How the model of the atom changed1. Dalton — atoms are solid spheres.
2. Thomson — the plum pudding model: positive charge with electrons embedded in it.
3. Rutherford — alpha particles fired at gold foil: most passed straight through, some were deflected and a few bounced back. This showed a small, dense, positively charged nucleus.
4. Bohr — electrons orbit in fixed shells (energy levels).
The three types of radiationRadioactive decay is random and spontaneous.
| Alpha (α) | Beta (β) | Gamma (γ) | |
|---|---|---|---|
| What it is | 2 protons + 2 neutrons (a helium nucleus) | A fast electron from the nucleus | An electromagnetic wave |
| Charge | +2 | −1 | 0 |
| Stopped by | Paper | Aluminium | Thick lead (reduced) |
| Ionising | Most | Medium | Least |
| Penetrating | Least | Medium | Most |
Nuclear equationsMass number and atomic number must balance on both sides.
Alpha decay — mass number goes down by 4, atomic number goes down by 2.
Beta decay — mass number stays the same, atomic number goes up by 1 (a neutron turns into a proton and an electron).