Barrons SAT Subject Test Chemistry, 13th Edition

(F, T) The first two principal energy levels fill up at 2 and 8 electrons,
respectively. That leaves 7 electrons to fill the 3s and 3p orbitals like this:
3 s^2 , 3p^5. With only one electron missing in the 3p orbitals, the most likely
oxidation number is −1.