If motion of electron is described on the bases of classical mechanics & electromagnetic theory. It does not explain the stability of atom.A very few α – particles bounced back which indicates that the whole mass is concentrated at the center of atom.A small fraction of α – particles were deflected from its original path which indicates that the positive charge is at the center of the atom and due to repulsive forces deflections takes places.Most of the α – particles passes through the foil without any deflection which indicates that most of the atom is empty.Limitations:Could not satisfactorily explain the results of scattering experiment carried out by Rutherford.Postulate:Atom is a sphere of positive charge in which number of electrons are embedded.THOMSAN MODEL OF ATOM:He proposed that atom is regarded as sphere of radius 10 – 8 cm (approx.) Carrying positive charge particles due to protons and in which negatively charged particles are equally embedded.By the bombardment of thin sheets of beryllium with fast moving α particles he observed that highly penetrating rays consist of neutral particles known as neutrons.The mass of neutron is 1.675 ×10 – 24 g or 1.008665 amu or u
DISCOVERY OF NEUTRON: It was discovered by CHADWICK. PROTON: The smallest & lightest positive ion H + charge = 1.6 ×10 – 19 coulomb. These rays were named as positive rays or anode rays or as canal rays. On reducing the pressure, he observed a new type of luminous rays passing through the holes or perforations of the cathode and moving in a direction opposite to the cathode rays. 
In 1836 the same experiment was modified the discharge tube by using a perforated cathode.
DISCOVERY OF PROTON: It was done by E. The mass of electron(me) = 9.1 × 10 – 31 kg. From the above points J.J.Thomson was able to determine the value of charge to mass ratio(e/m)= 1.7588 × 10 11cKg – 1. The deflection of electrons from its original path increases when voltage increases. Lighter the mass of the particle greater will be the deflection. Greater the magnitude of the charge on the particle greater is the deflection when electric and magnetic field is applied. Charge to mass ratio of Electron( e/ m) : The charge to mass ratio is known as specific charge J.J.Thomson measured e/m ratio of the electron based on following points. How many electrons does this oil drop has? Sol we know that q = ne,n = 5).i.e Charge of an electron (e) = – 1.6 ×10 – 19coulomb Millikan concluded that the magnitude of electrical charge 'q' on the droplets is always an integral multiple of the electrical charge 'e' that is q = ne, where n = 1,2,3.( An oil drop has 8.0 ×10 – 19C charge. Hence electrons are considered as constituent particles of all atoms. The nature of cathode rays is found to be independent of nature of the cathode material and nature of the gas in the tube. 
Cathode rays contain negatively charged particles called electrons. In the presence of electric field they are deflected towards anode and in magnetic field towards south pole. 
These rays travel in straight lines in the absence of electric and magnetic field. These rays are invisible but their behaviour can be observed with the help of fluorescent or phosphorescent materials, which glow when hit by them. The cathode rays move from cathode to anode. Some of the characteristics of cathode rays are:. These rays are known as cathode and anode rays. When high voltage is applied across the electronic current start flowing through stream of parts moving in the tube from cathode ( – ) to anode (+). In the experiment the electrical discharge tube is filled with gas at low pressure & high voltage. DISCOVERY OF ELECTRON: The Electron was discovered by J.J THOMSON by conducting a Cathode ray tube experiment. The particles are Electron, Proton and Neutron. later it was concluded that Atom is made up of 3 sub – atomic particles. The first atomic theory, proposed by John Dalton in 1808, regarded atom as the ultimate indivisible particle of matter. Atoms are the building blocks of elements.
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