Chemistry

1613 s and p – Block Elements(i) Phosphorus (III) oxidePhosphorus (III) oxide is a white solid, melting at 24°C andboiling at 173°C. The structure of its molecule is best worked outstarting from a P4 molecule, which is a little tetrahedron.Pull this apart so that you can see the bonds . . . . . . and thenreplace the bonds by new bonds linking the phosphorus atoms viaFig. 13.8 oxygen atoms. These will be in a V-shape (rather like in water). Thephosphorus is using only three of its outer electrons (the 3 unpaired p electrons) to form bondswith the oxygens.Fig. 13.9Fig 13.10Fig 13.11(ii) Phosphorus(V) oxidePhosphorus (V) oxide is also a white solid, subliming (turningstraight from solid to vapour) at 300°C. In this case, the phosphorususes all five of its outer electrons in the bonding.Solid phosphorus(V) oxide exists in several different forms -some of them polymeric. We are going to concentrate on a simplemolecular form, and this is also present in the vapour.This is most easily drawn starting from P4O6. The other four oxygensare attached to the four phosphorus atoms via double bonds.(b) The Sulphur OxidesSulphur has two common oxides, sulphur dioxide (sulphur(IV) oxide),SO2, and sulphur trioxide (sulphur(VI) oxide), SO3.(i) Sulphur dioxideSulphur dioxide is a colourless gas at room temperature with aneasily recognised choking smell. It consists of simple SO2 molecules.The sulphur uses 4 of its outer electrons to form the double bonds withthe oxygen, leaving the other two as a lone pair on the sulphur. Thebent shape of SO2 is due to this lone pair.(ii) Sulphur trioxidePure sulphur trioxide is a white solid with a low melting and boiling point.Gaseous sulphur trioxide consists of simple SO3 molecules in which all six of the sulphur'souter electrons are involved in the bonding.There are various forms of solid sulphur trioxide. The simplest one is a trimer, S3O9, wherethree SO3 molecules are joined up and arranged in a ring.There are also other polymeric forms in which the SO3 molecules join together in longchains. For example:The fact that the simple molecules join up in this way to make bigger structures is whatmakes the sulphur trioxide a solid rather than a gas.