Going from left to right across a period in the periodic table, the basicity decreases as the electronegativity of the atom possessing the lone pair of electron increases. Hence basicity of $$NH_2^-$$ is higher than F^-. In moving down a group, as the atomic mass increases, basicity decreases. Hence F^- is more basic than I^- and HO^- is more basic than HS-. Hence among the ionic species, $$NH_2^-$$ has maximum proton affinity.

$$\mathop {F - F}\limits_{38} \,\,\,\mathop {Cl - Cl}\limits_{57} \,\,\mathop {Br - Br}\limits_{45.5} \,\,\mathop {I - I}\limits_{35.6} $$
X - X bond dissociation energy (kcal/mol)

The lower value of bond dissociation energy of fluorine is due to the high inter-electronic repulsion between non-bonding electrons in the 2p-orbitals of fluorine. As a result F - F bond is weaker in comparison to Cl - Cl and Br - Br bonds.

The molar enthalpy change accompanying the addition of an electron to an atom (or ion) is known as electron gain enthalpy.
Generally in increases on moving from left to right in a period and in a group it decreases as the size increases.
Exception: Because of the small size of F, electron-electron repulsion present in its relatively compact 2p-subshell, do not easily allow the addition of an extra electron. On the other hand, Cl because of its comparatively bigger size than F, allows the addition of an extra electron more easily.
$$\mathop O\limits_{ - 1.48} < \mathop S\limits_{ - 2.0} < \mathop F\limits_{ - 3.6} < \mathop {Cl}\limits_{ - 3.8} $$

Ionic radius in the n^th orbit is given by

r_n = $${{{n^2}{a_0}} \over {{Z^*}}}$$ or, $${r_n} \propto {1 \over {{Z^*}}}$$

When n is the principal quantum number, a₀ the Bohr's radius of H-atom and Z^*, the effective nuclear charge.