Carbon can form a high energy C2 molecule, as well as the non-molecular graphite form and diamond form. Diamond and graphite differ in the respect of structure: graphite is non-molecular in 2 dimensions, whereas diamond is non-molecular in 3 dimensions. You should also look at buckminsterfullerene, C60; if you can describe the shape of a football, you can describe this molecule.
Phosphorus also has several allotropic forms. It can form a highly reactive P4 molecule, i.e. white phosphorus, which must be stored under water because of the strain imposed by the P4 tetrahedron; the material ignites in air. Red phosphorus, has a similar structure, however, in this allotrope there are no discrete molecules. There are a few reactions where red phosphorus can be used in place of white phosphorus provided that reaction times are extended.
Sulfur has several allotropic forms. Flowers of sulfur, a bright yellow powder, is what is normally supplied to laboratories; this is the S8 molecule, and is as soluble as a brick.
Selenium is again non-molecular and has 2-3 allotropes, i.e. black and red selenium.
At undergraduate and A level, the so-called allotropes of carbon, and phosphorus should be committed to memory.
