Some substances which emit radiation on their own are called radioactive substances and such an event is called radioactivity.
The event of radioactivity was observed by the chemist Henri Becquerel.
Generally, radioactive substances emit three types of radiations or rays –
Alpha rays : Particles in alpha rays are similar to He nucleus with positive charge.
Beta rays : Beta particles are negatively charged and they are similar to electrons.
Gamma rays : Gamma rays are similar to X-rays.
Penetrating power : The penetrating power of beta rays is 100 times more than that of alpha rays and the penetrating power of gamma rays is 1000 times more than that of alpha rays.