Structure of Atom

02 Discovery of Electron, Proton and Neutron

  • Electrons were discovered by J.J. Thomson in 1897.
  • J. Thomson’s experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons.
  • He performed an experiment on the cathode rays and showed that these are charged particles.
  • The velocity of cathode rays was much less as compared to the speed of light.
  • So, to measure the charge to mass ratio of cathode rays, he devised new ways.
  • Thompson therefore concluded that the cathode rays are 11000th mass of hydrogen ion (which is a proton). The negatively charged cathode ray particles came to be known as electrons which have negligible mass.

Characteristics of an Electron

  • Mass of an Electron:
    The relative mass of an electron is 11840 u

     

  • Charge of an Electron:
    The absolute charge on an electron is 1.6 × 1019 coulomb of negative charge.

     

DISCOVERY OF PROTON

 

  • The existence of proton in the atoms was shown by E. Goldstein in 1886.
  • When Goldstein passed electricity at high voltage through a gas at very low pressure taken in a discharge tube, streams of heavy particles were given out by the anode (positive electrode).
  • These streams of particles are called anode rays.
  • The anode rays obtained from hydrogen consist of protons.
  • A proton can be produced when we remove an electron from the hydrogen atom.
    H (Hydrogen atom)  H+ (Proton) + e (Electron)
  •  Thus, proton is a hydrogen ion ( H+)
  • He also found that the charge to mass ratio of the positive particles depend on the nature of the gas present in the discharge tube.
  • So, the charge to mass ratio (e/m) was different for different gases.
  • It was highest in case of the hydrogen gas mainly because hydrogen is the lightest atom meaning m will be the least hence e/m ratio will be highest in this case.
  • The proton is a positively charged particle found in the atoms of all the elements.
  • Protons reside inside neutron.

 

Characteristics of a Proton:

  • Mass of a Proton: The relative mass of a proton is 1 u.
  • Charge of a Proton: The charge of a proton is equal and opposite to the charge of an electron. The absolute charge of a proton is
    1.6 × 1019

    coulomb of positive charge.

DISCOVERY OF NEUTRON

 

  • The discovery of another subatomic particle by James Chadwick in 1932.
  • This particle is called neutron.
  • The neutron is a neutral particle found in the nucleus of an atom.

 

Characteristics of a neutron:

 

  • Mass of a Neutron: The relative mass of a neutron is 1 u.
  • Charge of a Neutron: Neutron has no charge.

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