Rutherford Atomic Model And Its Limitations

What is atom ?

Matter is made up of tiny particles called atoms . Atoms are indivisible particles which participate in chemical reactions also “they can neither be created nor destroyed in a chemical reaction”-(Law of conservation of mass) . They combine in small whole numbers to form compounds . Atoms of an element are identical in nature and chemical properties . The relative number and kinds of atoms are constant in a given compound .

RUTHERFORD’S EXPERIMENT :

Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. Rutherford designed an experiment for this. In this experiment, fast moving alpha (α)-particles were made to fall on a thin gold foil.

• He selected a gold foil because he wanted as thin a layer as possible. 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:

(i) Most of the fast moving α-particles passed straight through the gold foil . (ii) Some of the α-particles were deflected by the foil by small angles , Surprisingly one out of every 12000 particles appeared to rebound .

Rutherford concluded from the α-particle scattering experiment that–

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

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

(iii) A very small fraction of α-particles were deflected by 1800,indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

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

(i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.

(ii) The electrons revolve around the nucleus in circular paths.

(iii) The size of the nucleus is very small as compared to the size of the atom.

Drawbacks of Rutherford’s model of the atom :

The revolution of the electron in a circular orbit is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. We know that atoms are quite stable.

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Kind Regards,

Aryan Singh