AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Compounds contain two or more elements chemically combined in fixed…
Full specification wording
Atoms, elements and compounds: Compounds contain two or more elements chemically combined in fixed proportions and can be represented by formulae using the symbols of the atoms from which they were formed.
A compound is a substance made when two or more different elements are chemically combined.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 6 of 10
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A compound is a substance made when two or more different elements are chemically combined. The elements are joined in fixed proportions, so the ratio of atoms is always the same for that compound. For example, water always contains hydrogen and oxygen in a 2:1 atom ratio, giving the formula H₂O; carbon dioxide is always CO₂. A formula uses the chemical symbols of the elements and subscript numbers to show how many atoms of each element are in one unit. No subscript means one atom. The formula lets you count atoms, calculate relative formula mass and balance equations. Mixtures differ because elements are not chemically combined and proportions can vary.
What earns the marks
- A compound contains two or more different elements joined by chemical bonds, not just mixed together.
- The proportions of the elements in a given compound are fixed, so the atom ratio is always the same.
- A chemical formula uses element symbols with subscripts to show the number of each type of atom in one formula unit.
- A symbol without a subscript represents one atom of that element, for example the C in CO₂.
- Formulae allow counting atoms and calculating relative formula mass, and they support writing balanced equations.
Common mistakes and how to put them right
Calling a mixture such as air a compound.
Put it right: Air is a mixture because its components are not chemically combined and their proportions vary.
Reading subscripts as multipliers of the whole formula.
Put it right: In H₂O the 2 applies only to hydrogen, giving two hydrogen atoms and one oxygen atom.
Writing formulae with baseline numbers instead of subscripts.
Put it right: Use subscript notation such as H₂O and CO₂.
Exam tips
- Count atoms element by element when interpreting a formula, and state the number of each type clearly.
- Use the formula to calculate relative formula mass by adding the relative atomic masses in the correct proportions.
- When comparing compounds and mixtures, refer to fixed proportions and chemical combination as the key differences.
Try the lesson
The start of the Atoms, elements and compounds: Compounds contain two or more elements chemically combined in fixed proportions and can be represented by formulae using the symbols of the atoms from which they were formed. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following is the chemical formula of a compound made from two different elements?
Listen
A compound is a substance made when two or more different elements are chemically combined. The elements are joined in fixed proportions, so the ratio of atoms is always the same for that compound. For example, water always contains hydrogen and oxygen in a 2:1 atom ratio, giving the formula H₂O; carbon dioxide is always CO₂. A formula uses the chemical symbols of the elements and subscript numbers to show how many atoms of each element are in one unit. No subscript means one atom. The formula lets you count atoms, calculate relative formula mass and balance equations. Mixtures differ because elements are not chemically combined and proportions can vary. A strong answer earns marks for these points. A compound contains two or more different elements joined by chemical bonds, not just mixed together. The proportions of the elements in a given compound are fixed, so the atom ratio is always the same. A chemical formula uses element symbols with subscripts to show the number of each type of atom in one formula unit. A symbol without a subscript represents one atom of that element, for example the C in CO₂. A common slip: Calling a mixture such as air a compound. Instead, air is a mixture because its components are not chemically combined and their proportions vary. Exam tip: Count atoms element by element when interpreting a formula, and state the number of each type clearly.
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Key words
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Chemical combination of elements
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Fixed composition and atom ratios
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Formulae as symbolic representations
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Counting atoms and relative formula mass
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See every definition
- Chemical combination of elements
- A compound contains two or more different elements joined by chemical bonds, not just mixed together.
- Fixed composition and atom ratios
- The proportions of the elements in a given compound are fixed, so the ratio of atoms is always the same for that compound.
- Formulae as symbolic representations
- A chemical formula uses the chemical symbols of the elements and subscript numbers to show how many atoms of each element are in one unit.
- Counting atoms and relative formula mass
- The formula lets you count atoms, calculate relative formula mass and balance equations.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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