Unstable nuclei

If an atomic nucleus does not change or break down, we say that it is stable. Most nuclei are stable. However, some atomic nuclei are unstable and may change or break down. When this happens, they emit particles or rays, including:

Substances containing unstable atoms are described as radioactive. The particles or rays they give off are types of radiation.

Types of radiation

The table shows some features of the four main types of radiation:

Type of radiationNatureSymbol in a nuclear equationElectric charge
alpha, αTwo protons and two neutrons bound together (identical to a helium nucleus) _{2}^{4}\alpha+2
beta, βA fast-moving electron from a nucleus _{-1}^{0}\beta-1
neutron, n A neutral subatomic particle from a nucleus _{0}^{1}n0
gamma, γElectromagnetic radiation _{0}^{0}\gamma0

Penetrating properties

Radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air, and gamma radiation travels very large distances.

All types of radiation become less intense as they travel further away from the radioactive material. This is because the particles or rays become more spread out. The thicker the substance, the more the radiation is absorbed.

Alpha, beta and gamma radiations penetrate materials in different ways.

Alpha radiation

Alpha radiation is the least penetrating. It can be stopped (or absorbed) by a sheet of paper or a human hand.

Beta radiation

Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.

Gamma radiation

Gamma radiation is the most penetrating. It can penetrate air, paper or thin metal. It may only be stopped by many centimetres of lead or many metres of concrete.

Alpha, beta and gamma rays passing through a hand, beta and gamma rays passing through aluminium and gamma rays stopping at lead.