Answer
If one more electron is added to a half-filled sub-shell, electron-electron repulsions will increase because two electrons must occupy the same atomic orbital. This may result in a slight decrease in the stability of the atom.
Work Step by Step
If one more electron is added to a half-filled sub-shell, electron-electron repulsions will increase because two electrons must occupy the same atomic orbital. This may result in a slight decrease in the stability of the atom.