- The nucleus of an atom consists of two types of particles – protons and neutrons.
- The total number of protons in the nucleus is called atomic number (Z).
- The total number of nucleons (proton or neutrons) in the nucleus is called its mass number (A).
- If N is number of neutrons then,
|A = Z + N|
- Binding energy of a nucleus is the energy with which nucleons are bound in the nucleus.
Nuclear Fission Physics :
- It is the phenomenon of splitting a heavy nucleus into two or more smaller nuclei.
|92U235 + 0n¹ ⇒ 56Ba141 + 36Kr92 + 30n¹ + Q|
- where, Q = 200 MeV (nuclear energy)
- Nuclear reactor uses nuclear energy for peaceful purpose. It is based on phenomenon of controlled nuclear chain reaction.
- Moderators like heavy water; graphite and deuterium slow down neutrons. Rods of cadmium or boron serve as control rods. Ordinary water and heavy water serve as coolants.
Nuclear Fusion Physics :
- It is the phenomenon of fusing of two or more lighter nuclei to form a single heavy nucleus.
- Mass defect in the process appears as energy.
|41H¹ ⇒ 2He4 + 2 (+1e0) + 26.7 MeV|
Example Hydrogen bomb.
Hydrogen Bomb Physics :
- It is based on the phenomenon of nuclear fusion and was made in 1952 by American scientists.
- The central core of a hydrogen bomb is a uranium (or plutonium) fission bomb which is surrounded by a compound of heavy hydrogen, like lithium hydride (LiH2).
- When the fission bomb is exploded, it produces such a high temperature and pressure that the heavy – hydrogen nuclei come extremely close and fuse together, liberating – huge energy.
Memorable Points :
- Rutherford (1911), discovered nucleus by the scattering of α – particles from gold foil.
- Nucleus contains almost all the mass and all the positive charge of the atom.
- The nucleus is spherical in shape and its volume is nearly 10-15 times the volume of atom.
- The sum of number of proton and number of neutrons is called mass number; Neutron was discovered by Chadwick in 1932.
- Neutron is slightly heavier than proton.
- Since, neutron is neutral; it does not ionize the gas and hence does not produce any track in the Wilson Cloud Chamber.
- Penetrating power of neutrons is very high and it can penetrate through the nucleus of an atom. That is why neutron is responsible for the nuclear fission.
- Owing to the small size of nucleus, Fermi (F) is found to be convenient unit of length :
|1 Fermi (F) = 10-15m|
- Density of the nuclei of all the atoms is same, as it is independent of mass number.
- Lighter elements have equal, or nearly equal, number of protons and neutrons in their nuclei.
- As we go to heavier elements, the number of neutrons compared with the number of protons increases progressively. The number of neutrons is 20% larger in iron and 50% larger in uranium.
- The excess of neutrons over protons in the heavier nuclei is due to the fact that the protons repel each other electro – statically, and as such their relative number must be reduced in order to secure the stability of the nucleus.
- The nuclei for which Z ( number of protons ) is greater than 82 are unstable and undergo radioactive decay.
- Relative strength of the gravitational, coulomb’s and nuclear force is,
|Fg : Fc : Fn : 1 : 1036 : 1038|
- The minimum mass of uranium needed to sustain chain reaction is called critical mass. Critical mass of uranium is 10 kg.
- All the neutrons produced during fission cannot cause fission. Some of them escape out of the material.
- To form atom bomb two or more pieces of uranium or plutonium are used. The mass of each piece is less than the critical mass but combined mass is more than the critical mass. In controlled chain reaction only one neutron causes further fission. The rate of reaction remains constant.
- Fusion takes place at high temperature of 107 or 108 °C. The pressure required is also high.
- Fusion is possible only where there is an abundance of lighter elements.
- At high temperatures Carbon – Nitrogen cycle takes place in which four hydrogen nuclei fuse together to form a helium nucleus. In this reaction carbon and nitrogen act as catalysts.
- The first nuclear reactor was made by Fermi who used U235 as fuel.
- The size of a fission bomb cannot be increased beyond a limit because in it the fissionable substance is kept in two pieces, and the size of each piece should be less than the critical size.
- There is no such restriction on the size of hydrogen bomb. The substance to be fused may be taken in it in any quantity. Once the fusion is initiated, it can spread throughout any mass of the substance.
- Therefore, a fusion bomb is much more destructive than the fission bomb.