Structure, properties and uses

Simple molecular substances consist of molecules in which the atoms are joined by strong covalent bonds. However, the molecules are held together by weak forces so these substances have low melting and boiling points. They do not conduct electricity.

Giant covalent structures contain many atoms joined together by covalent bonds to form a giant lattice. They have high melting and boiling points. Graphite and diamond have different properties because they have different structures. Graphite conducts heat and electricity well because it also has free electrons.

Simple molecules

Covalent bonds form between non-metal atoms. Each bond consists of a shared pair of electrons, and is very strong. Covalently bonded substances fall into two main types:

1. Simple molecules
2. Giant covalent structures

Simple molecules

A molecule of carbon dioxide

These contain only a few atoms held together by strong covalent bonds. An example is carbon dioxide (CO₂), the molecules of which contain one atom of carbon bonded with two atoms of oxygen.

Properties of simple molecular substances

• Low melting and boiling points - This is because the weak intermolecular forces break down easily.
• Non-conductive - Substances with a simple molecular structure do notconduct electricity. This is because they do not have any free electrons or an overall electric charge.

Higher tier only

Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds. All have very strong bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. Simple molecular substances are gases, liquids or solids with low melting and boiling points.

The animation shows how the weak intermolecular forces between water molecules break down during boiling or melting:

Macromolecules

Macromolecules have giant covalent structures. They contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds. Their atoms are arranged into giant lattices, which are strong structures because of the many bonds involved. 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 3,600°C.

Diamond

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.

Graphite

Graphite

Graphite is a form of carbon in which the carbon atoms form layers. Each carbon atom in a layer is joined to only three other carbon atoms.The layers can slide over each other because there are no covalent bonds between them. This makes graphite much softer than diamond. It is used in pencils and as a lubricant. Graphite conducts electricity.

Silica

Silica

Silica, which is found in sand, has a similar structure to diamond. It is also hard and has a high melting point. However, it contains silicon and oxygen atoms instead of carbon atoms.