ATOMIC MODELS

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.

Thomson’s Model:

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.

1779_Thomson’s Model.JPG

This model could not satisfactorily explain the results of scattering experiment carried out by Rutherford who worked with Thomson.

Rutherford’s Model:

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].

1311_Rutherford’s Model.JPG

Observation:

  • 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)              

 Conclusions:

  •  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. 

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