Question

Why is ionisation potential of F- is less than Cl-

Answer 1

we know that ionisation potential of elements decreases as one moves down a group due to addition of extra shells. As the electrons are more far away from the nucleus ,less energy is required to remove electron(s).
Therefore, in group 17,chlorine has larger atomic radius than fluorine,so less amount of energy is required to remove an electron from chlorine as compared to fluorine. Thus,chlorine has a low ionisation energy as compared to fluorine