Valence Electrons and Ionic Compounds

Sodium has one valence electron sitting alone in its outer shell, far from a full shell by gain and only one electron away by loss. Losing that single electron is cheaper than gaining seven, so sodium always forms Na+. Magnesium has two valence electrons to shed and reaches the same stable, full-shell arrangement as neon by losing both, giving Mg2+. The pattern holds across the main groups: the number of valence electrons an atom has, or the number it needs to reach eight, sets the charge it carries once it becomes an ion.

To build a formula, match total charge, not atom count. Aluminum has 3 valence electrons to lose, forming Al3+. Oxygen needs 2 electrons to complete its octet, forming O2-. Electrons lost must equal electrons gained, so find the smallest numbers of each ion that balance: 2 aluminum atoms lose 3 electrons each (6 total), and 3 oxygen atoms gain 2 electrons each (6 total). That gives Al2O3, not AlO.

The octet rule is a guideline built from main-group behavior, not a law of physics. Hydrogen is stable with 2 electrons, not 8. Some atoms form stable ions or molecules with expanded or incomplete octets. Transition metals break the pattern most clearly: their valence electron counts don't map onto one predictable charge, since many lose electrons from more than one sublevel and form multiple possible charges, like iron as Fe2+ or Fe3+. That variability is why transition metal compounds need a different naming system, with the charge stated directly.