Discovery of Radioactivity - Structure Of Atom

The discovery of cathode rays and anode rays showed that atoms are divisible into subatomic particles. This was further supported by the discovery of radioactivity by a French scientist Henri Becquerel in 1896. Radioactivity is the phenomenon of spontaneous emission of active radiations by certain elements. The substances which emit such radiations are called radioactive substances. The common radioactive substances are uranium, radium, polonium, thorium, etc.

The effect of magnetic and electric fields on these radiations was studied by placing a small sample of radioactive substance, uranium, in a cavity of a block of lead. The radioactive radiations coming out from a narrow slit were allowed to pass through a strong electric or magnetic field. The deflections of these rays were recorded on a photographic plate. The radiations were found to split into three types of rays. These were :

Alpha rays. The rays which are deflected towards the negative electrode are called alpha rays.
Beta rays. The rays which are deflected towards the positive electrode are called beta rays.
Gamma rays. The rays which are not deflected at all and are therefore, neutral are called gamma rays.

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