In 1896 a French scientist, Henry Becquerel, was conducting some experiments on the compounds of Uranium in connection with X-ray. In those days there was no sunlight due to cloudy weather. So he kept this compound in a paper and placed it in a drawer where there were already placed some photographic plates covered in black paper. After a few days he found that the photographic plates carry some impressions. From it he concluded that the Uranium Salt, even in the absence of sunlight, emits radiations which pass through the wrapping paper and affect the photographic plates. In order to test his conclusion, he performed many experiments.
Becquerel took a closed box into which no light could enter and placed in it the uranium salt and the photographic plates near to each other and also separated by aluminum sheets. In both the cases, he obtained the same results. This showed that strong radiations are emitted by the uranium slat and they are not affected by the absence or presence of the sunlight. It does not matter even which compound of uranium is used. Whether this compound is in the solid or in the liquid state. Afterward, with the discovery of thorium, polonium and radium etc., and the same actions found in these elements, the phenomenon was termed as radioactivity.
Radioactivity is such a process in which the elements with the charge number greater than 82, naturally keep on radiating.
During that process three different types of radiations are emitted which are termed as Alpha (a), Beta (B) and Gamma (y) radiations. Since this is the natural property of elements are called natural radioactive elements. Now-a-days, using modern techniques, the elements. Which are much lighter than the natural radioactive elements, are made radioactive? Such elements are known as artificial radioactive elements. The atoms of radioactive elements change into atoms of the other elements due to the emission of radiations. This breaking process is spontaneous and continues for a long time and is irreversible.
The radioactive elements whose atomic numbers are the same but have different atomic mass numbers are called radioactive isotopes.
In order to study the, nature of radiations emitted due to radioactivity, Rutherford and others performed an experiment as described below.