Radioactivity was discovered by whom?

Hint:Radioactivity is the phenomenon by which a nucleus of an unstable atom loses energy by emitting particles of ionizing radiation. Ionizing radiation means the flow of energy in the form of atomic particles or electromagnetic waves that displace the electrons from atoms and cause the atom to ionize. We are going to know about the discovery of Radioactivity and who discovered the radioactivity phenomenon.

Complete step-by-step solution:
The discovery of X-rays is the reason for the discovery of radioactivity that is separated from X-rays. The X-rays were first discovered by Wilhelm Conrad Roentgen $\left( {1845 - 1923} \right)$ in the year $1895$.
Henri Becquerel learned about Roentgen's discovery of X-rays through the study of fluorescence salts. Becquerel took photographic plates with black paper and fluorescent salts. He paced the concealed paper in sunlight for the study, but there was dark in the sky at that time. So, he placed it in a dark drawer.
After some days, he opened the concealed paper and developed it for study. There is a lack of salts with a lack of energy source, continually fluorescence. Henry Becquerel discovered it was Radioactivity.
Radioactive elements give three types of radiation during emission, they are alpha $\left( \alpha \right)$, beta $\left( \beta \right)$, and gamma $\left( \gamma \right)$ radiations.
Antoine Henri Becquerel $\left( {1852 - 1908} \right)$ was the first to discover the phenomenon of Radioactivity and received a Nobel prize in Physics.
We came to know that Antoine Henri Becquerel was the first to discover radioactivity.

Note:After the discovery of radioactivity, Marie Curie $\left( {1867 - 1934} \right)$ has discovered Polonium and Radium. Pierre and Marie were awarded the Nobel Prize in physics in the year $1903$. In radioactive decay, a relatively large amount of energy is liberated during each disintegration. It is more than one million times the amount of energy produced in an exothermic reaction.