An atom is radioactive when it decays or changes to form another atom Something is always produced from the old atom it is by random or chaotic chance the process is independent of temperature, chemical composition & pressure.

The three main types are Alpha -  Beta -  Gamma - 

 alpha

 alpha Nucleus throws out a bundle of; 2 protons + 2 neutrons

 alpha Nucleus throws out a bundle of; 2 protons + 2 neutrons

 alpha Nucleus throws out a bundle of; 2 protons + 2 neutrons

 alpha Nucleus throws out a bundle of 2 protons + 2 neutrons

 beta

 beta Nucleus throws out a neutron.

 beta This breaks down to a proton and an electron.

 beta The proton is sucked back into the nucleus. This high speed electron is the  particle.

 beta

 beta

 gamma

 gamma Nucleus throws a bundle of pure energy. This is a short wavelength, electromagnetic wave.