AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Covalent bonding: deduce the molecular formula of a substance from a given model or diagram in these forms showing the atoms and bonds in the molecule.
A molecular model or diagram uses spheres or letters for atoms and lines or sticks for covalent bonds.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A molecular model or diagram uses spheres or letters for atoms and lines or sticks for covalent bonds. To deduce the molecular formula, identify each distinct element present, then count how many atoms of each element appear in one molecule. Write the element symbols with the counts as subscripts, using the conventional order and omitting the subscript 1. For example, a model with one carbon atom bonded to four hydrogen atoms gives CH₄; a model with two hydrogen atoms and two oxygen atoms gives H₂O₂. Check that every atom in the diagram has been counted exactly once, including atoms at the ends of chains and any atom shown without a label. The formula must describe one molecule, not the whole mixture or the number of bonds.
What earns the marks
- Identify every different element shown in the model or diagram before counting any atoms.
- Count the atoms of each element in a single molecule, including terminal atoms and atoms joined by multiple bonds.
- Write the correct element symbols in the accepted order, usually carbon first, then hydrogen, then other elements alphabetically.
- Use subscript numbers for counts greater than one and omit the subscript 1, for example H₂O not H₂O₁.
- Check the total number of atoms counted against the total number of spheres or labelled atoms in the diagram.
Common mistakes and how to put them right
Counting bonds instead of atoms.
Put it right by counting atom labels or spheres.
Forgetting terminal atoms in a chain.
Put it right by tracing every bond to its end atom.
Writing a subscript of 1, such as CO₁.
Put it right by omitting the 1 and writing CO.
Exam tips
- Annotate the diagram lightly with a tally for each element before writing the formula.
- If the diagram uses letters, treat each letter as one atom of that element unless a key states otherwise.
- Re-count after writing the formula to confirm the subscripts match the diagram.
Try the lesson
The start of the Covalent bonding: deduce the molecular formula of a substance from a given model or diagram in these forms showing the atoms and bonds in the molecule. lesson, made from checked teaching notes for this specification point.
Have a go first
A molecule is represented by the following structural diagram: H–O–O–H What is the molecular formula of this substance?
Listen
A molecular model or diagram uses spheres or letters for atoms and lines or sticks for covalent bonds. To deduce the molecular formula, identify each distinct element present, then count how many atoms of each element appear in one molecule. Write the element symbols with the counts as subscripts, using the conventional order and omitting the subscript 1. For example, a model with one carbon atom bonded to four hydrogen atoms gives CH₄; a model with two hydrogen atoms and two oxygen atoms gives H₂O₂. Check that every atom in the diagram has been counted exactly once, including atoms at the ends of chains and any atom shown without a label. The formula must describe one molecule, not the whole mixture or the number of bonds.
Key words
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Molecular formulae
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Representing molecules
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Counting atoms in models
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See every definition
- Molecular formulae
- Written using element symbols with counts as subscripts, in the conventional order and omitting the subscript 1, to describe one molecule.
- Representing molecules
- A model or diagram uses spheres or letters for atoms and lines or sticks for covalent bonds in the molecule.
- Counting atoms in models
- Identify each distinct element present, then count how many atoms of each element appear in one molecule, checking every atom is counted exactly once.
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