Development of the Atomic Model — AQA GCSE Physics

The model of the atom changed many times as new experimental evidence was gathered.

The stages

1. Solid sphere — atoms were thought to be tiny indivisible spheres (Dalton).
2. Plum pudding model — after the discovery of the electron (Thomson), the atom was pictured as a ball of positive charge with electrons embedded in it.
3. Nuclear model — the alpha particle scattering experiment (Rutherford). Alpha particles fired at thin gold foil: most passed straight through, but a few deflected or bounced back. This showed the atom is mostly empty space with a small, dense, positively charged nucleus.
4. Bohr model — Niels Bohr proposed that electrons orbit the nucleus at specific distances (energy levels), which agreed with experimental observations.
5. Later, experiments showed the nucleus contains positive protons, and James Chadwick found evidence for the neutron (about 20 years after the nucleus was discovered).

Why models change

Each new model came from new experimental evidence. This shows how scientific ideas are tested, refined and sometimes replaced.

Exam tips

• Learn the order: solid sphere → plum pudding → nuclear → Bohr.
• Describe the alpha scattering experiment and what each observation showed.
• Name the discoveries: electron (Thomson), nucleus (Rutherford), energy levels (Bohr), neutron (Chadwick).
• Use this as an example of models changing with evidence.