Heavy elements like uranium are unstable and emit invisible radiations spontaneously to gain stability. This phenomenon is called “natural radioactivity”. This was accidentally discovered by Becquerel in 1896 while studying the relation between X-rays discovered by Rontgen in 1895 and the phenomenon of fluorescence. He called them Becquerel rays.
Madame Curie and her husband Pierre Curie isolated radium and polonium from an ore of uranium called pitch-blend which showed much larger radioactivity than uranium. Later thorium and actinium possessing radioactivity were also discovered. Radiations from radioactive elements are called radioactive radiations.
Radioactive radiations are spontaneous and instantaneous and are not affected by pressure, temperature, electric and magnetic fields etc. Their emission rates also cannot be changed by any means not even by combining radioactive elements chemically with other elements to form different compounds.
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