Covalent Bonding and Simple Molecular SubstancesAQA GCSE Chemistry: Revision notes

There are three types of strong chemical bond:

• Ionic — transfer of electrons between oppositely charged ions
• Covalent — sharing of pairs of electrons between atoms
• Metallic — delocalised electrons shared throughout a structure

This subtopic focuses on covalent bonding and the substances it produces.

In covalent bonding, atoms share pairs of electrons so that each atom gets a fuller, more stable outer shell.

Dot and cross diagrams can be used to show this sharing for molecules such as hydrogen (H2), chlorine (Cl2), oxygen (O2), nitrogen (N2), hydrogen chloride (HCl), water (H2O), ammonia (NH3) and methane (CH4).

A single covalent bond can also be shown as a line between two atoms, e.g. H–H, Cl–Cl.

Substances made of small molecules held together by covalent bonds are called simple molecular substances (e.g. H2, Cl2, H2O, CO2). Within each molecule, the covalent bonds are very strong.

However, between molecules, only weak intermolecular forces act, and it is these weak forces — not the covalent bonds — that must be overcome to melt or boil the substance. This is why simple molecular substances have:

• Low melting and boiling points
• The ability to exist as gases or liquids at room temperature

Larger molecules have higher melting and boiling points, because larger molecules have stronger intermolecular forces to overcome.

Simple molecular substances do not conduct electricity, because the molecules have no overall charge and there are no free ions or electrons to carry current.

The three states of matter — solid, liquid and gas — can be explained using particle theory. Whether a substance is solid, liquid or gas at a given temperature depends on the strength of the forces holding its particles together, which in turn depends on its type of bonding and structure.

Simple molecular substances typically have weak forces between molecules, so many exist as gases or liquids at room temperature, and changes of state happen at relatively low temperatures.