The oxide of the second-row element beryllium, BeO, is amphoteric,
but MgO is basic. The bonds between Mg2+
and O2- ions in the crystal are
almost entirely ionic.
Crystalline magnesium oxide is a close-packed array of oxide ions,
held together by an equal number of tiny Mg2+
ions slipped into niches between layers of oxide ions.
The solubility of ionic salts represents a competition between
the attractions of ions for one another, and the attractions of
polar water molecules for both.
Ionic salts of magnesium generally are less soluble than their
sodium counterparts, because the smaller size and doubled charge
of the Mg2+ ion helps them to hold
the crystal together more tightly.MgO is nearly insoluble in water,
but soluble in acid:
The borderline between acidic and basic oxides of the third-row
elements comes at aluminum, and Al2O3
and Al(OH)3 are amphoteric.
Aluminum hydroxide is insoluble in pure water, but dissolves in
either acid or base: