Discovery of Neutron

Up to 1932 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. Mass of electrons is negligible. Mass of each protons is equal to 1.00376 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 discovered these neutral particles for which he was awarded the Nobel Prize in Physics in 1936. He obtained these neutral particles by bombarding light nuclei Li and Be with x-particles. He named these particles as neutrons on account of their neutral nature.

Definiton of A Neutron

Having known the mass and charge of a neutron, it can be defined as a sub-atomic particle which bears a mass equal to that of a proton (i.e. equal to 1 amu) and has no charge on it. For this reason a neutron is represented as
 1  n .
 0

Explanation of Production of Productive Rays

When a high voltage is applied across the metallic electrodes of a discharge tube filled with a gas under reduced pressure, the molecules of the gas are dissociated into atoms. Now when high speed electrons (i.e. cathode rays), originated from cathode strikes these atoms, the atoms are converted into cations which pass through the holes of the cathode and constitute positive rays. The electrons obtained in the conversion of the gaseous atoms into cations move towards anode as negative rays.

         M                 +          e-(originated from cathoode) =>                M+                    +               2e- 
Gaseous Atoms                                                                 Pass through the perforated           Move towards anode 
                                                                                                                          cathode as positive rays                  as negative rays

Now it is believed that the protons like electrons are also the common universal constituents of all atoms, excepting hydrogen which has no neutron.

Definition of Proton

Having known the mass and charge on a proton, it can be defined as a sub atomic particle which bears mass equal 1 amu and positive charge equal to +1 on it. In other words, a positive is a  sub atomic particle which bears one unit mass and one unit of positive charge on it.

Anode (or Positive) Rays-Discovery of Proton

Production of positive rays. We have seen that since an atom as a whole, is electricity neutral and it has negatively charged particles known as electrons, it should have some particles which should be positively charged. In order to show the presence of positively charged particles in an atom, attempts were made by many scientists. In 1886, German physicist, E. Goldstrein took a discharge tube provided with a cathode perforated with extremely fine holes and observed that when a large potential difference was applied between the electrodes, not only cathode rays of electrons were originated from the cathode, moving from cathode towards anode, but also new type of colored rays were rays were also simultaneously originated from the anode, moving from anode towards cathode through the fine holes or canals in the cathode and causing aglow on the wall of opposite to anode.

Initially these rays were called canal rays, since they pass through the canals or holes of the cathode. These are also called anode rays as they seem to originate from the anode, J.J Thomson has shown that these rays are positively charged and hence he has called them positive rays.

The origin of these rays can easily be understood. In a discharge tube when gas atom (say H atom) lose negatively charged electrons, they acquire a positive charge. These positively charged Particles then move from anode to cathode.

Electrons Are The Common Universal Constituents of All Atoms

It has been shown by Thomson that the electrons emitted in the cathode rays have the same e/m value (= -1.758*108 coulomb g.), irrespective of the nature of the material of the cathode and that of the gas taken in the discharge tube. Similarly it has been shown by Millikan that the value of e for the electrons is also the same (= -1.602*10-18 coulomb),irrespective of the gas used in the experiment. From these observations it can be concluded that electrons are present in all forms of matter and are common or universal constitutions of all atoms.

Definition of Electrons

Having known the mass and charge on an electron, it can be defined as a subatomic particle which bears mass equal to 0.0005493 amu (which is taken to be equal to zero) and negative charge equal to-1 on it. In other words an electron is a mass less particle which bears one unit of negative charge on it. An electron is indicated as -10e where zero indicates that it has no mass and -1 stands to show how the presence of one unit of negative charge on it.

Gathode (or Negative) Rays - Discovery of Electron

Production of Gathode Rays- The first indication of the complex nature of atom was obtained from a study of conduction of electricity through gases at reduced pressure by Julius Plucker (1850). He took a cylindrical glass tube, called discharge tube which was 30 cm long and 4cm in diameter, closed at both ends, provided with metallic electrodes pump and low pressure gauge. To obtain very low pressures, molecular pump and charcoal cooled by liquid air that has the property of absorbing most of the residual gas were used. The two electrodes were connected to a high voltage battery (voltage was the order of 10,000 volts and more). The electrode connected to the negative terminal of the battery is known as cathode (-) while that connected to the positive terminal is called anode (+) or anti-cathode. Plucker pumped out most of the gas of the tube by means of high vacuum pump and low pressure gauge and thus reduced the pressure of the gas in the tube to about 0.001 mm Hg. Now he observed that when he applied a high voltage of the order of 10,000 volts across the metallic electrodes , the walls of the tube began to glow with a bluish or greenish light depending on the composition of the glass of which the tube is made. It thus appeared that the invisible rays of light were emitted from the cathode and moved towards anode at right angles and with large velocity. These rays were called cathode rays by Goldstein (1876) since these originated from the cathode.