Our model of the atom has changed as new evidence was discovered.
1. Dalton (early 1800s) — atoms are tiny solid spheres that can't be divided.
2. Thomson (1897) — discovered the electron, leading to the plum pudding model: a ball of positive charge with electrons embedded in it.
3. Rutherford (1911) — the alpha-scattering experiment. Most alpha particles passed straight through gold foil, some were deflected and a few bounced back. This showed the atom is mostly empty space with a small, dense, positive nucleus — the nuclear model.
4. Bohr (1913) — electrons orbit the nucleus at fixed distances (energy levels or shells). His calculations agreed with experiments.
5. Later experiments showed the positive charge of the nucleus is made of smaller particles, each with the same positive charge — protons.
6. Chadwick (1932) — about 20 years after the nucleus was accepted, he provided evidence for neutrons in the nucleus.
Each new model replaced the old one because it explained the new experimental evidence better.