A level Physics · Edexcel 9PH0 · Topic 8A
The nuclear atom
Key points
- In Rutherford's alpha-scattering experiment, alpha particles were fired at thin gold foil.
- Most passed straight through, so the atom is mostly empty space; a few were deflected through large angles, so the positive charge and almost all the mass is in a tiny nucleus.
- The nucleus is about m across, compared with about m for the atom.
- Nucleon number = protons + neutrons; proton number ; written X. Isotopes have the same but different .
- Electron diffraction from nuclei gives a better measure of nuclear size because electrons are not affected by the strong force.
- The strong nuclear force holds nucleons together; it is attractive over about 3 fm but repulsive below about 0.5 fm.
- Thermionic emission produces free electrons from a heated filament; they can be accelerated by a p.d. to energy .
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Our picture of the atom as a tiny, dense nucleus surrounded by mostly empty space came from firing particles at thin foils and watching where they went. The same idea, scattering a beam off a target, is still how physicists probe the…
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