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Thursday, February 6, 2014


Which Science Nerd has not heard the name of Sir Ernest Rutherford,the New-Zealand born physicist who is famous for the famous alpha particles scattering experiment or sometimes known as Rutherford's Experiment. So, first of all Let us follow a brief chronology. In 1886, E.Goldstein discovered the presence of new radiations in a gas discharge and called them canal rays.In 1900,Joseph John Thomson (J J Thomson) discovered electrons, or the fundamental particle present in the atoms having negative charge and a negligible mass - the mass being 1/2000th of that of the proton.(You can read the post about Thomson's Model of an atom here.In 1919,Rutherford did his experiment which had remarkably helped the world of Nuclear Physics. 

Sir Ernest Rutherford

The famous alpha particle scattering experiment

                                                   The Summarisation of the experiment

Rutherford selected a thin gold foil for the experiment.Why? This is because he wanted a layer as thin as possible. This gold foil was about 1000 atoms thick.
a-particles are doubly and positively charged Helium nucleus. As they have a mass of 4 u,fast moving a-particles have a considerable amount of energy. 
It was expected that a-particles would be deflected by the sub-atomic particles in the gold atoms. As the a-particles were much heavier than the protons,Rutherford did not expect to see much large deviations. 
But the experiment gave very unexpected results. In Rutherford's words - "This result was almost as incredible as if you fire a 15 inch shell at a tissue paper and it comes back and hits you."
  • Most of the fast moving a-particles went in straight through the gold foil.
  • Some of the a-particles were deflected by small angles.
  • One out of every 12000 particles appeared to rebound.

The conclusions of the experiment - 
  • Most of the space inside the atom is empty because most of the a-particles passed in straight through the gold foil without getting deflected.
  • Very few particles were deflected by small angles indicating that the positive charge of the atom occupies very little space.
  • A very small number of a-particles were deflected by 180 degree, indicating that the positive charge and mass of the gold atom occupy very little volume within the atom.
Rutherford's Nuclear Model of Atom - 
  • There is a positively charged centre in an atom called the nucleus. Nearly all the mass of the atom resides in the nucleus.
  • The electrons revolve around the nucleus in well-defined orbits.
  • The size of the nucleus is very small as compared to the size of the atom. The size of the radius of the atom is about 10^5 times than that of the radius of the nucleus.
Disclaimer - The pictures used in the post do not belong to me.Information is collected from various sources and are authentic.

Demerits of Rutherford's Nuclear Model of Atom (Or Rutherford's Model of Atom) -

  • If a body is in circular motion,it will go under acceleration. If a charged particle goes under acceleration,it will radiate energy.This will lead to almost every form of matter being unstable
  • But,we know that in reality,matter is quite stable.

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