Give a brief explanation about radioactivity. 

Becquerel discovered radioactivity in $1896$ .

While studying the fluorescence and phosphorescence of compounds irradiated with visible light, Becquerel observed an phenomenon. After illuminating some pieces of uranium - potassium sulphate with visible light he wrapped them in black paper and separated the package from a photographic plate by a piece of silver when after several hours of exposure the photographic plate was developed it showed blackening.

This blackening is due to something that must have been emitted by the compound. The phenomenon of radiation emission from a compound is known as radioactivity. And emitted radiation called radioactive radiation and elements contained in the compound is known as radioactive element.

The following are the notable facts of this phenomenon are :

$(i)$ The emission of radioactive radiation is spontaneous and continuous. It is not affected by external factors like change in temperature, pressure, presence of electric field or magnetic field. Such parameters cannot stop the emission of radioactive radiations or cannot change the rate of emission.

$(ii)$ Even by chemically combining a radioactive element with any other element the rate of emission of radiations is not affected.

These two points show that radioactivity is a nuclear phenomenon in which the nuclei of heavy elements are unstable and during their attempts to acquire stability they emit radioactive radiations.

Three types of radioactive decay occur in nature :

$(i)$ $\alpha$-decay in which a helium nucleus $\left({ }_{2}^{4} \mathrm{He}\right)$ is emitted.

$(ii)$ $\beta$-decay in which electrons or positrons (particles with the same mass as electrons but with a charge exactly opposite to that of electron) are emitted.

$(iii)$ $\gamma$-decay in which high energy (hundreds of $\mathrm{KeV}$ or more) photons are emitted.