Development of the Atomic Model — AQA GCSE Chemistry

Our model of the atom has changed many times as scientists gathered new experimental evidence.

The stages of the model

Solid sphere model — atoms were first thought to be tiny, solid spheres that could not be divided (Dalton, early 1800s).
Plum pudding model — the discovery of the electron (by J.J. Thomson) showed atoms contain smaller particles. The plum pudding model pictured a ball of positive charge with negative electrons embedded in it.
Nuclear model — the alpha particle scattering experiment (Rutherford) fired positive alpha particles at thin gold foil. Most passed straight through, but a few were deflected or bounced back. This showed the atom is mostly empty space with a small, dense, positively charged nucleus.
Bohr model — Niels Bohr proposed that electrons orbit the nucleus at specific distances (energy levels). This matched experimental observations.
Later work showed the nucleus contains positively charged protons, and James Chadwick provided evidence for the neutron (about 20 years later).

Each change came from new experimental evidence. This is a good example of how scientific theories are tested and modified over time.

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