Chemical bonds: Covalent bonding occurs in most non-metallic elements and in compounds of non-metals.

The idea

Covalent bonding occurs when non-metal atoms share pairs of electrons. This happens in most non-metallic elements, such as hydrogen (H₂), oxygen (O₂), nitrogen (N₂) and chlorine (Cl₂), and in compounds of non-metals, such as water (H₂O), carbon dioxide (CO₂) and methane (CH₄). Each shared pair of electrons is attracted to the nuclei of both atoms, holding them together. Atoms share electrons so that each atom achieves a full outer shell, often eight electrons, except hydrogen which needs two. The number of covalent bonds an atom forms depends on how many electrons it needs to complete its outer shell.

For example: Oxygen needs two more electrons, so it forms two covalent bonds, as in O₂ or H₂O. Covalent substances can be small molecules or giant covalent structures, and their properties depend on their structure and the strength of the forces between particles.

What earns the marks

• Identify covalent bonding as sharing pairs of electrons between non-metal atoms.
• Give examples of non-metallic elements that are covalently bonded, such as H₂, O₂, N₂ or Cl₂.
• Give examples of non-metal compounds that are covalently bonded, such as H₂O, CO₂ or CH₄.
• Draw or interpret dot-and-cross diagrams showing shared electron pairs and full outer shells.
• Relate the number of covalent bonds formed by an atom to the number of electrons needed to complete its outer shell.

Common mistakes and how to put them right

• Stating that covalent bonds involve transfer of electrons.

  Put it right by saying electrons are shared between non-metal atoms.

• Drawing dot-and-cross diagrams with unpaired electrons in the outer shell of a bonded atom.

  Put it right by ensuring each atom has a full outer shell where possible.

• Confusing covalent compounds with ionic compounds.

  Put it right by checking that all elements involved are non-metals.

Exam tips

• Check that every element in the substance is a non-metal before deciding that covalent bonding is present.
• In dot-and-cross diagrams, use different symbols for electrons from different atoms and show the shared pair in the overlap region.
• When explaining properties of simple molecular substances, refer to weak intermolecular forces rather than breaking covalent bonds.