Radioactive Decay Types

When nuclei have too many or too few neutrons, they become unstable and undergo radioactive decay to reach a more balanced state. The type of decay depends on what's wrong with the nucleus.

Alpha decay occurs when nuclei have too few neutrons relative to protons. An alpha particle contains 2 protons and 2 neutrons, so when it's emitted, the mass number drops by 4 and the atomic number decreases by 2. Alpha particles are quite hefty but don't travel far - only about 5 cm in air before being stopped by something as thin as paper or your skin.

Beta decay happens when there are too many neutrons. A neutron transforms into a proton and releases a fast-moving electron called a beta particle. This increases the atomic number by 1 (since there's now an extra proton) but keeps the mass number the same. Beta particles travel further than alpha - about 1 metre in air - and need 3mm of aluminium to stop them.

Key Point: Alpha particles have high ionising power (they knock electrons off atoms easily), while beta particles have low ionising power.

Gamma rays are electromagnetic waves released when nuclei "cool down" after alpha or beta decay. They don't change the atomic or mass numbers at all. Gamma rays travel the furthest - up to 1 km in air - and have very low ionising power, but they're the hardest to stop, requiring thick lead or concrete.

Half-life is the time needed for half the unstable isotopes in a sample to decay, or for the activity to halve. Scientists use this concept to date archaeological samples, though it's worth remembering that radioactive decay is completely random - you never know when a specific atom will decay.