Silicon dioxide

Much of the silicon and oxygen in the Earth’s crust is present as the compound silicon dioxide also known as silica.

Silicon dioxide has a giant covalent structure. Part of this structure is shown in the diagram - oxygen atoms are shown as red, silicon atoms shown as brown:

Each silicon atom is covalently bonded to four oxygen atoms. Each oxygen atom is covalently bonded to two silicon atoms. This means that, overall, the ratio is two oxygen atoms to each silicon atom, giving the formula SiO₂.

Silicon dioxide is very hard. It has a very high melting point (1,610 °C) and boiling point (2,230 °C), is insoluble in water, and does not conduct electricity. These properties result from the very strong covalent bonds that hold the silicon and oxygen atoms in the giant covalent structure.

Silicon dioxide is found as quartz in granite, and is the major compound in sandstone. The sand on a beach is made mostly of silicon dioxide.