Some nuclei are unstable and break down by giving out radiation. This is called radioactive decay, and it happens whatever the temperature or chemical state of the atom.
Three types of radiation
| Type | Nature | Stopped by |
| alpha (α) | helium nucleus (2 protons, 2 neutrons) | a sheet of paper |
| beta (β) | fast-moving electron | a few mm of aluminium |
| gamma (γ) | electromagnetic wave | thick lead / concrete |
Alpha is the most strongly ionising but the least penetrating; gamma is the least ionising but the most penetrating. In a magnetic or electric field, alpha and beta bend in opposite directions (opposite charges) while gamma is undeflected.
Key idea
Alpha decay: A falls by 4, Z falls by 2. Beta decay: a neutron becomes a proton, so Z rises by 1 and A is unchanged.
Worked example
A nucleus 22688Ra emits an alpha particle. The new nucleus has nucleon number 226 − 4 = 222 and proton number 88 − 2 = 86, giving 22286Rn (radon).
Remember
- Background radiation comes from rocks (radon), cosmic rays and food.
- A Geiger-Müller tube detects and counts radiation.
- Decay is random: we cannot predict which nucleus decays next.