Covalent bonding: Students should be able to recognise common substances that consist of small molecules from their chemical formula.

The idea

A small-molecule covalent substance consists of a discrete, finite number of non-metal atoms joined by covalent bonds, such as H₂O, CO₂, NH₃, CH₄, O₂, N₂, HCl, Cl₂ and C₆H₁₂O₆. To recognise one from its formula, check that all elements are non-metals and that the formula represents a discrete particle. A formula with a specific number of atoms, whether a few (like H₂O) or several (like C₆H₁₂O₆), indicates a small molecule. This contrasts with giant covalent structures (like SiO₂), which use empirical formulae for continuous lattices, or polymers, which show repeating units with a subscript n. For example, CO₂ contains one carbon and two oxygen atoms, so it is a small molecule; NaCl contains a metal, so it is ionic. The skill involves inspecting the formula to identify discrete molecules.

What earns the marks

• Identify all elements present in the formula and confirm they are non-metals before deciding that the substance is covalent.
• Recognise that a formula showing a specific, finite number of atoms (e.g. H₂O, CO₂, NH₃, CH₄ or C₆H₁₂O₆) indicates a discrete small molecule.
• Recognise common small molecules from their formulae, including H₂, O₂, N₂, Cl₂, HCl, H₂O, CO₂, NH₃ and CH₄.
• Distinguish small molecules from giant covalent structures (e.g. SiO₂) which form continuous lattices, and from polymers which have repeating units.
• Explain that the formula of a small molecule shows the actual number of atoms covalently bonded together in one discrete particle.
• Use the presence of a metal combined with a non-metal as evidence that a substance is ionic, not a small covalent molecule.

Common mistakes and how to put them right

• Treating any formula containing only non-metals as a small molecule, including giant covalent substances such as SiO₂.

  Put it right: Check whether the structure is discrete or giant, noting that SiO₂ is a giant covalent lattice.

• Assuming that a formula with several atoms, such as C₆H₁₂O₆, must be a polymer or giant structure.

  Put it right: C₆H₁₂O₆ represents a discrete small molecule, whereas a polymer contains many repeating units denoted by 'n'.

• Classifying NaCl as a small molecule because it is written with two symbols.

  Put it right: Sodium is a metal, so NaCl is an ionic compound, not a covalent molecule.

Exam tips

• Underline the element symbols in a formula and label each as metal or non-metal before deciding on the type of substance.
• Learn a short list of common small molecules and their formulae so recognition is quick and accurate.
• If asked to explain, state that the formula shows a specific number of non-metal atoms covalently bonded in a discrete molecule.