Atom bomb is a nuclear weapon which makes a huge explosion powerful enough to destroy a city. It is based on uncontrolled fission chain reaction. When the nucleus of a heavy element like uranium is bombarded by a neutron, it breaks into two large fragments and either two or three fresh neutrons are produced. These neutrons further collide with other nuclei causing more fissions. In this process large amount of energy is released. If the chain reaction continues, tremendous, devastating explosion will take place. This is what we call as atomic explosion.
To make an atom bomb critical mass of uranium – 235 or plutonium – 239 undergoes fission chain reaction by stray neutrons. In this process tremendous energy is released.
Based on fission chain reaction, the first atomic bomb was developed during the Second World War by a team of American scientists. The first atomic bomb made of uranium – 235 was dropped on August 6, 1945 on Hiroshima and three days later the second bomb based on plutonium was dropped on Nagasaki (Japan). Those have been most tragic events in the history of mankind. Destruction caused by these bombs ended Second World War.
After this Russia, Britain, China and France also tested their atom bombs.