The History of the Atom
Get to grips with The History of the Atom for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Know the sequence: Dalton's spheres, Thomson's plum-pudding model, Rutherford's nuclear model, Bohr's shells and Chadwick's neutron.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The model of the atom has changed over time as new experimental evidence emerged. This is a good example of how scientific models develop.
Before the electron was discovered, atoms were thought to be tiny solid spheres that could not be divided. The discovery of the electron led to the plum pudding model, in which the atom was a ball of positive charge with negative electrons embedded in it.
The alpha particle scattering experiment changed this. Most alpha particles passed straight through the foil, so the atom is mostly empty space. A few were deflected, and very few bounced back, showing that the mass and positive charge are concentrated in a tiny nucleus. This gave the nuclear model.
Revision notes
Early models
Atoms were first thought to be tiny solid spheres that could not be divided.
The discovery of the electron led to the plum pudding model: a ball of positive charge with negative electrons embedded within it.
The alpha scattering experiment
Alpha particles were fired at thin gold foil. Most passed straight through, showing the atom is mostly empty space.
Some were deflected, showing the centre has a positive charge. A very few bounced straight back, showing the mass is concentrated in a tiny nucleus. This replaced the plum pudding model with the nuclear model.
Refining the nuclear model
Niels Bohr proposed that electrons orbit the nucleus at specific distances, which agreed with experimental observations.
Later work showed the positive charge of the nucleus is made of particles with the same charge, called protons. About twenty years later James Chadwick provided evidence for neutrons.
Key points
- Atoms were first thought to be indivisible solid spheres.
- The plum pudding model followed the discovery of the electron.
- Most alpha particles passed straight through the foil.
- This showed the atom is mostly empty space.
- A few bounced back, showing a tiny dense nucleus.
- Bohr proposed electrons orbit at specific distances.
Worked examples
Example 1
State what the plum pudding model suggested about the atom. [2 marks]
Model answer
It suggested the atom was a ball of positive charge (1 mark) with negative electrons embedded within it (1 mark).
Example 2
Explain what the alpha particle scattering experiment showed about the structure of the atom. [3 marks]
Model answer
Most alpha particles passed straight through the foil, showing that the atom is mostly empty space (1 mark). Some were deflected, showing the centre of the atom has a positive charge (1 mark). A very few bounced straight back, showing the mass is concentrated in a very small nucleus (1 mark).
Example 3
Name the scientist who provided evidence for the existence of neutrons. [1 mark]
Model answer
James Chadwick (1 mark).
Common mistakes
Saying the plum pudding model had a nucleus.
It had no nucleus — that came from the alpha scattering experiment.
Not linking each observation to its conclusion.
Each of the three observations gives a separate conclusion and a separate mark.
Saying alpha particles were deflected because of electrons.
The deflection was caused by the positive charge of the nucleus.
Confusing Bohr and Chadwick.
Bohr proposed electron orbits; Chadwick provided evidence for neutrons.
Exam tips
- Pair each experimental observation with the conclusion it supports.
- Learn the order in which the models developed.
- Name the scientists associated with each stage.
- Use this topic as your example of models changing with evidence.
Key terms
- Plum pudding model
- A model of the atom as positive charge with embedded electrons.
- Alpha scattering
- The experiment that led to the nuclear model.
- Nuclear model
- A model with a tiny dense nucleus surrounded by electrons.
- Neutron
- An uncharged particle in the nucleus, evidenced by Chadwick.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.