A network solid or covalent network solid is a chemical compound (or element) in which the atoms are bonded by covalent bonds in a continuous network extending throughout the material. In a network solid there are no individual molecules, and the entire crystal may be considered a macromolecule. ${\mathrm{SiO}}_2$ is a network solid. ${\mathrm{Li}}_2\mathrm{O}$ and $\mathrm{NaCl}$ are ionic crystals. ${\mathrm{H}}_2\mathrm{O}$ is a covalent liquid and ${\mathrm{CO}}_2$ is a covalent gas.