»   Radioactivity is the sending out of harmful radiation or particles, caused when atomic nuclei breakup spontaneously.
»   Radioactivity was discovered by Henry Becquerel, Madame Curie and Pierre Curie for which they jointly win Noble prize.
»   The nucleus having protons 83 or more are unstable. They emit a, P and y particles and become stable. The elements of such nucleus are called radioactive elements and the phenomenon of emission of α, β and γ particles is called radioactivity.
»   γ rays are emitted after the emission of α and β rays.
»   Robert Pierre and his wife Madame Curie discovered a new radioactive element radium.
»   The rays emitted by radioactivity were first recognized by Rutherford.
»   The end product of all natural radioactive element after emission of radioactive rays is lead.
»   With the emission an α -particle, atomic number is decreased by 2 and mass member is decreased by 4.
»   With the emission of a β -particle atomic number is increased by one and mass number does not change.
»   The effect on the mass number and atomic number with the emission of α, β and γ rays is decided by Group-displacement law or Soddy-Fajan Law.
»   Radioactivity is detected by G.M. Counter.
»   The time in which half nuclei of the element is decayed is called half life of the radioactive substance.

Nuclear Fission and Fusion

A nucleus is represented as zXA where Z is the number of proton (called atomic number), A is sum of number of protons (Z) and number of neutrons (N) and is called mass number.
Number of neutrons N = A - Z


The nuclear reaction in which a heavy nucleus splits into two nuclei of nearly equal mass is nuclear fission. The energy released in the nuclear fission of nuclear energy.

Nuclear fission was first demonstrated by Strassmann and O. Hahn. They found that when U235 nucleus is excited by the capture of a neutron, it splits into two nuclei Ba143 and K92 .

Chain Reaction

When uranium atom is bombarded with slow neutrons, fission takes place. With the fission of each uranium nucleus, on the average 3 neutrons and large energy is released. These neutrons cause further fission. Clearly neutrons a chain of fission of uranium nucleus starts which continues till whole of uranium is exhausted. This is called chain reaction.

Chain reaction is of the following two types :
1. Uncontrolled chain reaction
2. controlled chain reaction

Uncontrolled chain reaction

In each fission reaction, three more neutrons may caused. These three neutrons may cause the fission of three other U235 nuclei producing 9 neutrons and so on. As a result the number of neutron goes on increasing till the whole of fissionable material is consumed. This chain reaction is called uncontrolled or explosive chain reaction. This reaction proceeds very quickly and a huge amount of energy is liberated in a short time.

Atom bomb : Atom bomb is based on nuclear fission. U235 and Pu239 are used as fissionable material. This bomb was first used by USA against Japan in second world war (6th August, 1945 at Hiroshima & 9th August, 1945 at Nagashaki).

Controlled Chain Reaction

A fission chain reaction which proceeds slowly without any explosion and in which the energy released can be controlled is known as controlled reaction. Actually in this situation only one of the neutrons produced in each fission is able to cause further fission. In controlled chain reaction, the rate of reaction remains constant.

Nuclear Reactor or Atomic Pile

Nuclear reactor is an arrangement in which controlled nuclear fission reaction takes place.

» First nuclear reactor was established in Chicago University under the supervision of Prof. Fermi.

Uses of nuclear reactor
1. To produce electrical energy from the energy released during fission.
2. To produce different isotopes which can be used in medical, physical and agriculture science.

Fast Breeder Reactor

A nuclear reactor which can produce more fissionable fuel than it consumes is called a fast breeder reactor.

Nuclear Fusion

When two or more light nuclei combined together to form heavier nucleus, tremendous energy is released. This phenomenon is called nuclear fusion.
» The energy released by sun and other stars is by nuclear fusion.
» For the nuclear fusion, a temperature of the order of 108 K is required.

Hydrogen bomb

Hydrogen bomb was made by American scientists in 1952 This is based on nuclear fusion. It is 1000 times more powerful than atom bomb.