 Silicon dioxide

Silicon dioxide is $\mathrm{Si}{\mathrm{O}}_2$, a network-forming oxide occurring in crystalline forms such as quartz and in amorphous silica. Its solid is not a collection of isolated linear $\mathrm{Si}{\mathrm{O}}_2$ molecules.

The connectivity and arrangement of silicon-oxygen units affect density, optical behavior, and mechanical properties. This makes crystalline quartz and silica glass chemically similar but structurally distinct materials.