Question

What is electron configuration of fluoride ion?

Answer 1

`F^- : 1s^2 2s^2 2p^6`

alternatively:
`F^- : [Ne]`

Explanation:

Elemental Fluorine has an electron configuration of `1s^2 2s^2 2p^5` and needs 1 more electron to complete its `2p` orbital which it will acquire in formation of the fluoride ion.

Thus it gains an electron when forming the fluoride ion, and becomes isoelectronic to neon.