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CBSE - IX
Atomic Models
Atomic Models
Lesson Demo
Matter is made of atoms and all atoms consist of charged particles.
According to Dalton’s atomic theory an atom was indivisible and indestructible.
Gold stein’s work on the canal rays led to the discovery of protons. The mass of a proton was about 2000 times that of an electron and it carried a positive charge.
J. J.Thomson in his experiments with cathode ray tube found that the particles were attracted to the positive terminal of the tube. Thomson concluded that the particles must be negatively charged and called these ‘electrons’. An electron has a negligible mass and has a charge of minus one.
Thomson model was also called water melon model or Christmas pudding model.
Thomson’s model.
- An atom is a positively charged sphere. Electrons are set within the sphere.
- An atom is electrically neutral.
Rutherford’s Experiment
- Most alpha particles passed through the gold foil
- Some particles were deflected slightly
- A few particles appeared to rebound
Rutherford’s Model
- Positively charged centre in an atom is called a nucleus.
- Electrons revolve around the nucleus in circular orbits.
- A nucleus is much smaller than the atom.
Drawbacks of Rutherford’s Model
- Electrons move in their orbits
Atoms are unstable
Bohr’s Model
- Electrons revolve in discrete orbits called shells.
- Electrons do not radiate energy. Within an orbit, the energy of an electron is constant.
All the three scientists Thomson, Rutherford and Bohr received Nobel prizes for their theories.