Covalent bonding - giant covalent structures

Giant covalent structures contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds [covalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ]. The atoms are usually arranged into giant regular lattices [lattice: A lattice is a regular grid-like arrangement of atoms in a material. ] - extremely strong structures because of the many bonds involved. The graphic shows the molecular structure of diamond and graphite: two allotropes [allotropes: Allotropes are structurally different forms of an element. They differ in the way the atoms bond with each other and arrange themselves into a structure. Because of their different structures, allotropes have different physical and chemical properties. ] of carbon, and of silica (silicon dioxide).

From left to right - graphite, diamond, silica

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 [conduct: To allow electricity, heat or other energy forms to pass through. ] electricity. Graphite contains free electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ], so it does conduct electricity. Silicon is semi-conductive - that is, midway between non-conductive and conductive.

Graphite

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 [lubricant: A substance used to reduce the friction between two solid surfaces. ]. Each carbon atom in a layer is joined to only three other carbon atoms. Graphite conducts electricity.

Diamond

Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure. As a result, diamond is very hard and has a high melting point. It does not conduct electricity.

Silica

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.

Buckminsterfullerene

Structure of a buckminsterfullerene molecule - a large ball of 60 atoms

Buckminsterfullerene is yet another allotrope of carbon. It is actually not a giant covalent structure, but a giant molecule in which the carbon atoms form pentagons and hexagons - in a similar way to a leather football. It is used in lubricants.