Question 3
A sodium atom has the configuration 2, 8, 1. Which route to a stable valence shell does it actually take?
The right answer is the one marked above.
Losing one electron. That leaves 2, 8 — a full outer shell, reached by moving a single electron.
Gaining seven would also give a full shell, at 2, 8, 8, but it means moving seven electrons instead of one, so it does not happen. Gaining one gives 2, 8, 2, which is not stable, and sodium has only one outer electron to lose, not eight.