Radioactivity is the spontaneous and random emission of radioactive rays from unstable radioactive materials after which they become more stable. There are three different types of radioactive emissions, Alpha particle- a, Beta particle- B, Gamma ray-r.
The process is said to be spontaneous because it is not influenced by any physical factors such as temperature, pressure, time, etc. A nucleus is unstable if it is too big. All nuclei with z > 83 or A> 209 are unstable.