Phosphorus(III) oxide

Phosphorus(III) oxide is a white solid, melting at 24°C and boiling at 173°C. The structure of its molecule is best worked out starting from a P₄ molecule which is a little tetrahedron.

Pull this apart so that you can see the bonds . . .

. . . and then replace the bonds by new bonds linking the phosphorus atoms via oxygen atoms. These will be in a V-shape (rather like in water), but you probably wouldn't be penalised if you drew them on a straight line between the phosphorus atoms in an exam.

The phosphorus is using only three of its outer electrons (the 3 unpaired p electrons) to form bonds with the oxygens.