Hydrogen and chlorine can each form one covalent bond, oxygen two bonds, nitrogen three, while carbon can form four bonds
A covalent bond forms when two non-metal atoms share a pair of electrons. The electrons involved are in the highest occupied energy levels - or outer shells - of the atoms. An atom that shares one or more of its electrons will complete its highest occupied energy level.
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.
How many bonds?
Atoms may form multiple covalent bonds - that is, share not just 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 number of covalent bonds is equal to eight minus the group number . The table below gives more detail on this rule:
Group 4 Group 5 Group 6 Group 7 example carbon nitrogen oxygen chlorine number of bonds 8 - 4 = 4 8 - 5 = 3 8 - 6 = 2 8 - 7 = 1
Double and triple bonds
Note that molecules can have a double covalent bond - meaning they have two shared pairs of electrons - or a triple covalent bond - three shared pairs of electrons. A double covalent bond is shown by a double line, and a triple bond by a triple line.
When drawing dot-and-cross diagrams for covalent bonds, you only need to show the electrons in the highest occupied energy level, as only these are involved.