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Rutherford's Model of Atom


In our previous blog we talked about Thomson's Model of Atom. From the blog we also know that, his model was the first atomic model ever proposed. But we all know that this atomic model was unable to explain lots of results of experiments carried out by other scientists. Today we are going to talk about one of those experiments and its results.

In 1911, Ernest Rutherford overturned Thomson's model with his well-known "Gold Foil Experiment". In this experiment, he made fall fast moving alpha (α)-particles on a thin gold foil. He selected a gold foil because he wanted as thin a layer as possible in this experiment. This gold foil was about 1000 atoms thick. α-particles are doubly-charged helium ions. Since they have a mass of 4 u, the fast-moving α-particles have a considerable amount of energy. It was expected that α-particles would be deflected by the sub-atomic particles in the gold atoms. Since the α-particles were much heavier than the protons, he did not expect to see large deflections. But, the α-particle scattering experiment gave totally unexpected results.

The following observations were made:

  • Most of the fast moving α-particles passed straight through the gold foil.
  • Some of the α-particles were deflected by the foil by small angles.
  • Surprisingly one out of every 12000 particles appeared to rebound.
  • In the words of Rutherford, “This result was almost as incredible as if you fire a 15-inch shell at a piece of tissue paper and it comes back and hits you”.

 On the basis of experiment Rutherford concluded the:

1.    Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.

2.    Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

3.    A very small fraction of α-particles were deflected by 180 degree, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

4.    From the data he also calculated that the radius of the nucleus is about 105 times less than the radius of the atom.

 

On the basis of his experiment, Rutherford put forward the nuclear model of an atom, which had the following features:

  • There is a positively charged center in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
  • The electrons revolve around the nucleus in circular paths.
  • The size of the nucleus is very small as compared to the size of the atom.

 Why Thomson's model of atom failed at the first place.
As we know, Thomson proposed that, atom consist of positively charged sphere. In Rutherford experiment, α-particles were used to made incident on gold foil. α-particles are positively charged ions, they should be deflected by the positive sphere of the protons. How every, α-particles were massive and possessing a considerable amount of energy, they had to be deflection even it’s a small deflection. But Rutherford observed that, most of α-particles passed without deflection, showing that protons are not the sphere of the atom. But they are concentrated inside a nucleus.

However, Rutherford's model explained the position of protons and proposed a more specific atomic model, it failed to explain the stability of the atom. Can you propose something to make the atom stable. What do you think. Let us know in the comment box. We will explain this in our upcoming blogs. Until then try to figure it out on your own.


Fore more details refer to Beyond Reaction: Fundamental Chemistry




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