Subatomic Particles

In the physical sciences, subatomic particles are particles much smaller than atoms. The two types of subatomic particles are: elementary particles, which according to current theories are not made of other particles and composite particles

There are three subatomic particles in an atom:

  • Electrons
  • Protons
  • Neutrons

A. Electrons

1. Discovery of Electrons - Cathode ray tube experiment

A cathode ray tube is made of glass containing two thin pieces of metal, called electrodes, sealed in it

Electron, Gun, Emitter, Cathode, Ray, Tube, Crt

At very low pressures and at extremely high voltages, we can observe the electrical discharge through the gases.   

When a high voltage is applied across, current starts flowing from cathode to anode terminal through a stream of particles present.

However when we make a small hole in the anode part and cover the background area with a florescent material, we start observing bright spots on it. 

Observations from the above CRT experimenter were:

  • The cathode rays start from cathode and move towards the anode.
  •  These rays themselves are not visible but their behavior can be observed with the help of certain kind of materials (fluorescent or phosphorescent) which glow when hit by them. 
  • In the absence of electrical or magnetic field, these rays travel in straight lines
  • In the presence of electrical or magnetic field, the behavior of cathode rays are similar to that expected from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles, called electrons
  • The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube

  • Thus from this experiment the presence of negatively charged ELECTRONS was made, a basic constituent of all the atoms

    2. Charge to Mass Ratio of Electron

    British physicist J.J. Thomson conducted an experiment to calculate the ratio of electrical charge (e) to the mass of electron (me ). He used an apparatus of CRT and used electrical and magnetic fields (perpendicular to each other) to deflect the electrons.     

    Observations from the experiment that JJ Thompsons made:

    • The magnitude of the negative charge on the particle, greater the magnitude of the charge on the particle, greater is the interaction with the electric or magnetic field and thus greater is the deflection
    • With respect to the mass of the particle - lighter the particle, greater the deflection
    • The strength of the electrical or magnetic field — the deflection of electrons from its original path increases with the increase in the voltage across the electrodes, or the strength of the magnetic field.

    When he applied only electrical field to the rays, the deflected rays hit point A. When only magnetic field was applied, rays hot point C.

    When both feilds were applied in balance, the resulting rays hit point B in center

    Thus through the experimentation, he concluded that 

    where e = charge of an electron = 

    m(e)= mass of an electron = 

    Online Instructor

    Kunal Kapoor

    Related Tutorials