CoachThe noble gases, with a full outer shell, hardly react: their configuration is especially stable. When atoms of the first elements form ions, they gain or lose electrons so as to reach the configuration of the nearest noble gas: two electrons in shell 1, like helium, for the lightest atoms — the duet rule — and eight electrons in the outer shell, like neon or argon, for the others — the octet rule.
Examples
Example 23.9 (Ions predicted by the rule)
Sodium, , loses its single valence electron: , , like neon. Magnesium, , loses two: . Aluminium, , loses three: . Chlorine, , gains one: , , like argon. Oxygen, , gains two: . Sulfur, : . Lithium, , loses one and keeps , like helium: .