Some elements are naturally radioactive (they are called radionuclides) and are unstable. In order to become more stable they decay emitting energy and form other elements. The energy is emitted as radiation in three main forms:
alpha particles, beta particles and gamma photons. The new element formed through decay is termed a daughter product. For example Radium-226 decays to Radon-222 emitting an alpha particle and gamma photons. In this example, Radon-222 is the daughter of Radium-226. These radiations can interact with surrounding matter to produce positively and negatively charged particles called ions. This process is called ionisation, hence the term
Radiation that does not produce sufficient energy to ionise matter is termed
non-ionising. Examples include: ultraviolet radiation, light, infrared radiation and radio frequency radiation.