Period 3 oxides and physical properties

Last time

Before we start: last time, in Period 3 chlorides, radius and prediction, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A whole period in a row of oxides

Burn each element in Period 3 and you get an oxide. Sodium oxide dissolves to give a strong alkali. Sulfur trioxide dissolves to give a strong acid. In between, magnesium oxide, aluminium oxide and silicon dioxide change steadily from basic to acidic, and the pattern is explained by structure and bonding.

Making the oxides

Sodium burns to give sodium oxide: 4 Na plus O two goes to 2 Na two O. Phosphorus burns to give phosphorus(V) oxide: 4 P plus 5 O two goes to P four O ten. Sulfur burns to give sulfur dioxide: S plus O two goes to SO two. Aluminium forms Al two O three. The maximum oxidation state in the oxides rises across the period, from plus 1 for sodium to plus 7 for chlorine, because the number of outer electrons rises.

Structure and bonding of the oxides

Sodium and magnesium oxides are giant ionic lattices, with strong electrostatic attraction between ions. Aluminium oxide is ionic with some covalent character. Silicon dioxide is a giant covalent network. The oxides of phosphorus, sulfur and chlorine are simple molecular, with weak intermolecular forces.

Melting points

Magnesium oxide melts at 2852 degrees Celsius, aluminium oxide at 2072, silicon dioxide at 1610, sodium oxide at 1132. Ionic oxides melt high because much energy is needed to overcome strong electrostatic attraction. Magnesium oxide's ions carry two plus and two minus, so it is higher than sodium oxide. Silicon dioxide melts high because strong covalent bonds must be broken in the giant structure. Phosphorus(V) oxide at 340 degrees, sulfur trioxide at 17, and sulfur dioxide at minus 73 are simple molecular, with weak forces overcome on melting.