Discovery of Cathode Rays and Electrons

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Historical Concept of an Atom:

  • 600 B.C. Indian saint and philosopher Maharshi Kanad proposed that matter is made up of smallest individual particles. He called these particles as ‘paramanu’.
  • Around 400 B.C. The Greek Philosopher Democritus suggested that all the matter is composed of tiny, discrete, indivisible particles. He called these particles ‘at0ms’.
  • Both the ideas are of conceptual nature and didn’t have any experimental evidence.

Assumptions of Dalton’s atomic theory:

  • Every element is made up of extremely small particles called an atom.
  • The atoms are indivisible and they can neither be created nor be destroyed. Atoms of the same element resemble each other in all respects but differ from the atoms of other elements.
  • When chemical compounds are formed they do so by the combination of atoms of different elements in a simple proportion of whole numbers.
  • Atoms of different elements may combine in more than one proportion to form different compounds.

Evidences of Electrical Nature of Matter:

  • When a glass is rubbed with silk or ebonite is rubbed with fur, electricity is generated. electricity gets transferred from one point to another point through certain substances. This phenomenon indicated that the matter has electrical nature.
  • Michael Faraday in 1832 passed electricity through the solution and he called the phenomenon as electrolysis. He observed that charged particles migrate towards oppositely charged electrodes. During this process, they accumulate on the electrode or escape out as a gas at the electrode. On the basis of his experiments, he proposed the laws of electrolysis.
  • These laws of electrolysis given by Michael Faraday provide a relation between matter and electricity. These laws assumed discrete nature of electricity. These discrete particles are called electrons by Loney.

Fundamental particles of an atom:

  • Protons, neutrons, and electrons that make up an atom are known as the fundamental subatomic particles.Protons and neutrons are present in the nucleus of an atom, they are called intranuclear particles or nucleons. Electrons revolve around the nucleus in a circular orbit. They are called extra-nuclear particles.

Discovery of  Electrons:

Discovery of Electrons 01

  • Electrons were the first of sub-atomic particles to be discovered, by J.J. Thomson in 1859.
  • J.J. Thomson made a detailed study of the discharge of electricity through gases under very low pressure. This experiment was performed using cathode ray tube (Crooke’s tube).  It consists of a glass tube connected to two metal electrodes at two ends.  There is a side tube connected to a vacuum pump to reduce pressure.
  • When a gas is subjected to a high potential (5000 to 10000 V) at low pressure, the glass wall of tube glows with fluorescent light.  This glow is because bombardment of glass by rays emitted from the cathode.  As the rays arise from the cathode, they are called cathode rays. These rays were found to consist of negatively charged particles with a negligible but definite mass.
  • Stoney suggested name ‘electron’ for the particles of constituting these rays.
  • Thomson also showed that the electrons are present in atoms of all elements. Thus electrons are the fundamental particles of atom.

Characteristics of Cathode Rays:

  • The cathode rays are emitted normally from the surface of the cathode irrespective of the position of the anode.
  • The cathode rays travel in straight lines and cast sharp shadows of objects in their paths.
  • They are able to produce fluorescence and phosphorescence in many substances when they fall on them.  The colour of fluorescence depends upon the nature of substance e.g. Willemite emits green colour. Alumina emits a red colour.
  • The cathode rays produce a blackening on a photographic plate when they are incident on it.
  • The cathode rays consist of negatively charged particles.
  • Cathode rays generate heat when they strike a target
  • The cathode rays are deflected by electric and magnetic fields and the direction of deflection shows that they are negatively charged particles.
  • Cathode rays can ionize the gases through which they pass.
  • Cathode rays produce X – rays when stopped by a target

Charge to Mass Ratio of Electron:

  • J. J. Thomson measured the ratio of electrical charge (e) to mass (m) of cathode ray particles using specially designed cathode ray tube.

Discovery of Electrons 02

  • Construction :

    • The apparatus consists of discharge tube containing gas at a very low pressure about 0.01 mm of mercury. The discharge tube has cathode C at one end and fluorescent screen S at the other. Anode A consists of a cylinder with a fine bore.
    • The electric field can be applied between the plates P1 and P2. Plate P1 is positive and plate P2 is negative
    • Magnetic field can be applied perpendicular to the electric field and perpendicular to the plane of the diagram and into the plane.
  • Working :

    • Cathode emits electrons and they are collimated by cylindrical fine bore anode. The velocity of electrons depends on the potential difference between the cathode and the anode.
    • When no field is applied. The electrons move in a straight line and forms spot at O at the centre of the screen.
    • When cathode rays are passed through an electric field created by applying a potential across the plates P1 and P2 only. it is found that the cathode rays particles get deflected towards the positive plate.
    • When cathode rays are passed through the magnetic field created by applying strong magnetic field only, it is found that the cathode rays particles get deflected in a circular path.
    • Thomson applied both the fields simultaneously and adjusted its value such that the spot remains at the centre of screen at O. Knowing values of V, B and d and using following formula the e/m ratio can be determined.

Discovery of Electrons 03Where E = V/d. The value of e/m is 1.758820 x 1011 C kg-16.

  • Thomson repeated the experiment for different materials of the cathode and found that the e/m ratio is always the same. From this he concluded that the particles present in cathode rays (electrons) are fundamental particles of any atom of all matter.

The charge on an Electron:

  • Scientist R. A. Millikan in his oil drop experiment determined the charge on the electron and he found that the charge on an electron is 1.6022 x 10-19 C.

Mass of an Electron:

  • Using e/m ratio and charge on the electron, the mass of an electron is found to be  9.1094 x 10-19 kg.

Characteristics of Electrons:

  • Electrons are negatively charged.
  • They are revolving in circular orbits around the nucleus.
  • They have mass 0f 0.00055 a.m.u.  (9.11 ×  10 -31 Kg.). This mass of electron is negligible compared to other particles. Hence it is taken as zero.
  • Its mass is 1/1850 times that of the proton.
  • Electrons carry negative charge of  1.6 ×  10 -19 C. This charge carried by the electron is considered to be a unit negative charge.
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