Properties of Giant Covalent Structures
Very high melting points – Substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken. Graphite, for example, has a melting point of more than 3,600ºC.
Variable conductivity – Diamond does not conduct electricity. Graphite contains free electrons, so it does conduct electricity. Silicon is semi-conductive – that is, midway between non-conductive and conductive.
Graphite is a form of carbon in which the carbon atoms form layers. These layers can slide over each other, so graphite is much softer than diamond. It is used in pencils, and as a lubricant. Each carbon atom in a layer is joined to only three other carbon atoms. Graphite conducts electricity.
Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure. It is the best example of covalent solid, which is usually octahedral in shape. As a result, diamond is very hard and has a high melting point. It does not conduct electricity. Diamond is the hardest substance known.
Silica, which is found in sand, has a similar structure to diamond. It is also hard and has a high melting point, but contains silicon and oxygen atoms, instead of carbon atoms.
The fact that it is a semi-conductor makes it immensely useful in the electronics industry: most transistors are made of silica.