Substances with many covalent bonds

Covalent bonding leads to the formation of substances with different types of structures, for example:

  • small molecules, which contain a fixed number of atoms joined by covalent bonds
  • giant covalent substances, which contain many atoms joined by covalent bonds

An example - Silicon dioxide

Silicon dioxide (often called silica) is the main compound found in sand. It is an example of a substance with a giant covalent structure. It contains many silicon and oxygen atoms. All the atoms in its structure are linked to each other by strong covalent bonds. The atoms are joined to each other in a regular arrangement, forming a giant covalent structure. There is no set number of atoms joined together in this type of structure.

Covalent structure of silicaSilica has a giant covalent structure containing silicon atoms (grey) and oxygen atoms (red)

High melting points and boiling points

Substances with giant covalent structures are solids at room temperature. They have very high melting points and boiling points. This is because large amounts of energy are needed to overcome their strong covalent bonds to make them melt or boil.

Conduction of electricity

Most substances with giant covalent structures have no charged particles that are free to move. This means that most cannot conduct electricity. Graphite, a form of carbon which can conduct electricity, is an exception.

Question

State three properties that are typical of substances with giant covalent structures.

They have high boiling points, high melting points and they cannot conduct electricity.