Beginning of Essay
Most simple chemical compounds consist of crystals. If you were to examine a lump of crystalline material very closely, you would be able to see tiny individual crystals. All crystals have a definite geometric shape, determined by the way the atoms are linked toge ....

Middle of Essay
.... evaporates. Crystals may also form when a vapor or molten substance solidifies. Allotropes are different forms of the same element. For Example, oxygen has two allotropes, normal oxygen and ozone. Pure carbon also has two allotropes, diamond and graphite. A crystal of diamond is in fact a single giant molecule in which every carbon atom is linked to four other, by four equal, strong bonds. These bonds are arranged tetrahedrally round each atom and there are no planes along which the giant molecule can easily be split. This quality is what gives diamond its tremendous hardness. Graphite on the other hand, consists of flat sheets of carbon atoms ....