The Physics Question Bank

Our ideas about the atom have changed over time. Each model was accepted until new evidence showed that it needed to be improved.

At first, Dalton described atoms as tiny solid spheres. He thought atoms were very small particles that could not be divided. This was a useful early model, but it did not explain what was inside the atom.

Later, J. J. Thomson discovered the electron. This showed that atoms were not solid balls, because they contained smaller negatively charged particles. Thomson suggested the plum pudding model. In this model, the atom was a ball of positive charge with negative electrons spread through it, a bit like fruit in a pudding.

Rutherford then carried out the alpha scattering experiment. Alpha particles were fired at very thin gold foil. Most of the alpha particles passed straight through, but some were deflected and a few bounced back.

This evidence showed that the plum pudding model was wrong. If most alpha particles passed through, the atom must be mostly empty space. If some were deflected or bounced back, the positive charge and most of the mass must be concentrated in a tiny central nucleus.

Rutherford suggested the nuclear model of the atom. In this model, the atom has a tiny, dense, positively charged nucleus, with electrons around it.

Bohr then improved the model by suggesting that electrons orbit the nucleus in fixed energy levels, or shells. This helped explain why electrons could only have certain amounts of energy.

Later, Chadwick provided evidence for the neutron. This helped explain why the nucleus had more mass than could be explained by protons alone.

Order to remember

Dalton: atoms are tiny solid spheres.

Thomson: electrons are discovered, leading to the plum pudding model.

Rutherford: alpha scattering shows the atom is mostly empty space with a small positive nucleus.

Bohr: electrons orbit in fixed energy levels or shells.

Chadwick: neutrons are discovered in the nucleus.

6-mark answer split into marks

1 mark: Dalton described atoms as tiny solid spheres.

2 mark: Thomson discovered the electron, showing that atoms contain smaller negatively charged particles.

3 mark: Thomson suggested the plum pudding model, with electrons spread through a ball of positive charge.

4 mark: Rutherford’s alpha scattering experiment showed that the atom is mostly empty space.

5 mark: Rutherford concluded that the positive charge and most of the mass are concentrated in a tiny nucleus.

6 mark: Bohr suggested electrons are in fixed energy levels, and Chadwick later provided evidence for the neutron.