Models of the atom
From solid spheres to the plum pudding
Scientific models change when new evidence doesn't fit the old one.
- Early 1800s – solid spheres: John Dalton described atoms as tiny spheres that could not be divided.
- J. J. Thomson discovered the , so atoms must contain smaller particles. His model was a with negative electrons embedded in it, like fruit in a pudding.
Key terms
The alpha particle scattering experiment
Around 1909, Geiger and Marsden, working in Rutherford's lab, fired at a very thin sheet of gold foil. What they saw:
| Observation | What it showed |
|---|---|
| Most alpha particles went straight through | The atom is mostly empty space |
| Some were deflected | The centre has a , which repels alpha particles |
| A very few bounced back | The mass is concentrated in a tiny centre |
This led to the : a small, positively charged with electrons around it. It replaced the plum pudding model.
Key terms
Bohr, protons and Chadwick
- About 1913 – Bohr: Niels Bohr suggested that electrons orbit the nucleus at , or . His calculations agreed with experimental results.
- Later experiments showed that the positive charge of the nucleus is made of smaller particles, each with the same positive charge: .
- 1932 – Chadwick: James Chadwick showed that the nucleus also contains , with no charge.
Remember
The order is Dalton → Thomson → Rutherford → Bohr → Chadwick. Bohr's energy levels came before Chadwick's neutron.