Rutherford’s Model

 Rutherford’s Model

Rutherford conducted his famous alpha-scattering experiment which was the first definitive experiment to establish the basic structure of atom, specially the discovery of nucleus containing protons and neutrons. To check the conclusions of Thomson’s model Rutherford bombarded a thin sheet of Gold by fast moving alpha particles coming from a radioactive source and observed their deviations after passing through the foil. α-particles are positively charged helium nuclei with atomic mass 4 a.m.u.
Following observations were made:
1. Most of the particles pass through the gold foil without any deviation.
2. Very few particles went through a deviation by small angle.
3. Even fewer particles (1/20000) suffered a deflection of 900 or more and even complete rebound.
Following conclusions were made on the basis of the above observations:
1. Most of the space inside an atom is empty.
2. As some particles deviated and alpha particles being positively charged helium nuclei, can be deviated by a positive charge, so there must be some positively charged region in the atom.
3. As only a few particles deviated, it was concluded that the positively charged region was very small as compared to the atom that is the positive charge of the atom is concentrated in a very small space called “The Nucleus”. The order of diameter of a nucleus is 10-15 m and that of an atom is about 10-10m.
Thus nucleus was discovered which contained protons and neutrons and it was proposed that the electrons revolved around the nucleus in circular orbits. This conclusion regarding electrons was not supported with enough evidence which eventually led to failure of this model. The biggest achievement of the model was the discovery of the nucleus.
The alpha particle scattering experiment proved that the neutrons are located inside the nucleus because had they been present outside, the alpha rays colliding with them must have suffered a deviation as their masses were comparable to those of the alpha particles.
The alpha particles on collision with electrons do not suffer much deviation from their straight line path as their momentum is negligible compared to that of the electrons.
Rutherford’s model is similar to our solar system where the nucleus is like the sun and the electrons are like planets. That is why these electrons are called planetary electrons

Failure of Rutherford’s Model

According to classical theory of electromagnetic waves, (Maxwell’s theory) when a charge is subjected to a continuous motion around an opposite charge it radiates energy continuously in form of radiations and thus will lose energy. That’s why the electron in Rutherford’s Model should lose energy and follow a spiral path and collapse into the nucleus in 10–8s. Although this does not happen but Rutherford had no explanations for the same.


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