The Nuclei of some atoms are radioactive. This means that they are unstable and will break-up over time. Most elements have unstable forms and are called radioisotopes. As they break-up they give out three types of radiation: alpha, beta and gamma rays.
The alpha rays are streams of positively charged particles made up of two neutrons and two protons.
The beta rays are streams of electrons.
The gamma rays are electro-magnetic waves and are the most penetrating type of radiation.
Radiation can be very dangerous.