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Discovery of Electron, Proton and Neutron

Dalton's concept of the indivisibility of atom was disproved by experimental evidences obtained by scientists towards the end of the 19th century. At ordinary pressure conditions, the gases are poor conductors of electricity. However, they start conducting if their pressure is reduced to about 10-2 atm and a very high voltage (5000-10,000 V) is applied at the electrodes. 

The gas emits characteristic light under these conditions. If the pressure of the gas is further reduced to about 10-4 atm, the emission of light ceases but the gas continues to conduct electricity. Under these conditions, a beam of rays moving in straight lines is emitted from the cathode, which on striking the glass tube produces a faint greenish light. Also, if an object is placed in the path of these rays, a sharp shadow of the object is produced on the wall of the glass tube. These rays were named cathode rays.
Rutherford, through his experiments, showed that atomic nuclei are very heavy, positively charged centres of atoms. A few years later, Moseley developed a technique to measure the magnitude of the positive charge of atomic nuclei. He found that the magnitude of positive charges carried by the atomic nuclei always happened to be integral multiples of the unit positive charge. He was also successful in arranging all known elements in the order of increasing nuclear (positive) charge. Since the nuclear charges of all atomic nuclei are integral multiples of unit charge, the atomic nuclei must always contain an integral number of protons. Further, an atom must have an equal number of protons and electrons.

Subsequent studies of these rays revealed that they are negatively charged particles as they can be deflected by applying electrical and magnetic fields. The splitting up of the gaseous atoms by the discharge produced these particles. At low pressures, the cathode rays did not recombine with the remaining fragments and were emitted from the cathode.


This lesson is going to give us the complete understanding of the following concepts
  1. Different Models of an atom, their postulates and limitations
  2. Dual nature of matter and debroglie wave equation
  3. Quantum mechanical nature of electrons (schordinger waves)
  4. Heisenberg's uncertainty principle and its mathematical aspect
  5. Principles relating to electronic configuration of atoms

Existence of Neutrons in the Nucleus of Atom

In 1932, Chadwick carried out an experiment in which α-particles were bombarded on beryllium. There was an emission of particles which were unaffected by electric and magnetic fields. 

These particles were called neutrons and their origin was explained on the basis of the following reaction: 


where the superscript refers to the mass number (sum of the protons and neutrons in the nucleus) and the subscript to the atomic number.

The formation of isotopes with the same atomic number and different mass numbers were explained on the basis that nuclei of different isotopes possess the same number of protons but different number of neutrons.

The mass of a neutron was found to be 1.6748 10-27 kg which is 1839 times heavier than the electron.

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