Covalent bonding: When atoms share pairs of electrons, they form covalent bonds. These bonds between atoms are strong.

The idea

Covalent bonding happens between non-metal atoms that need more electrons to complete their outer shells. Instead of transferring electrons, the atoms share one or more pairs. A single shared pair is one covalent bond, shown as a line in a displayed formula. For example, two hydrogen atoms each contribute one electron to a shared pair, giving H₂, where each H has the electron arrangement of helium. In H₂O, oxygen shares one pair with each hydrogen, giving two single bonds and two lone pairs on oxygen. Each shared pair is attracted to both nuclei, so the bond holds the atoms together strongly. This strong attraction is the covalent bond; it is not a physical force between molecules.

What earns the marks

• Covalent bonds form between non-metal atoms, for example H and O or C and H.
• A covalent bond is a shared pair of electrons, with each atom contributing at least one electron to the pair.
• Atoms share electrons to achieve a full outer shell, often eight electrons (or two for hydrogen and helium).
• A single covalent bond is one shared pair; double and triple bonds involve two and three shared pairs respectively.
• The shared pair is attracted to both nuclei, which makes the bond strong.
• Displayed formulae use lines to represent shared pairs, for example H—H, and dot-and-cross diagrams show the origin of each electron.

Common mistakes and how to put them right

• Thinking atoms transfer electrons in covalent bonding.

  Put it right by stressing that covalent bonding involves sharing, while ionic bonding involves transfer.

• Drawing a bond as a line but forgetting the shared pair of electrons.

  Put it right by linking the line to one pair of electrons.

• Confusing the strength of the covalent bond with the strength of forces between molecules.

  Put it right by stating that covalent bonds are strong attractions within a molecule, whereas intermolecular forces are much weaker.

Exam tips

• When asked to explain bonding, name the atoms involved, state that electrons are shared, and say how this gives a full outer shell.
• Use dot-and-cross diagrams to show which atom supplies each electron in a shared pair, and check that outer shells are complete.
• If a question asks why a bond is strong, refer to the attraction between the shared pair and both nuclei, not to the molecule as a whole.