Definition of radioactivity:

The phenomenon of emission of active radiations by an element was termed radioactivity. The element exhibiting this property was called radioactivity element.

Natural radioactivity is a spontaneous and self disruptive activity exhibited by a number of heavy elements occurring in nature. The word spontaneous means by its own or by shear out of nature of the element and self disruptive means that the element disintegrates itself i.e. no external provocation influences the emission. Radioactivity is, therefore, the property by virtue of which a heavy element disintegrates itself without being forced by any external agent to do so.

The phenomenon was discovered by a French Physicist, Henry Becquerel in 1896. He observed that uranium salts possessed a peculiar property of affecting a photographic plate even when the plate was in a light proof package. This he thought, must be due to certain active radiations emitted by uranium salts. These radiations were called Becquerel rays. The total number of radioactivity elements known at present is about 40. For instance, natural elements with atomic number greater than 82 are all radioactive. Their nuclei are thus unstable nuclei. Some examples are Radium, Thorium, Actinium, Polonium etc.

Effect of external conditions on radioactivity:

The experiments show that the phenomenon of radioactivity is not at all affected by the imposed conditions of temperature, pressure, chemical combination etc. Therefore, electrons orbiting the nucleus were not responsible for radioactivity. Hence, the radioactivity must be property of heavy nuclei only.

This is the introduction of radioactivity.

Brain Teaser: 

1. In radiactivity, what happens:

a) lighter atoms joins

b) heavy element disintegrates into lighter atoms with the help of external force

c) heavy element disintegrates into lighter atoms without the help of external force

d) both b) and c)

Please submit your answer in comment section.