Question

Why does oxygen have a lower ionization energy than oxygen?

Answer 1

Because nitrogen does not have any paired valence `2p` electrons that can be removed, while oxygen does.

Coulombic repulsion energy decreases the energy input required to remove a `2p` valence electron from `"O"` atom, i.e. decreases the ionization energy of `"O"` atom compared to `"N"` atom.

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Write out the electron configurations.

`"N"`: `1s^2 2s^2 color(red)(2p^3)`

`ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))`
`underbrace(" "" "" "" "" "" "" "" ")`
`" "" "" "2p`

`ul(uarr darr)`
`2s`

`"O"`: `1s^2 2s^2 color(red)(2p^4)`

`ul(uarr color(red)(darr))" "ul(uarr color(white)(darr))" "ul(uarr color(white)(darr))`
`underbrace(" "" "" "" "" "" "" "" ")`
`" "" "" "2p`

`ul(uarr darr)`
`2s`

You can see that oxygen has a paired `2p` valence electron. That adds electron repulsion, which makes it easier for the electron to get removed.

Thus, oxygen atom has a lower (less positive) ionization energy than nitrogen atom.