Nucleus consists of proton and neutron. These proton and neutron remains under the action of two competitive forces; attractive force also called nuclear force and electrostatic repulsive force. Nuclear force exists between the nucleolus. (Neutron and proton). Electrostatic force of repulsion exists between the protons. Hence, for a nucleus to remain stable required certain number of proton and neutron. However, such condition doesn’t hold in every element especially with heavy element whose atomic number is greater than 82. The elements which are unstable emit radiation spontaneously without using any external agent. This activity of a nucleus is known as radioactivity and the elements which show this behavior are known as radioactive elements. The behaviors of radioactivity of radioactivity don’t depend upon temperature, pressure and chemical reactions. The radioactive elements emit radiation in different forms which are known as alpha particle emission, beta particle emission and gamma rays emission.
The figure above shows the condition for the stability of nucleus and dotted line indicate the condition of instability.