Uupto 1982 it was postulated that an atom is composed of only protons and electrons, and that the mass of an atom is due to the mass of protons present in the nucleus, since the mass of electrons is negligible. Maas of each proton is equal to 1.00375 atomic mass unit (a.m.u)
Rutherford noticed that the atomic masses of different atoms could not be explained if it is supposed that the atom is composed only of protons and electrons and therefore, he predicted in 1920 that some kind of neutral particles having mass equal to protons must be present in an atom It was in 1932 that Chadwick discoved Thomson's a atomic model these neutral particles for which he was awarded the Nobel Prize in Physis is 1935. He obtained these neutral particles by bombarding light nuclei like Li and Be with alpha particles. He named these particies ns neutrons on account of their neutral nature.
Experiment:
In his axperiment, Chadwick directed a stream of o-particles
(2He) produced from polonium source at a berryllium target (4Be isotope) beyond which was placed a charge detector . He found that penetrating radiations are produced. He named these rudiations as neutrons on account of the fset that they have no charge on them as indicated by charge detector
The reaction which occurred in Chadwick's experiment is an example of artificial transmutation where an atom of berylltum (4B isotope) into a carbon atom through the nuclear reaction shown below is converted into a carbon atom through the nuclear reaction shown below:
Mass and charge of a neutron
The mass of a neutron is equal to that of a proton. On the relative scale, since the mass of a proton is taken to be equal to 1 amu, although the exact mass of a neutron is 1.00898 0.m.u. In grams, the mass of a neutron is equal to 1.672 10g. Neutron has no charge on it
Definition of a Neutron
Having known the mass and charge of a neutron, it can be defined as a sub stomic particle which bears a mass equal to that of a proton (Le. equal to 1 amu) and has no charge on it. For this reason a neutron is represented as 1n
Properties of Neutron
I) Decay of neutron. A free neutron (n) decays into a proton with the emission of an electron (1) and a neutrino.
ii) The neutrons are very penetrating. Their intensity of penetration is reduced to one-half in passing through lead of thickness of about 1.5 cm. Neutrons are less penetrating shan cosmic rays.
iii) Neutrons have the property of expelling high speed protons from hydrogenous matter like paraffin, water, paper, cellophane etc. From the rang of protons knocked out from paraffin the energy of neutrons has been calculated.
iv) Neutrons resemble X-rays and y-rays in so far as they do not produce a track in a Wilson cloud chamber but differ in one respect that neutrons produce no ionisation.
v) Fast, slow and thermal neutrons. Neutrons with energies above 1.2 MeV are called fast nautroiss hecoute 12 MeV is the minimum energy for fiasion of Neutrons with energies below 1 eV are called slow neutrons. Neutrons with energies between 18V and 1.2 MeV are called epi-thermal neutrons. Those neutrons which are in thermal equilibrium with the matter through which they pass and have a Maxisellian distribution of energies with a most probable value depending upon temperature are called thermal neutrons. The most probable energy of thermal neutrons at 27°C is 0.0026
Slow neutrons are usually more effective than fast ones, because high-speed neutrons may pass through a neucleus without bringing about transmutation
Uses of Neutrons
I) Almost all the elements can be transmulated by neutrons Some of the elements require fast neutrons whereas other require alow ones. Thus a fast neutron ejects an a-particle from a nitrogen nucleus, thereby converting it into a boren nucleus.
N+B+He
When slow moving neutrons fall on certain substances, they are captured with the enussion of high energy particle from a nitrogen nucleus, thereby converting it into a boron nucleus.
Cu+Cu+hu
Radioactive
ii) The neutrons are now being widely used in biological and medical sciences Because of their intense biological effects they are now being used in the treatment of cancer.
| Particle | Mass (g) | Mass (kg) | Mass (amu) | Charge (e.s.u.) | Charge (Coulomb) | Charge (units) | Symbol | Discoverer | Location |
|---|---|---|---|---|---|---|---|---|---|
| Electron | 9.108 x 10^-28 | 9.108 x 10^-31 | 0.005493 | -4.8 x 10^-10 | -1.602 x 10^-19 | -1 | e | J.J. Thomson | Outside the nucleus |
| Proton | 1.672 x 10^-24 | 1.672 x 10^-27 | 1.005757 | 1.602 x 10^-10 | 1.602 x 10^-19 | +1 | p | J.J. Thomson | In the nucleus |
| Neutron | 1.672 x 10^-24 | 1.672 x 10^-27 | 1.00898 | 0 | 0 | 0 | n | Chadwick | In the nucleus |