 Covalent compounds, on the other hand, are either individual molecules or indefinitely repeating structures. In either case, Lewis thought that they are formed from atoms bound together by shared pairs of electrons. Hydrogen gas is said to consist of molecules composed of two hydrogen atoms held together by a single, covalent bond; oxygen gas, of molecules composed of two oxygen atoms and a double bond; methane, of molecules composed of four equivalent carbon-hydrogen single bonds, and silicon dioxide (sand) crystals of indefinitely repeating covalently bonded arrays of SiO2 units.