The nuclear model of the atom, isotopes, radioactivity and types of decay, half-life, and the Solar System.
The nuclear model of the atom
An atom has a tiny, dense, positively charged nucleus (protons and neutrons) surrounded by electrons. The Geiger–Marsden (Rutherford) scattering experiment — most alpha particles passing straight through gold foil, a few deflected sharply — showed the atom is mostly empty space with a small, massive, positive nucleus.
| Particle | Relative charge | Relative mass |
|---|---|---|
| proton | +1 | 1 |
| neutron | 0 | 1 |
| electron | −1 | ~ |
The proton (atomic) number is the number of protons; the nucleon (mass) number is protons + neutrons. Isotopes are atoms of the same element (same ) with different numbers of neutrons (different ).
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