Which of these elements has a less favorable (more positive) `Delta_(eg)H`?

Options

A. `B`
B. `C`
C. `N`
D. `O`

Answer

C

Explanation

Magnitude of an element's `Delta_(eg)H` depends on the element's valence shell electron configuration. The electron configuration of `B,C, N`, and `O` are as follows:
`{:(B:1s^(2)2s^(2)2p^(1),,,C:1s^(2)2s^(2)2p_(x)^(1)2p_(y)^(1),),(N:1s^(2)2s^(2)2p_(x)^(1)2p_(y)^(1)2p_(z)^(1),,,O: 1s^(2)2s^(2)2p_(x)^(2)2p_(y)^(1)2p_(2)^(1),):}`
Carbon has only two electrons in its `2p` subshell and can radialy accept another in its vacent `2p_(z)` orbital to give a stable electron configuration. nitrogen has a stable electron configuration in the from of a half-filled `2p` subshell. Thus, teh additional electron must pair up in one fo the `2p` orbitals, where it fells a repulsion form the electron already present. Thus, the electron gain enthalpy of `N` is less favorable. Oxygen also must add an electron to a `p` orbital that already has one electron, but the additional stabilizing effect of increased `Z_(eff)` across the periodic table counteracts the effect of electron repulsion, resulting in a more favorable `Delta_(eg)H` for `O`.