Predict from each set, the element which has the more negative electron gain enthalpy `(Delta_(eg)H^(ɵ))`. Give reasons:
a. `C` or `Si` , b. `F` or `Cl` , c. `O` or `S`
e. `F, Cl S, P`
f. (i) `[Ne] 3s^(2) 3p^(5)` , (ii) `[Ne]3s^(2) 3p^(4)` , (iii) `[Ne] 3s^(2) 3p^(3)`

Explanation

a. `Delta_(eg)H^(ɵ)` of `C gt Si` (more negative), due to small size of `C` than `Si`. [Since they contain only `4e^(-)'s` in the outermost shell, so electron repulsion in these atoms are not very large and therefore are not considered.]
b. `Delta_(eg)H^(ɵ)` of `Cl gt F` (more negative). Because adding an electron to the larger `3p`-orbitals of `Cl` leads to lesser interelectronic repulsion than adding an electron to smaller `2p`-orbitals of `F`.
c. `Delta_(eg)H^(ɵ)` of `O gt N`
Note: `Delta_(eg)H^(ɵ)` of `O` is highly negative, while that of `N` is slightly positive.
Since `N` is quite stable due to half-filled `2p`-orbitals therefore it is difficult to add an extra electron to `N` and thus energy is required to add an extra electron.
Whereas O atom has smaller size and higher nuclear charge than `N` and therefore it has a high tendency to accept an extra electron. Thus energy is released.
d. `Delta_(eg)H^(ɵ)` of `S gt O` [Same explanation as in part (b) above].
e. `Delta_(eg)H^(ɵ)` of `(Cl gt F)/("Group" 17) gt (S)/("Group" 16) gt (P)/("Group" 15)`
`Delta_(eg)H^(ɵ)` of `Cl gt F` [as explained in part (b) above]
`Delta_(eg)H^(ɵ)` becomes more and more negative along the period `(rarr)`.
`:. Delta_(eg)H^(ɵ)` of `S gt P`
f. Electronic configuration of (i) corresponds to `Cl (Z = 17)`.
Electronic configuration of (ii) corresponds to `S (Z = 16)`.
Electronic configuration of (iii) corresponds to `P (Z = 15)`.
All of them belong to the 3rd period and `Delta_(eg)H^(ɵ)` becomes more and more negative along the period `(rarr)`.
`:. Delta_(eg)H^(ɵ)` of `Cl gt S gt P`
`Cl` contains one electron less than the stable inert gas configuration, i.e. `[Ne] 3s^(2) 3p^(6)` and hence has a strong tendency to accept an electron to acquire the stable inert gas configuration so the highest energy is released (more negative value).