Structure of Atom

• 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 11000^th 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:
  $\mathrm{The} \mathrm{relative} \mathrm{mass} \mathrm{of} \mathrm{an} \mathrm{electron} \mathrm{is} \frac{1}{1840} \mathrm{u}$

   

• Charge of an Electron:
  $\mathrm{The} \mathrm{absolute} \mathrm{charge} \mathrm{on} \mathrm{an} \mathrm{electron} \mathrm{is} –1.6 \times {10}^{–19 }\mathrm{coulomb} \mathrm{of} \mathrm{negative} \mathrm{charge}.$

   

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.
  $\mathrm{H} (\mathrm{Hydrogen} \mathrm{atom}) \to {\mathrm{H}}^{+} \left(\mathrm{Proton}\right) + {\mathrm{e}}^{–} \left(\mathrm{Electron}\right)$
• $\mathrm{Thus}, \mathrm{proton} \mathrm{is} \mathrm{a} \mathrm{hydrogen} \mathrm{ion} ( {\mathrm{H}}^{+})$
• 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
  $1.6 \times {10}^{–19}$

  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.