Atomic Model of Rutherford
In 1911, Lord Rutherford made an experiment to construct an atomic model. He took a thin gold foil and bombarded it with α-particles. Gold foil was surrounded by photographic film.
He observed in the alpha scattering experiment that :
- Most α-rays passed through gold foil un-deflected.
- Only a few were deflected at an angle greater than 90o.
- Very few were bounced (i.e. turned back).
Rutherford Experiment Conclusions :
On the bases of these alpha scattering experiment observations, Rutherford made the following conclusions.
- As the majority of α-rays passed un-deflected so most of the atomic volume is empty space.
- As only a few α-rays deflected from their straight path so some positive portion is there in the atom which has repelled +ve α-rays. This portion is very small in size so occasionally it comes in front of some α-rays.
- As very few α-rays were bounced so some massive portion is there in the atom which has pushed back the α-rays. This portion is very small in size so occasionally it comes in front of some α-rays.
This heavy and positive portion is given the name “Nucleus” by Rutherford.
- All the mass of the atom concentrates in the nucleus.
- The atomic system resembles a solar system in which the nucleus is like the sun and electrons are like planets. Electrons are revolving around the nucleus as planets around the sun.
- The number of protons and electrons is equal so the overall atom is neutral.
Defects of Rutherford Model:
The following defects were pointed out in Rutherford’s atomic model
- According to Rutherford, the atom resembles a solar system so laws of gravitation should be applicable to it but these laws do not apply to the atom as it consists of charged particles.
- According to Maxwell’s classical electromagnetic theory, if a charged particle revolves around an opposite charge particle it radiates continuous energy. And if it is so then the electron should fall into the nucleus following a spiral path and the atom should collapse. But it does not happen in atoms.
- If electrons emit continuous energy then the atomic spectrum should be continuous but it is a line spectrum.
- There were no defined orbits for electrons.
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