The Development of the Model of the Atom
Build confidence in The Development of the Model of the Atom for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. The alpha scattering experiment replaced the plum pudding model with the nuclear model of a small, dense, positively charged nucleus.
Free downloads
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, and this is a standard example of how scientific models develop.
Before the electron was discovered, atoms were thought to be tiny indivisible spheres. The discovery of the electron produced the plum pudding model — a ball of positive charge with negative electrons embedded in it.
The alpha particle scattering experiment overturned this. Most alpha particles passed straight through the gold foil, showing the atom is mostly empty space. Some were deflected, showing a concentrated positive charge, and a very few bounced back, showing the mass is concentrated in a tiny nucleus. Bohr then showed electrons orbit at specific distances, and Chadwick later provided evidence for neutrons.
Revision notes
Early models
Atoms were first thought to be tiny 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
Most alpha particles passed straight through the foil, showing the atom is mostly empty space.
Some were deflected, showing the positive charge is concentrated in one place. A very few bounced back, showing the mass is concentrated in a tiny nucleus. This replaced the plum pudding model with the nuclear model.
Refining the model
Niels Bohr calculated that electrons must orbit at specific distances from the nucleus, and this agreed with experimental observations.
Later work showed the positive charge is made of particles called protons. About twenty years later James Chadwick provided evidence for neutrons.
Key points
- Atoms were first thought to be indivisible spheres.
- The plum pudding model followed the electron's discovery.
- 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.
- Chadwick provided evidence for neutrons.
Worked examples
Example 1
Describe the plum pudding model of the atom. [2 marks]
Model answer
The atom was thought to be a ball of positive charge (1 mark) with negative electrons embedded within it (1 mark).
Example 2
Explain what each observation in the alpha scattering experiment showed. [3 marks]
Model answer
Most alpha particles passed straight through, showing the atom is mostly empty space (1 mark). Some were deflected, showing the centre has a concentrated positive charge (1 mark). A very few bounced straight back, showing the mass is concentrated in a very small nucleus (1 mark).
Example 3
State the contribution made by Niels Bohr to the model of the atom. [2 marks]
Model answer
He calculated that electrons orbit the nucleus at specific distances (1 mark), and his theoretical calculations agreed with experimental observations (1 mark).
Common mistakes
Saying the plum pudding model had a nucleus.
It had no nucleus — that came from the alpha scattering experiment.
Not pairing each observation with its conclusion.
Each of the three observations gives a separate conclusion and a separate mark.
Saying alpha particles were deflected by electrons.
The deflection was caused by the concentrated positive charge of the nucleus.
Confusing Bohr and Chadwick.
Bohr proposed electron orbits; Chadwick evidenced neutrons.
Exam tips
- Pair each observation with the conclusion it supports.
- Learn the models in chronological order.
- Name the scientists with their specific contributions.
- Use this as your example of models changing with evidence.
Key terms
- Plum pudding model
- Positive charge with embedded electrons, and no nucleus.
- Alpha scattering
- The experiment that revealed the nucleus.
- Nuclear model
- A model with a tiny dense positive nucleus.
- Chadwick
- The scientist who provided evidence for neutrons.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.