AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
The discovery of the electron led to the plum pudding model of the atom.
Full specification wording
The development of the model of the atom (common content with physics): The discovery of the electron led to the plum pudding model of the atom.
When J. J. Thomson investigated cathode rays in the late 1890s, he showed that atoms contain small, negatively charged particles called electrons.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 3 of 14
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When J. J. Thomson investigated cathode rays in the late 1890s, he showed that atoms contain small, negatively charged particles called electrons. This discovery meant atoms could no longer be solid indivisible spheres. Thomson proposed the plum pudding model, in which the atom is a sphere of positive charge with negative electrons scattered through it, like fruit in a pudding. The model kept the atom electrically neutral overall because the positive and negative charges balance. It explained why atoms can lose or gain electrons to form ions. However, it could not explain the results of the alpha particle scattering experiment carried out by Geiger and Marsden in Rutherford's group. That experiment showed that positive charge and mass are concentrated in a tiny central nucleus, so the plum pudding model was replaced.
What earns the marks
- State that the electron was discovered through experiments on cathode rays.
- Describe the plum pudding model as a positive sphere with negative electrons embedded in it.
- Explain that the model accounts for the atom being electrically neutral overall.
- Recognise that the model was later disproved by alpha particle scattering results.
- Link the discovery of the electron to the need for a new atomic model.
Common mistakes and how to put them right
Saying the plum pudding model has a nucleus.
Put it right by stating the positive charge is spread throughout the sphere.
Claiming electrons orbit the nucleus in this model.
Put it right by describing them as embedded in the positive sphere.
Thinking the model was proposed before the electron was discovered.
Put it right by placing the electron discovery first.
Exam tips
- Name the scientist associated with the plum pudding model, J. J. Thomson, if the question asks for it.
- Use the phrase embedded electrons to match the specification wording.
- When comparing models, state one clear difference rather than listing many vague points.
Try the lesson
The start of the The development of the model of the atom (common content with physics): The discovery of the electron led to the plum pudding model of the atom. lesson, made from checked teaching notes for this specification point.
Have a go first
Which model of the atom was proposed directly as a result of the discovery of the electron?
Listen
When J. J. Thomson investigated cathode rays in the late 1890s, he showed that atoms contain small, negatively charged particles called electrons. This discovery meant atoms could no longer be solid indivisible spheres. Thomson proposed the plum pudding model, in which the atom is a sphere of positive charge with negative electrons scattered through it, like fruit in a pudding. The model kept the atom electrically neutral overall because the positive and negative charges balance. It explained why atoms can lose or gain electrons to form ions. However, it could not explain the results of the alpha particle scattering experiment carried out by Geiger and Marsden in Rutherford's group. That experiment showed that positive charge and mass are concentrated in a tiny central nucleus, so the plum pudding model was replaced. A strong answer earns marks for these points. State that the electron was discovered through experiments on cathode rays. Describe the plum pudding model as a positive sphere with negative electrons embedded in it. Explain that the model accounts for the atom being electrically neutral overall. Recognise that the model was later disproved by alpha particle scattering results. A common slip: Saying the plum pudding model has a nucleus. Instead, stating the positive charge is spread throughout the sphere. Exam tip: Name the scientist associated with the plum pudding model, J. J. Thomson, if the question asks for it.
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Key words
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Discovery of the electron
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Plum pudding model
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Electrical neutrality of atoms
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See every definition
- Discovery of the electron
- Experiments on cathode rays showed that atoms contain small, negatively charged particles called electrons.
- Plum pudding model
- A model in which the atom is a sphere of positive charge with negative electrons scattered through it, like fruit in a pudding.
- Electrical neutrality of atoms
- The atom is neutral overall because the positive and negative charges balance.
Label the diagram
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1Box 1
2Box 2
3Box 3
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