A picture that had to change
Show on a big screenFour pictures of the atom have come one after another.
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- Dalton: an atom is a solid ball.
- Thomson: positive charge spread out, electrons inside.
- Rutherford: a tiny nucleus, with electrons circling it.
- Bohr: electrons only in fixed shells.
The idea
Scientists change a model when new evidence does not match it. Evidence is what experiments show. Each model is still useful. Together they show how science moves one step at a time.
| Model | Idea | What it could not explain |
|---|---|---|
| Dalton, 1808 | Atoms are solid and cannot be divided | Radioactivity and electrons |
| Thomson | Electrons inside a positive sphere | The gold foil results |
| Rutherford, 1911 | A tiny nucleus, electrons circling it | Why atoms are stable |
| Bohr, 1913 | Electrons in fixed shells | Atoms with many electrons |
Rutherford's model had a problem. A circling electron keeps changing direction. Known physics said a charge that changes direction gives out energy. So the electron should lose energy and spiral into the nucleus. Atoms would collapse. But atoms are stable.
Bohr proposed a fix. He said electrons move only in fixed paths. These are shells, or stationary states. In a shell, an electron does not lose energy.
Even Bohr's model was later improved. Electrons are now described as clouds around the nucleus.
Try the lab to finish this lesson.