Structure of Atom

04 Rutherford’s Experiment – Discovery of Nucleus

  • In Rutherfords experiment, fast moving alpha (α)  particles were made to fall on a thin gold foil.
  • He selected a gold foil because he wanted as thin a layer as possible which was about atoms thick. The -particles are doubly-charged helium ions.

Observation:

  • Most of the fast moving α-particles passed straight through the gold foil.
  • Some of the  (α) particles were deflected by the foil by small angles.
  • Surprisingly one out of every 12000 particles appeared to rebound.
rutherfordObser

Conclusions:

  • Rutherford’s alpha-particle scattering experiment shows the presence of a nucleus in the atom.
    • Since most of the alpha particles pass straight through the gold foil without any deflection, it shows that there is a lot of empty space in the atom.
    • Observation that a very few alpha particles completely rebound on hitting the gold foil shows that the nucleus is very dense and hard which does not allow the alpha particles to pass through it and occupies very less space.
    • A very small fraction of (α) particles were deflected by  , indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.
  • From these conclusions, he calculated that the radius of the nucleus is around 105 times less than that of the atom.
  • So according to Rutherford, features of nucleus include:
    • Nucleus of an atom is positively charged.
    • Nucleus of an atom is very dense and hard.
    • Nucleus of an atom is very small as compared to the size of the atom as a whole.

Rutherford’s Model of the Atom:

  • There is a positively charged, dense centre in an atom. The protons and neutrons reside in the nucleus. Protons have positive charge whereas neutrons have no charge. Almost the entire mass of an atom is concentrated in the nucleus.
  • The negatively charged electrons surround the nucleus along the circular paths .The electrons are revolving round the nucleus in circular paths at very high speeds. The circular paths of the electrons are called orbits.
  • The number of protons and electrons in an atom is equal. Hence, an atom is electrically neutral. The electrostatic attraction between the positively charged nucleus and negatively charged electrons holds the atom together.
  • Most of the atom is empty space.

Drawback of Rutherford’s Model of the Atom:

  • Rutherford’s model does not explain the stability of the atom.
  • The revolution of the electron in a circular orbit is not expected to be stable.
  • According to the electromagnetic theory of physics, if a charged particle undergoes accelerated motion, then it must radiate energy (or lose energy) continuously.
  • This means that the negatively charged electrons revolving around the nucleus with accelerated motion will lose their energy continuously by radiation.
  • The electrons will then be attracted more strongly by the oppositely charged nucleus.
  • And ultimately the electrons should fall into the nucleus by taking a spiral path.

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