What's the Atomic Energy Level Configuration for Atomic Numbers, 9, 12, & 20?

Answer 1

Element 9 is Fluorine
[He] 2s2, 2p5 = 9 electrons

Element 12 is Magnesium
[Ne] 3s2 = 12 electrons

Element 20 is Calcium
[Ar] 4s2 = 20 electrons

Answer 2

Atomic number 9:
1s^2 2s^2 2p^5

Atomic number 12:
1s^2 2s^2 2p^6 3s^2

Atomic number 20:
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2