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Atomic Models

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Atomic Models - Lesson Summary

Different models were proposed to explain the distribution of sub atomic particles in an atom. Of these models, the models proposed by J.J. Thomson and Ernest Rutherford are of great significance as they paved the way for modern structure of an atom.J.J. Thomson, in 1898, proposed the atomic model called Thomson's atomic model soon after the discovery of electrons.

According to this model, an atom is a sphere consisting of uniformly spread mass of positive charge with negatively-charged particles called electrons embedded in it. In this model, the negatively charged particles that are embedded in evenly spread positive charge this model is also called plum pudding, raisin pudding or watermelon model.


Although this model successfully explained the overall neutrality of the atom, it could not explain the results of the scattering experiment conducted by Ernest Rutherford.

In order to validate Thomson's atomic model in 1911, Rutherford conducted an experiment called the gold foil experiment or alpha particle scattering experiment.

In this experiment, a stream of high-energy alpha-particles from a radioactive source was bombarded on a very thin gold foil.


According to Thomson's model, all the alpha particles should pass through the gold foil almost undeflected, but the observations were quite contrary to these expectations.

The following observations made in this experiment:

  • Most of the alpha-particles passed through the gold foil undeflected.
  • A small fraction of the alpha-particles were deflected by small angles.
  • A few alpha particles were deflected by large angles.
  • A very little amount of alpha-particles, one in twenty thousand, were bounced back to the source that is nearly by 180°.


Based on these observations, Rutherford concluded that as most of the alpha-particles passed undeflected, most of the space inside the atom is empty.

Only a small number of deflected particles suggested that the positive charge of the atom is not spread throughout the atom but is concentrated at the centre in a very small volume.

Rutherford named the positively charged centre present in an atom as nucleus.

Based on the observations and conclusions of the gold foil experiment, Rutherford proposed a new atomic model called Rutherford's Nuclear Model of Atom. According to the Rutherford's Nuclear Model of Atom:

  • Atom consists of a tiny dense positively charged centre called nucleus.
  • Entire positive charge and most of the atomic mass of an atom is concentrated in the nucleus.
  • Electrons are found outside the nucleus. They revolve around the nucleus with high velocities in circular paths so as to counterbalance the electrostatic forces of attraction between the protons and electrons.
  • As electrons move in circular paths around the nucleus just like planets revolve around the sun in the solar system, Rutherford's atomic model is also called planetary model.

Limitations:
According to the principle of electrodynamics, a charged particle in circular motion emits energy continuously. Hence an electron revolving round the nucleus also loses energy. As a consequence of this, the electron follows the spiral path towards the nucleus and this ultimately results in collapse of an atom.

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