Give a brief explanation about radioactivity.

(N/A) Becquerel discovered radioactivity in $1896$.
While studying the fluorescence and phosphorescence of compounds irradiated with visible light,Becquerel observed a 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. After several hours of exposure,when 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. The emitted radiation is called radioactive radiation,and the elements contained in the compound are known as radioactive elements.
The following are the notable facts of this phenomenon:
$(i)$ The emission of radioactive radiation is spontaneous and continuous. It is not affected by external factors like changes in temperature,pressure,or the presence of an electric or magnetic field. Such parameters cannot stop the emission of radioactive radiations or 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 $\left({ }_{2}^{4} He\right)$ 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 an electron) are emitted.
$(iii)$ $\gamma$-decay,in which high-energy (hundreds of $KeV$ or more) photons are emitted.