# Unstable nuclei

If an atomic does not change or break down, we say that it is stable. Most 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 are described as . The particles or rays they give off are 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}n$0
gamma, γElectromagnetic radiation$_{0}^{0}\gamma$0

## Penetrating properties

Radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, 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.