Silicon Dioxide

Silicon dioxide, or silica, (SiO₂) is another important example of a macromolecular solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions.. Silica can exist in six different crystalline forms. The best known of these is quartz, whose crystal structure shown previously is shown again below.

A portion of the giant covalent molecule (SiO₂)_n. The lattice shown would extend indefinitely in all directions in a macroscopic crystal. Each silicon atom (light color) is covalently bonded to four oxygen atoms (dark color). Each oxygen bonds to two silicons. The ratio of silicon to oxygen is 2:4 or 1:2, in accord with the formula. Computer-generated. (Copyright © 1976 by W. G. Davies and J. W. Moore.)

Sand consists mainly of small fragments of quartz crystals. Quartz has a very high melting pointThe temperature at which a solid becomes a liquid. Also called freezing point., though not so high as diamond.

If you refer back to the examples on silicon, you can remind yourself of the reason that SiO₂ is macromolecular. Silicon is reluctant to form multiple bonds, and so discrete molecules, analogous to , do not occur. In order to satisfy silicon’s valence of 4 and oxygen’s valence of 2, each silicon must be surrounded by four oxygens and each oxygen by two silicons. This can be represented schematically by the Lewis diagram