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Covalent bonds are those in which the atoms reach a stable configuration (filled outer shell) by sharing valence electrons. Unlike ionic bonds, which are nondirec-tional, covalent bonds act between specific pairs of atoms and thus form mole¬cules. Covalent bonds are most prevalent in gas molecules. Covalent bonding also results in the formation of very large molecules which are present as solids rather than as liquids and gases. Diamond, silicon, and silicon carbide are examples of such covalent-bonded solids. They are characterized by high strength and melting temperature and low ductility. The atoms in the diamond structure are arranged on two interpenetrating face-centered cubic lattices. The entire crystal is composed of only one molecule, and in order to fracture the crystal, the strong covalent inter¬atomic bonds must be broken.