A covalent bond is a shared pair of electrons. Covalent bonding results in the formation of molecules or giant structures. Substances with small molecules have low melting and boiling points and do not conduct electricity. Giant covalent substances have very high melting points.
Polymers have very large molecules. The atoms in a polymer molecule are joined together by strong covalent bonds in long chains. There are variable numbers of atoms in the chains of a given polymer. One example of a polymer is poly(ethene).