We know the fundamental particles of the atom. Now let us see, how these particles are arranged in an atom to suggest a model of the atom.
J.J. Thomson, in 1904, proposed that there was an equal and opposite positive charge enveloping the electrons in a matrix. This model is called the plum – pudding model after a type of Victorian dessert in which bits of plums were surrounded by matrix of pudding.
This model could not satisfactorily explain the results of scattering experiment carried out by Rutherford who worked with Thomson.
α– particles emitted by radioactive substance were shown to be dipositive Helium ions (He++) having a mass of 4 units and 2 units of positive charge.
Rutherford allowed a narrow beam of α–particles to fall on a very thin gold foil of thickness of the order of 0.0004 cm and determined the subsequent path of these particles with the help of a zinc sulphide fluorescent screen. The zinc sulphide screen gives off a visible flash of light when struck by an a particle, as ZnS has the remarkable property of converting kinetic energy of α particle into visible light. [For this experiment, Rutherford specifically used α particles because they are relatively heavy resulting in high momentum].
- Majority of the a–particles pass straight through the gold strip with little or no deflection.
- Some α–particles are deflected from their path and diverge.
- Very few α–particles are deflected backwards through angles greater than 90°.
- Some were even scattered in the opposite direction at an angle of 180°[Rutherford was very much surprised by it and remarked that “It was as incredible as if you fired a 15–inch shell at a piece of tissue paper and it came back and hit you”]. There is far less difference between air and bullet than there is between gold atoms and α-particle assuming of course that density of a gold atom is evenly distributed. The distance of nucleus from where the α - particle returns back through 180° is called distance of closet approach and is given by
ro = q1q2 / 4πεo ( 1/2mv2)
- The fact that most of the α - particles passed straight through the metal foil indicates the most part of the atom is empty.
- The fact that few α - particles are deflected at large angles indicates the presence of a heavy positively charge body i.e., for such large deflections to occur α - particles must have come closer to or collided with a massive positively charged body.
- The fact that one in 20,000 have deflected at 180° backwards indicates that volume occupied by this heavy positively charged body is very small in comparison to total volume of the atom.