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Q.1 : What are cathode rays, and why were they significant in the discovery of the electron?
Ans: Cathode rays are streams of particles emitted from cathode terminals in vacuum tubes. They were significant because their properties led scientists like J.J. Thomson to propose the existence of the electron, challenging the prevailing atomic theory.
Q. 2 : How did J.J. Thomson contribute to the discovery of the electron?
Ans. J.J. Thomson conducted experiments with cathode ray tubes equipped with magnetic and electric fields. By measuring the deflection of cathode rays under these fields, he determined the charge-to-mass ratio (e/m) of electrons, providing evidence for their existence.
Q.3 : What was the significance of Thomson's experiments with cathode rays?
Ans. Thomson's experiments demonstrated that cathode rays were composed of negatively charged particles with a much smaller mass than that of atoms, leading to the proposal of the electron as a fundamental constituent of matter.
Q.4 : What was the purpose of Robert Millikan's oil drop experiment?
Ans. Robert Millikan's oil drop experiment aimed to measure the charge of a single electron accurately. By suspending tiny oil droplets in a chamber and observing their motion under an electric field, Millikan determined the precise value of the electron's charge.
Q.5 : How did Millikan's oil drop experiment contribute to our understanding of the electron?
Ans. Millikan's experiment provided the first accurate measurement of the electron's charge, helping to confirm its existence and paving the way for further research into its properties and behaviour.
Discovery Of Electron In Chemistry, Cathode Ray Tube, Mass Of Electron, Important Topics JEE 2025
Discovery Of Electron In Chemistry : The discovery of the electron in chemistry revolutionized our understanding of atomic structure, leading to the development of quantum theory and modern physics.
Shrivastav 13 May, 2024
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Discovery Of Electron In Chemistry :
John Dalton 1808, believed that matter is made up of extremely minute indivisible particles, called atom which can take part in chemical reactions. These can neither be created nor be destroyed. However, modern research has conclusively proved that atom is no longer an indivisible particle. The modern structure of atom is based on Rutherford’s scattering experiment on atoms and on the concepts of quantization of energy.
The works of J.J. Thomson and Ernst Rutherford actually laid the foundation of the modern picture of the atom. It is now believed that the atom consists of several sub-atomic particles like electron, proton, neutron, positron, neutrino, meson etc. Out of these particles, the electron, proton and the neutron are called fundamental subatomic particles and others are non-fundamental particles.
Discovery of Electron (Cathode Ray Tube Experiment)
Discovery of Electron (Cathode Ray Tube Experiment):
Cathode rays were discovered by William Crooke's & J.J. Thomson (1880) using a cylindrical hard glass tube fitted with two metallic electrodes. The tube has a side tube with a stop cock. This tube was known as a discharge tube. They passed electricity (10,000V) through a discharge tube at very low pressure (10
–2
to 10
–3
mmHg ). Blue rays were emerged from the cathode. These rays were termed as Cathode rays.
Properties of Cathode rays
Cathode rays travel in a straight line.
Cathode rays produce mechanical effect, as they can rotate the wheel placed in their path.
Cathode rays consist of negatively charged particles known as electron.
Cathode rays travel with high speed approaching that of light.
Cathode rays can cause fluorescence.
Cathode rays heat the object on which they fall due to transfer of kinetic energy to the object.
When cathode rays fall on solids such as Cu, X rays are produced.
Cathode rays possess ionizing power i.e., they ionize the gas through which they pass.
The cathode rays produce scintillation the photographic plates.
They can penetrate through thin metallic sheets.
The nature of these rays does not depend upon the nature of gas or the cathode material used in discharge tube.
The e/m (charge to mass ratio) for cathode rays was found to be the same as that for an electron ( – 1.7 × 108coloumb per gm). Thus, the cathode rays are a stream of electrons.
Important Points For Cathode rays
When the gas pressure in the discharge tube is 1 atmosphere no electric current flows through the tube. This is because the gases are poor conductor of electricity.
The television picture tube is a cathode ray tube in which a picture is produced due to fluorescence on the television screen coated with suitable material. Similarly, fluorescent light tubes are also cathode rays tubes coated inside with suitable materials which produce visible light on being hit with cathode rays.
Measurement of e/m for Electron:
Measurement of e/m for Electron :
J.J. Thomson determined the e/m value (charge/mass) of the electron by studying the deflection of cathode rays in electric & magnetic fields.
The value of e/m has been found to be
–1.7588 × 10
8
coulomb/g.
By performing a series of experiments, Thomson proved that whatever gas be taken in the discharge tube and whatever be the material of the electrodes the value of e/m is always the same.
Electrons are thus common universal constituents of all atoms.
Millikan's oil Drop Experiment (Determination of Charge of Electron)
Millikan's oil Drop Experiment :
The absolute value of the charge on an e
–
was measured by R.A. Millikan by the Millikan's oil drop experiment.
The apparatus used by him is shown in figure.
An oil droplet falls through a hole in the upper plate. The air between the plates is then exposed to X-rays which eject electrons from air molecules.
Some of these e
–
are captured by the oil droplet and it acquires a negative charge.
The metal plates were given an electric charge, and as the electric field between the plates was increased, it was possible to make some of the drops travel upwards at the same speed as they were previously falling.
By measuring the speed, and knowing things like the strength of the field and the density of the oil, radius of oil drops, Millikan was able to calculate the magnitude of the charge on the oil drops.
He found that the smallest charge to be found on them was approximately
1.59×10
–19
C
. This was recognized as the charge on an e
–
. The modern value is
1.602×10
–19
C
.
Mass Of The Electron
Mass of the Electron :
Mass of the e
–
can be calculate from the value of e/m and the value of electron.
or
This is termed as the mass of the electron i.e. mass of electron when moving with low speed.
The mass of a moving e
–
may be calculate by applying the following formula.
Mass of moving
Where v is the velocity of the e
–
and c is the velocity of light
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