AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The development of the model of the atom (common content with physics): Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.
Early atomic theory grew from chemical evidence that substances combine in fixed ratios.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Early atomic theory grew from chemical evidence that substances combine in fixed ratios. John Dalton pictured atoms as solid, indivisible spheres, like tiny billiard balls, and this model explained why elements react in definite proportions. The word atom comes from the Greek idea of something that cannot be cut. This model treated atoms as the smallest possible particles, with no internal structure. It was useful because it accounted for the law of conservation of mass and fixed composition, but it could not explain electrical effects. When Thomson discovered the electron in 1897, the indivisible-sphere model had to be revised, because atoms clearly contained smaller, negatively charged particles. The sphere model is therefore a historical stage, not the modern view.
What earns the marks
- State that before electrons were discovered, atoms were modelled as tiny solid spheres.
- Explain that these spheres were thought to be indivisible, meaning they had no smaller parts.
- Link the model to Dalton's atomic theory and its success in explaining fixed ratios in chemical reactions.
- Explain that the discovery of the electron showed atoms contain smaller negatively charged particles, so the sphere model was replaced.
- Recognise that the model was useful at the time even though it is now known to be incomplete.
- Compare the early sphere model with the modern model, which has a nucleus containing protons and neutrons surrounded by electrons.
Common mistakes and how to put them right
Saying the early model included electrons or a nucleus.
Put it right: It had no internal structure and no subatomic particles.
Claiming the sphere model was simply wrong and useless.
Put it right: It explained chemical combining ratios and was a reasonable model for the evidence then available.
Confusing indivisible with indestructible in a chemical reaction.
Put it right: Indivisible means the atom was thought to have no smaller constituent particles.
Exam tips
- Use the term 'indivisible' when describing the pre-electron model, as it captures the key idea.
- Contrast the early sphere model with the nuclear model in one sentence to show progression.
- Mention Dalton or the fixed ratios of chemical combination to justify why the model was accepted.
Try the lesson
The start of the The development of the model of the atom (common content with physics): Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided. lesson, made from checked teaching notes for this specification point.
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Before the discovery of the electron, how were atoms described?
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Early atomic theory grew from chemical evidence that substances combine in fixed ratios. John Dalton pictured atoms as solid, indivisible spheres, like tiny billiard balls, and this model explained why elements react in definite proportions. The word atom comes from the Greek idea of something that cannot be cut. This model treated atoms as the smallest possible particles, with no internal structure. It was useful because it accounted for the law of conservation of mass and fixed composition, but it could not explain electrical effects. When Thomson discovered the electron in 1897, the indivisible-sphere model had to be revised, because atoms clearly contained smaller, negatively charged particles. The sphere model is therefore a historical stage, not the modern view. A strong answer earns marks for these points. State that before electrons were discovered, atoms were modelled as tiny solid spheres. Explain that these spheres were thought to be indivisible, meaning they had no smaller parts. Link the model to Dalton's atomic theory and its success in explaining fixed ratios in chemical reactions. Explain that the discovery of the electron showed atoms contain smaller negatively charged particles, so the sphere model was replaced. A common slip: Saying the early model included electrons or a nucleus. Instead, it had no internal structure and no subatomic particles. Exam tip: Use the term 'indivisible' when describing the pre-electron model, as it captures the key idea.
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Key words
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indivisible
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Dalton's atomic theory
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electron
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sphere model
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See every definition
- indivisible
- Thought to have no smaller parts, so atoms were pictured as tiny spheres that could not be divided.
- Dalton's atomic theory
- Pictured atoms as solid, indivisible spheres, like tiny billiard balls, and explained why elements react in definite proportions.
- electron
- A smaller, negatively charged particle discovered in 1897, showing that atoms clearly contained smaller particles.
- sphere model
- A historical stage, not the modern view, in which atoms were treated as the smallest possible particles with no internal structure.
Label the diagram
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1Box 1
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3Box 3
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