Discovery of Neutrons


 
 
Concept Explanation
 

Discovery of Neutrons

Discovery of Neutron : The helium atom has 2 electrons and 2 protons. So, the mass of the helium atom was expected to be twice the mass of the hydrogen atom, which has 1 electron and 1 proton. But, actually the mass of the helium atom is four times that of the hydrogen atom. It was found that atoms of all the elements (except hydrogen) were at least twice as heavy as could be explained by the number of protons they had. To explain this it was predicted that other particles with no charge, but mass equal to that of a proton, must be present in all atoms except hydrogen. This prediction was proved to be correct with the discovery of the neutron.

In 1932, James Chadwick bombarded the element beryllium with alpha-particles. He observed the emission of a radiation with the following properties.

  • The radiation was highly penetrating.
  • The radiation remained unaffected in an electric or magnetic field, i.e., the radiation was neutral.
  • The particles constituting the radiation had the same mass as that of the proton.
  • Thus the relative mass of such a particle = 1 amu and the absolute mass =1.62times10^{-27}g. Because of their electrical neutrality, these particles were called neutrons.
  • It is now well-established that the neutron, like the electron and the proton, is a fundamental constituent of an atom. Neutrons are located in the nucleus along with the protons.
  • Sample Questions
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    Question : 1

    What is the number of neutrons present in a nitrogen atom?

    Right Option : C
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    Question : 2

    Which of the following elements bombarded with alpha particles by James Chadwick, during the discovery of neutron?

    Right Option : B
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    Question : 3

    What is the mass of neutron ?

    Right Option : A
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