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(i) O and F are the elements of the same period in the periodic table. An F-atom is having 1 electron and 1 proton more than that of O-atom as the electron is added in the same shell, thus the atomic size of O-atom is larger than F-atom. As O-atom is having 1 proton less than F-atom. So, the nucleus of O-atom cannot attract an incoming electron that strongly as that of an F-atom. Also, F-atom requires only 1 electron to achieve a stable inert gas configuration. So, the electron affinity of F(Fluorine) is more negative than that of O(oxygen).
(ii) F and Cl are the elements of the same group in the periodic table. On moving down the group the electron affinity becomes less negative. Here, the value of electron affinity of F is less negative than that of Cl. It is because the atomic size of Cl is larger than that of F. In Cl, the electron will be added to n = 3 quantum level, whereas in F, the electron will be added to n = 2 quantum level. Thus, as the electron-electron repulsion is reduced in Cl so an extra electron can easily be accommodated. So, the electron affinity of Cl is more negative compared to that of F.