Radioactive decay
Some nuclei are unstable. They break down on their own and give off radiation. This is radioactive decay. Unlike a chemical reaction, it changes the nucleus, so one element can turn into another.
| Type | What is emitted | Effect on the nucleus |
|---|---|---|
| Alpha | A helium nucleus: 2 protons and 2 neutrons | Atomic number down 2, mass number down 4 |
| Beta | An electron, as a neutron becomes a proton | Atomic number up 1, mass number unchanged |
| Gamma | High-energy light | No change in either number |
Alpha particles are stopped by paper. Beta particles are stopped by thin metal. Gamma rays need thick lead or concrete.
In a nuclear equation, the mass numbers balance on both sides and so do the atomic numbers.
Worked example: alpha decay of uranium-238
Uranium has atomic number 92. An alpha particle carries away 2 protons and 4 mass units.
- New atomic number: 92 - 2 = 90, which is thorium.
- New mass number: 238 - 4 = 234.
Uranium-238 decays to thorium-234.
Worked example: beta decay of carbon-14
Carbon has atomic number 6. A neutron becomes a proton, so the atomic number rises by 1 and the mass number stays 14.
Carbon-14 decays to nitrogen-14.
Common mistake
Thinking beta decay removes an electron from the electron cloud. The electron comes from the nucleus, created when a neutron turns into a proton.