What is radioactivity?

What is radioactivity?
The atoms making up matter are generally stable, but some of them are spontaneously transformed by emitting radiations which release energy. This is called radioactivity.
Atoms: In nature, matter - whether water, gas, rocks, living beings - consists of molecules, which are combinations of atoms. Atoms include a positively charged nucleus, around which move negatively charged electrons. The atom is neutral.
The nucleus of the atom includes positively charged protons as well as neutrons. It is this nucleus that is transformed when a radiation is emitted by radioactivit…