The element carbon exists in more than one form. Graphite and diamond are different forms of carbon. Both are made only of carbon atoms, but the atoms are arranged differently, so the two forms have different structures and different properties.
Both are examples of giant covalent substances. They are not made of small separate molecules. Instead, huge numbers of atoms are joined by strong covalent bonds in one continuous network. Because many strong covalent bonds must be broken to melt them, both diamond and graphite have very high melting points, and neither dissolves in water.
Diamond is hard and does not conduct electricity. Graphite is soft and does conduct electricity. The next lessons explain why.