(i) O and F are present in the same period of the periodic table. An F atom has one proton

and one electron more than O and as an electron is being added to the same shell, the

atomic size of F is smaller than that of O. As F contains one proton more than O, its nucleus

can attract the incoming electron more strongly in comparison to the nucleus of O atom.

Also, F needs only one more electron to attain the stable noble gas configuration. Hence,

the electron gain enthalpy of F is more negative than that of O.

(ii) F and Cl belong to the same group of the periodic table. The electron gain enthalpy

usually becomes less negative on moving down a group. However, in this case, the value

of the electron gain enthalpy of Cl is more negative than that of F. This is because the

atomic size of F is smaller than that of Cl. In F, the electron will be added to quantum level

n = 2, but in Cl, the electron is added to quantum level n = 3. Therefore, there are less

electron- electron repulsions in Cl and an additional electron can be accommodated easily.

Hence, the electron gain enthalpy of Cl is more negative than that of F.