Our idea of the atom has changed over time as new experiments gave new evidence. Each scientist built on and corrected the model before it. This is a good example of how science develops.
The main models in order
- John Dalton — the atom is a tiny, solid, indivisible sphere.
- J. J. Thomson — discovered the electron; proposed the "plum pudding" model: a sphere of positive charge with electrons dotted inside.
- Ernest Rutherford — the alpha-particle scattering experiment showed a small, dense, positively charged nucleus at the centre, with electrons around it.
- Niels Bohr — electrons move around the nucleus in fixed shells (energy levels).
- James Chadwick — discovered the neutron in the nucleus.
Worked example / 例题
In Rutherford's experiment, most alpha particles passed straight through a thin gold foil, but a few bounced back sharply. This showed the atom is mostly empty space with a tiny, dense, positive nucleus — the parts that bounced back had hit the nucleus. The plum-pudding model could not explain this, so it was replaced.
Key idea / 要点
An atomic model is changed or replaced when a new experiment produces results the old model cannot explain. Science advances through evidence, not opinion.
Why the models kept changing
Dalton could not know about electrons because they had not been discovered. Once Thomson found electrons, the solid-sphere idea was no longer enough. Rutherford's results then forced the plum-pudding model to be replaced by the nuclear model, and Bohr and Chadwick added further detail. Today's model uses electron shells around a nucleus of protons and neutrons. Each new discovery did not simply erase the one before it; instead it kept the parts that still worked and corrected the parts that the new evidence showed to be wrong. This step-by-step improvement, driven by experiment, is exactly how scientific knowledge grows more accurate over time.
Remember / 记住
- Dalton → Thomson → Rutherford → Bohr → Chadwick.
- Thomson found the electron; Chadwick found the neutron.
- Rutherford discovered the nucleus.