Sulphides and Oxides

The enthalpy of formation of the Sulphide ion, S^2-, is +330 kJmol^-1, whereas that of the Oxide ion, O^2-, is +630 kJmol^-1.

However, the lattice energy of ionic compounds containing the oxide ion is greater than that of those containing the sulphide ion, due to the much smaller size of the oxide ion.

This means that sulphides do not follow the ionic model very well. Binary sulphides tend to have a high degree of covalent character, and the high oxidation states achieved by metals with oxygen have no sulphide analogs, eg. PbO₂, but not PbS₂.

Hydrides

H₂S is gaseous, toxic, acidic, and a mild reducing agent. This is markedly different from water. This is due to the extensive hydrogen bond network which is present in water but which is not present further down the group.

The H-E-H bond angle approaches 90^o as the group is descended. There is a cross over from the sp³ hybridization in H₂O, resulting in a bond angle near the tetrahedral angle of 109.5^o, to the situation in Se and Te, where there is no such mixing and the H-E bonds are formed by p-orbitals, which are orthogonal and hence have a H-E-H angle of 90^o. Sulphur is some way between these two extremes.