Rutherford Atomic Model:
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 alpha scattering experiment that :
- Most of 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).
On the bases of these alpha scattering experiment observations Rutherford made following conclusions.
- As majority of α-rays passed un-deflected so most of 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 come in front of some α-rays.
This heavy and positive portion is given the name “Nucleus” by Rutherford.
- All the mass of atom concentrates in nucleus.
- Atomic system resembles solar system in which nucleus is like sun and electrons are like planets. Electrons are revolving around nucleus as planets around sun.
- Number of protons and electrons are equal so overall atom is neutral.
Defects of Rutherford Model:
Following defects were pointed in Rutherford atomic model
- According to Rutherford, atom resembles solar system so laws of gravitation should be applicable to it but these laws do not apply to atom as it consists of charged particles.
- According to Maxwell classical electromagnetic theory, if a charge particle revolves around oppositely charge particle it radiates continuous energy. And if it is so then electron should fall in nucleus following spiral path and atom should collapse. But it does not happen in atom.
- If electrons emit continuous energy then atomic spectrum should be continuous but it is line spectrum.
- There were no defined orbits for electrons.