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A covalent bond forms when two non-metal atoms share a pair of electrons. The electrons involved are in the outer shells of the atoms. An atom that shares one or more of its electrons will complete its outer shell.

Covalent bonds are strong – a lot of energy is needed to break them. Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water. These substances have strong covalent bonds within the molecules (between the atoms), but weak intermolecular forces between the molecules. This means that only a small amount of heat energy is required to separate the molecules from each other.

The slideshow shows a covalent bond being formed between a hydrogen atom and a chlorine atom, to form hydrogen chloride.

After bonding, the chlorine atom is now in contact with eight electrons in its outer shell, so it is stable. The hydrogen atom is now in contact with two electrons in its outer shell, so it is also stable.

Both nuclei are strongly attracted to the shared pair of electrons in the covalent bond, so covalent bonds are very strong and require a lot of energy to break.

How many bonds?

Atoms may form multiple covalent bonds – they share not only one pair of electrons, but two or more pairs. Atoms of different elements will form either one, two, three or four covalent bonds with other atoms.

There is a quick way to work out how many covalent bonds an element will form.

The table gives more detail on this rule.

Hydrogen forms one covalent bond. The noble gases in Group 0 do not form any.