Historical development of atomic models
- In olden days there was an idea that all matter is made of tiny indivisible particles.
- When Thomson discovered electron, this idea became value less.
- In 1898 Thomson proposed his model of atom
- According to this model positive charge is distributed uniformly over the entire body of the atom while the electrons are embedded in it.
- However this model failed as it could not explain the observed spectral lines emitted by an atom.
- Later in 1903 Lenard proposed his atomic model.
- According to him a lot of empty space exists in an atom. He could not explain the stability and mass of an atom.
- In 1911 Rutherford proposed his model of atom. According to him positively charged mass is situated at the centre and electrons are revolving around it.
- This model failed to explain the stability of an atom.
- In 1912, in order to account for the stability of atom, Neils Bohr, proposed a different model.
- According to this model
i ) Electrons in an atom revolve around the nucleus in circular orbits just as the planets revolve round the sun.
ii) The electrons revolve only in certain permitted orbits called ‘stationary orbits' and do not radiate energy while in such orbits.
iii) Whenever an electron jumps from one stationary orbit to another, absorption or emission of energy takes place. This theory could explain the spectral lines emitted by different elements.