron gain enthalpy is less than that of chlorine?
The reason for this is that the size of Fluorine atom is very small and hence there is very high inter-electronic repulsion among the electrons of fluorine.
This makes incoming of another electron not very favourable.
Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. As we go across a period the atomic radii of the atoms decreases. As we go down a group the atomic radii of the atom increases (even though the nuclear charge also increases, the atomic radii increases to a greater extent which practically cancels out the effect of the increased nuclear charge). So, fluorine is more electronegative than chlorine.