Nuclear Fission for std 11 & 12 GSEB course Chemistry

Uranium-235 is highly unstable. When a neutron collides with a uranium-235 nucleus, the nucleus breaks up or gets fissioned into two smaller nuclei of comparable masses along with the liberation of a large amount of energy (Fig. 12.1). The product nuclei of fission reaction are called fission fragments.
The reaction in which a heavy nucleus splits up into two medium nuclei with the liberation of enormous amount of energy, is called nuclear fission.

Note  
The German scientists Otto Hahn and Fritz Strassmann discovered unclear fission in 1938.
Dr. H. J. Bhabha (1909-1966) an Indian physicist started the atomic energy era in India.
He is known as the architect of India’s atomic energy programme. His work was followed up by Dr. Vikram A Sarabhai (1919-1971) and other eminent Indian scientists. 

Nuclear reaction

Example : 

When uranium-235 atoms are bombarded with slow moving neutrons, fission takes place. It is represented as follows. 

Fission fragments need not always be barium and krypton. Sometimes, uranium splits upto into xenon and strontium and sometimes into lanthanum and bromine. 

The energy released by fission of 1 g of U-235 is equal to that due to combustion of 50 millions tons of coal; it is about 8.5 x 1010 J.
During fission, 2 or 3 neutrons, g -rays and enormous amount of heat energy and light energy are released. The fission fragments are radioactive and they decay with the emission of
  alpha-rays, beta-rays and gama- rays. 

CHAIN REACTION
During each fission of U-235, 2 or 3 neutrons are released. These neutrons may cause further fission. The reactions continue further and further in the form of a chain with a multiplying effect. Such a self sustained reaction is called a chain reaction (Fig. 12.2).
Chain reaction is a continuous unclear fission reaction.
In a chain reaction if fission neutrons are built upto a certain level and thereafter the number of fission producing neutrons, is kept constant, then it is called controlled chain reaction. In this case, energy is generated at a definite rate. Controlled chain reaction is the principle of a nuclear reactor.

Chain Reaction of nuclear energy

Critical size : 
It the size of the fissionable material is smaller than a certain minimum, fission neutrons escape through its surface and the chain reaction is not possible. At a particular size called critical size, fission neutrons are retained to continue the chain reaction. The minimum size of a fissionable material required to sustain fission chain reaction, is called critical size.
Atom bomb (Nuclear bomb)
An atom bomb consists of two separate pieces of fissionable material, Each of these is smaller than the critical size. When they are brought together, the resulting lump is much larger than the critical size. Therefore chain reaction builds up rapidly resulting in an explosion.
Atom bomb was dropped on the Japanese city of Hiroshima, on 6th Aug 1945. Nearly 50% of the city was destroyed killing about 80,000 people. Three days later another bomb was dropped on Nagasaki destroying half the city and killing thousands of people. One bomb hand done more damage than 20,000 tons of TNT. Unbelievable destruction and horror was unleashed.

Radioactivity for std 11 & 12 GSEB course based

Review
An atom is made up of three fundamental particles namely proton, neutron and electron.
Protons and neutrons are bound together is the central part of an atom, called the nucleus. 
Electrons revolve round the nucleus in different imaginary orbits. 
The number of protons in an atom is called its atomic number, represented by Z.

Protons and neutrons is a nucleus have a common name ‘nucleons’ the total number of nucleons in an tom is known as the atomic mass number represented by A. Mass number is very close to the atomic mass.
Electron (-1eo) is a negatively charged particle with negligible mass.
Proton (+1p1) is a positively charged particle with unit mass considering the mass of hydrogen atom to be ‘one unit’.
Neutron (0N1) is an electrically neutral particle with unit mass.

INTRODUCTION

In 1896, Henri Becquerel (1852-1908) a French scientist, accidentally discovered radioactivity. He found that a photographic plate, wrapped well to protect is from visible light, was blackened by a mineral containing uranium. He concluded that the mineral emitted a new kind of radiations which occurred without any external cause. The emission of these radiations is not affected by external factors like temperature, pressure, electric field and magnetic field. The radiations were called Becquerel rays.

Radioactivity

All elements which emit Becquerel rays are said to be radioactive and the elements are called radioactive elements. 

All the elements from atomic number 81 to 92 are found to be radioactive. Experiments show that atoms of radioactive elements disintegrate into atoms of other elements. It became clear that the radiations must emanate from the nucleus and radioactivity is a nuclear phenomenon. We can conclude that the phenomenon is due to disintegration of an unstable nucleus accompanied by some type of radiation. 

The property of spontaneous disintegration of certain unstable. This is referred to as natural radioactivity. Radioactivity produced artificially or induced, is called artificial or induced radioactivity. The radiations emitted by radioactive elements are called radioactive radiations.

Note:

In 1898, Marie Curie (1867-1934) and her husband Pierre Cune (1959-1906) discovered a new element named Polonium and later on, another element named Radium, In 1903 Nobel prize in physics was shared by H. Becquerel, M. Curie and P. Curie, for their discovery of radioactivity. In 1911, M. Curie was awarded Nobel prize in chemistry for the extraction of radium in its purest form.