How atomic models developed
Ideas about atomic structure changed when new experiments produced evidence that earlier models could not explain. The most important development for nuclear physics was the replacement of a model with positive charge spread throughout the atom by one with that charge concentrated at its centre.
| Stage | Model | Main idea |
|---|---|---|
| Dalton, 1803 | Solid atom | Atoms were treated as tiny solid particles that could not be divided further. Atoms of one element were regarded as alike, while different elements contained different atoms; chemical reactions rearranged them. |
| J. J. Thomson, 1897 | Plum pudding | After discovering the electron, Thomson proposed a sphere of distributed positive charge containing negative electrons, so the atom was neutral overall. |
| Rutherford, Geiger and Marsden, 1909–1911 | Nuclear model | Alpha-particle scattering showed that positive charge and most of the atomic mass are concentrated in a very small central region. |
| Bohr, 1913 | Shell model | Electrons were described as occupying definite shells or energy levels around the nucleus. |
| Schrödinger, 1926 | Quantum model | Electron position was treated probabilistically, producing the idea of an electron cloud whose density represents the likelihood of finding an electron. |
| Chadwick, 1932 | Neutrons in the nucleus | The discovery of the neutron completed the basic picture of a nucleus containing protons and neutrons, surrounded by electrons. |
The decisive change
In Thomson's model, positive charge occupied the whole atomic volume. Rutherford's experiment instead required a small central concentration of positive charge.
The evidence therefore replaced the basic structure of the plum pudding model rather than merely refining it.