Atomic Models

Thompsons model of an Atom

JJ Thompsons proposed that an atom possesses a spherical shape (radius approximately 10–10 m) in which the positive charge is uniformly distributed. The electrons are embedded into it in such a manner as to give the most stable electrostatic arrangement

It was also named as Plum pudding. An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over the atom.

It was not consistent with the experimental results conducted later on, but was able to explain the net neutrality of an atom.

Rutherford’s Nuclear Model of Atom

Rutherford conducted an experiment famously called as Rutherfords alfa particles scattering experiment. He used alpha particles and 

When a beam of alpha particles ( positively charged ) is passed through a thin gold leaf, most of the particles get passed without any deflection in straight direction. But some particles get deflected in other directions. 

  • Maximum particles passed through the foil in straight line without deflection
  • Some particles deflected at small angles
  • Very few particles get deflected at 180 

Thus Observations made from the following experiment were:

  • Most of the space in the atom is empty as most of the α–particles passed through the foil undeflected.
  • A few positively charged α– particles were deflected. The deflection must be due to enormous repulsive force showing that the positive charge of the atom is not spread throughout the atom as Thomson had presumed. The positive charge has to be concentrated in a very small volume that repelled and deflected the positively charged α– particles
  • Calculations by Rutherford showed that the volume occupied by the nucleus is negligibly small as compared to the total volume of the atom. The radius of the atom is about 10–10 m, while that of nucleus is 10–15 m. One can appreciate this difference in size by realising that if a cricket ball represents a nucleus, then the radius of atom would be about 5 km.

Thus based on the above experimental observations, Rutherford concluded the the following model of an atom:

  • The positive charge and most of the mass of the atom was densely concentrated in extremely small region. This very small portion of the atom was called nucleus by Rutherford
  • The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular paths called orbits. Thus, Rutherford’s model of atom resembles the solar system in which the nucleus plays the role of sun and the electrons that of revolving planets
  • Electrons and the nucleus are held together by electrostatic forces of attraction.

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