Although florine is more electonegative than chlorine, fluorobenzene has lower dipole moment `(mu = 1.63D)` than chlorobenzene `(mu = 1.75D)`.

Explanation

Both fluorine in fluorobenzene and chlorine in chlorobenzene withdraw electrons from the ring by - I effect and donate electrons to the ring by + R effect. Because of smaller size of fluorine, the + R effect involving orbitals of similar sizes (2p of both F and C) is much stronger. So the moment due to stronger - I effect of fluorine is considerably neutralised by the moment due to + R effect. Hence, fluorobenzene possesses a net dipole moment which is relatively low (1.63D). On the orther hand, because of larger size of chlorine, the + R effect involving orbitals of dissimilar sizes (3p of CI and 2p of C) is much weaker than the - I effect which is somewhat lower due to lower electronegativity of chlorine. So, the moment due to + R effect is much smaller than the moment due to - I effect. Hence, chlorobenzen possesses a relatively high net dipole moment (1.75D).