Melting Points of Period 3 Oxides

The melting points of the Period 3 oxides follow from their structure and bonding.Ionic oxides (high melting points). Na₂O, MgO and Al₂O₃ have giant ionic lattices, and melting means overcoming strong electrostatic attraction between oppositely charged ions. MgO melts higher than Na₂O because Mg²⁺ has a higher charge (and is smaller) than Na⁺, so it attracts O²⁻ ions more strongly. MgO has the highest melting point of the Period 3 oxides (about 2850 °C).Al³⁺ has an even higher charge, but Al₂O₃ melts lower than MgO. The small, highly charged Al³⁺ ion polarises the oxide ions, giving the bonding some covalent character, so the ionic attraction is weaker than expected.Macromolecular oxide (high melting point). To melt SiO₂, many strong covalent bonds in the giant network must be broken, which needs a lot of energy.Molecular oxides (low melting points). P₄O₁₀, SO₃ and SO₂ only need weak intermolecular forces (van der Waals and permanent dipole-dipole forces) to be overcome; the covalent bonds inside the molecules do not break. P₄O₁₀ melts higher than the sulfur oxides because its larger molecules have more electrons and stronger van der Waals forces. SO₂ is a gas at room temperature (melting point about −73 °C).