AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to recognise common substances that consist of…
Full specification wording
Covalent bonding: Students should be able to recognise common substances that consist of small molecules from their chemical formula.
A chemical formula shows the number and type of atoms in a substance.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 4 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A chemical formula shows the number and type of atoms in a substance. If the formula contains only a few non-metal atoms, the substance is likely to consist of small molecules. For example, H₂O, CO₂, NH₃, CH₄, HCl, O₂, N₂, H₂ and Cl₂ are all small molecules. However, you must memorise exceptions like SiO₂ (silicon dioxide) and diamond/graphite (C), which are giant covalent structures despite having simple empirical formulas. By contrast, NaCl represents an ionic lattice because it contains a metal and a non-metal. To identify small molecules, check that all elements are non-metals and ensure the substance is not a known giant covalent exception. This skill helps you predict low melting points, low boiling points and no electrical conductivity.
What earns the marks
- A chemical formula shows the type and number of atoms in a substance.
- Small molecules contain only a few non-metal atoms, for example H₂O, CO₂, NH₃, CH₄, HCl, O₂, N₂, H₂ and Cl₂.
- Formulas such as SiO₂ represent giant covalent structures, not small molecules, even though they contain only non-metals.
- Formulas such as NaCl represent ionic compounds, not small molecules, because they contain a metal and a non-metal.
- Recognising small molecules from formulas allows prediction of low melting and boiling points and no electrical conductivity.
Common mistakes and how to put them right
Treating every formula containing non-metals as a small molecule.
Put it right by memorising exceptions like SiO₂ which is a giant covalent structure.
Assuming that a formula with two elements is always a small molecule.
Put it right by identifying ionic compounds such as NaCl.
Counting atoms incorrectly in formulas with subscripts.
Put it right by multiplying the subscript by any number outside a bracket, as in Ca(OH)₂.
Exam tips
- Check that all elements are non-metals and confirm the substance is not a known giant covalent exception (like SiO₂) before deciding it is a small molecule.
- Learn a short list of common small molecules, including H₂O, CO₂, NH₃, CH₄, HCl, O₂, N₂, H₂ and Cl₂.
- If a formula represents a giant covalent or ionic structure, do not describe it as a small molecule.
Try the lesson
The start of the Covalent bonding: Students should be able to recognise common substances that consist of small molecules from their chemical formula. lesson, made from checked teaching notes for this specification point.
Have a go first
Which chemical formula represents a common substance that consists of small molecules?
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Key words
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Chemical formula
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Small molecules
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Giant covalent structures
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Ionic lattice
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See every definition
- Chemical formula
- Shows the number and type of atoms in a substance.
- Small molecules
- Contain only a few non-metal atoms, for example H₂O, CO₂, NH₃, CH₄, HCl, O₂, N₂, H₂ and Cl₂.
- Giant covalent structures
- Structures such as SiO₂ and diamond/graphite (C) that have simple empirical formulas but are not small molecules.
- Ionic lattice
- Represented by NaCl because it contains a metal and a non-metal.
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