The atom that would gain two electrons to fill its valence energy level is S(sulfur)
This is because s (sulfur) is in atomic number 16 with 2.8.6 of [Ne] 3s^2 2p^4 electronic configuration. This implies that sulfur has 6 valence electron and therefore it require two electron to fill its valence energy level and obtain 18 rule electrons.
1 mole ------------- 22.4 L ( at STP )
?? mole ---------- 12 L
12 x 1 / 22.4 => 0.5357 moles
hope this helps!
I’d say market and distribution would be the next step
core electrons are those occupying the innermost or lowest energy levels
