Giant covalent structures

In a giant covalent structure, atoms are joined by covalent bonds in a continuous network that extends throughout the solid. A covalent bond is a strong bond formed when atoms share electrons. The structure is not made of separate small molecules: covalent bonds hold atoms together across the giant network, or lattice.

Melting and boiling points

A giant covalent structure contains very many strong covalent bonds. To melt or boil the substance, enough of these bonds must be broken to allow the atoms to separate and move more freely. Breaking the bonds requires a large amount of energy.

Giant covalent substances are therefore solids with very high melting points and usually very high boiling points.

Recognising giant covalent structures

A giant covalent structure consists of an extended network of atoms joined by covalent bonds. In a diagram, look for a repeating arrangement of bonded atoms that continues across the structure, rather than separate groups of atoms forming individual molecules. A diagram usually shows only a small part of the extended structure.

Giant covalent structures in diagrams

• Diamond: Carbon atoms are joined by covalent bonds in a three-dimensional network extending throughout the structure.
• Graphite: Carbon atoms are joined by covalent bonds in extended sheets. The sheets form layers; the covalent bonds shown in the network are within each sheet.
• Silicon dioxide: Silicon and oxygen atoms are linked by covalent bonds throughout a three-dimensional network. Its formula, $$\ce{SiO2}$$, gives the ratio of the atoms; it does not represent a separate small molecule in the structure.

The arrangements look different, but all three contain extended covalent frameworks, not collections of small, discrete molecules.