Alpha particle scattering and Rutherford's nuclear model of atom



In 1911, Rutherford, H.Geiger and Marsden performed a historical alpha particle scattering experiment which provided remarkable information regarding the arrangement of constituent particles in atom and lead to the birth of new nuclear model of atom.


J.J Thomson plum-pudding model of atom

In 1897 first nuclear model of atom was proposed by J.J Thomson, famously known as plum-pudding or water mellon model. In which he prescribed that atom is  postively charge and in which electrons are embedded. He gave the example of watermelon for the embedding of electron in the atom. He also states that due to the equal number of positive and negative charges make the atom electrically neutral.



But he failed to explain the subsequent experiments done on atom and conclusions derived from them.

Hence, the model could not stand for a longer time and was soon rejected and it leads to the bearth of  Alpha scattering and Rutherford's nuclear model.



Alpha particles scattering experiment

They took a thin gold foil having a thickness of 2.1×10^-7 m and placed it in the centre of the rotatable detecter made up of zinc sulphide and microscope. Then they direct a beam of 5.5MeV alpha particles emitted from the radioactive particle to the foil. Lead bricks collimated these alpha particles as they passed through them.

After hitting the gold foil these alpha particles studied by the brief flashes on the screen. Rutherford and his team expected to learn more about the structure of atom from the result of this experiment.





Observations

Here is what they found from this alpha particle scattering experiment :-

  • Most of the alpha particles (about 90%) passed through the gold foil undetected.

  • Some alpha particles got deflected through small angels.

  • A few alpha particles got deflected through very large angles. About 1 in 20,000 suffered a deflection of even 180° and came straight back.

These observations leads to many arguments and conclusions and it leads to the formation of new nuclear model of atom.




Conclusions of alpha particle scattering experiment :-


  • Most part of the atom is hollow. The empty space in the atom allows the clean passage of those alpha particles which posses though it and remain undeflected. Since, the number of such particles is very large, most part of the atom should be empty.
  • The positive charge is connecentrated somewhere in the atom. The alpha particles are postive charged. When they pass in the vicinity of this concentrated positive charge of atom, they get repelled and suffer deflection.
  • There is something very heavy in the atom. The alpha particles posses considerable mass. The returning back of some alpha particles indicates that they have collieded with some thing which is quite heavy. Moreover, since the number of such alpha particles is quite smal, the space occupied by the heavy part of the atom must be very small.


Rutherford nuclear model

On the basis of the conclusions derived from alpha particles scattering experiment (discourse above), Rutherford purposed a model of atom. This model is known as Rutherford Nuclear Model Of Atom. The silent features of this model are as follows :-


  • Almost all the mass and entire positive charge of an atom is concentrated into its centre called nucleus.

  • The size of the nucleus is extremely small as compared to the size of atom.
  • The nuclei of the atoms of different elements posses different nuclear charge.
  • Electrons move at very high speeds around the nucleus in orbits or closed circular paths.


Rutherford's nuclear model is comaperd with solar system. In a solar system, various planets revolve around the sun in orbits in almost the same way as the electrons revolve in orbits around the nucleus. Due to this anology, electrons of an atoms are also referred to as planetary electrons.

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