In this chapter, we will understand the groundbreaking discovery of electron, which is the first fundamental subatomic particle. In 1803, an English Chemist, Dalton proposed the first scientific atomic theory, known as Dalton’s atomic theory according to which all matter is made up of smallest indivisible and indestructible particle which he named Atoms.
However, this concept did not hold long and it was proved by the experiment conducted by brilliant research scientists like J.J. Thomson (1897), Ernest Rutherford (1911), Niels Bohr (1912), James Chadwick (1932) and others that atom consists of several particles, called subatomic particles like electron, proton, neutron, positron, neutrino, meson, etc. Out of these, electrons, protons, and neutrons are known as fundamental subatomic particles and are the building blocks of the atoms..
Let’s discuss the discovery of first fundamental subatomic particle, electron and its different properties.
Table of Contents
Discovery of Electron – Cathode Rays Discharge Tube Experiment
Production of Cathode Rays: The electron was discovered by experiments on conduction of electricity through gases. A German physicist and mathematician, Julius Plucker, in 1859 initiated the study of conduction of electricity through gases at low pressure (10-4 atm) in a discharge tube.
A common discharge tube consists of a hard glass cylindrical tube (approximately 50 cm long) with two metal electrodes fitted at the opposite ends. The positive electrode is called anode, which is connected to the positive (+ve) terminal of the battery. The negative electrode is called cathode, which is connected to the negative (-ve) terminal of the battery.
This discharge tube was connected through a vacuum pump which a maintained a very low pressure around 10-4 atm. Air was almost completely removed from the discharge tube and a very low pressure around 10-4 atm was maintained.
When a high voltage of the order of 10,000 volts or more, is applied across the electrodes in the discharge tube at a very low pressure, some invisible rays are emitted from the negative electrode (cathode) to the positive electrode (anode) as shown in the below figure. Since the negative electrode is named to as cathode therefore, these rays were called cathode rays because they originate at the cathode.
Properties of Cathode Rays
In 1897, J. J. Thomson extended these investigations to study the nature of cathode rays more deeply, leading to the discovery of electron. By conducting several experiments and carefully observing the deflection of electric and magnetic fields, he concluded that cathode rays consist of a stream of fast moving negatively charged particles. These particles were later named electrons. Cathode rays exhibit the following properties. They are as:
(1) The cathode rays travel in straight lines originating from the negative electrode (cathode) at very high speed ranging from 109 – 1011 cm per second. So, they cast a shadow on the wall opposite to the cathode when a metallic object is placed in their path, showing that they travel in a straight line.
(2) Cathode rays produce mechanical effects. If a small mice paddle wheel is placed between the electrodes, it rotates. This indicates that the cathode rays consist of material particles, which have mass and velocity.
(3) An electric field deflects cathode rays towards the positive plate. When cathode rays are passed in the presence of an electric field (i.e. when the rays are passed between two electrically charged plates), they get deflected towards the positively charged plate. They indicate that the particles making up cathode rays are negatively charged. These negatively charged particles have been named electrons.
(4) Cathode rays also get deflected towards the N pole when placed in the presence of the magnetic field. In 1895, J.B. Perrin demonstrated that bringing the cathode rays near a magnet caused them to deflect towards which the negative charges would be deflected. This experiment also shows that cathode rays are composed of negatively charged particles.
(5) These rays produce heat energy when they hit with an object. This shows that cathode rays have the kinetic energy, which gets converted into heat energy when stopped by an object. Thus, the cathode rays possess heating effect.
(6) Cathode rays produce X-rays when they strike against hard materials such as tungsten, copper, etc.
(7) When cathode rays strike the glass wall of the discharge tube beyond the anode, they produce a green glow. This glow is due to the interaction of the cathode rays (which are streams of electrons) with the materials in the glass, causing them to fluoresce.
(8) When cathode rays hit a screen coated with zinc sulphide placed inside the discharge tube, they cause the screen to emit light. This is because the zinc sulphide fluoresces when it is struck by the high-energy electrons of the cathode rays.
(9) Cathode rays ionize the gas through which they pass and make it to conduct the electricity. Normally, gases are the poor conductor of electricity at high pressure. But gases can conduct the electricity at very low pressure and high voltage because at very low pressure and high voltage, gases are ionized (i.e. changed into ions) and hence conduct electricity. For example, when hydrogen gas is filled in the discharge tube, the gas ionizes into H → H + e-.
(10) Cathode rays can penetrate through the thin sheets of aluminum and other metals.
(11) These rays affect the photographic plates.
(12) The nature of the cathode rays is independent of the nature of the cathode and the nature of gas present in the discharge tube. In other words, the properties of cathode rays do not depend upon the material of the electrodes and the nature of the gas present in the discharge tube.
(13) The ratio of charge to mass (e/m) for the particles in cathode rays was found to be the same as that for an electron (approx. 1.76 * 108 coulomb per gram). This observation demonstrates that the cathode rays consist of negatively charged particles with an e/m ratio equal to that of an electron.
Thus, cathode rays are a stream of electrons and were discovered as the first subatomic particles by J. J. Thomson. He played a crucial role in the discovery of electron through his pioneering experiments on cathode rays.
Determination of Charge to Mass (e / m) Ratio of an Electron
The determination of the charge to mass (e/m) ratio of an electron is a fundamental experiment that explained the properties of an electron. In 1897, J. J. Thomson first conducted this experiment, marking a pivotal moment in the discovery of electron and the development of atomic physics.
He determined the charge-to-mass ratio (e/m) of an electron by studying the deflections of cathode rays in both electric and magnetic fields. The value of e / m has been found to be -1.7588 x 108 coulomb/g. The path of an electron in an electric field is parabolic and is given by the following formula:
y = (eE / 2mv2) * x2
Here,
- y = deflection in the path of electron in y-direction
- e = charge on electron
- E = intensity of applied electric field
- m = mass of electron
- v = velocity of electron
- x = distance between two parallel electric plates within which electron is moving.
The path of an electron in a magnetic field is circular with radius “r” and is given by the following formula:
r = mv / eB
where,
- m= mass of electron
- v = velocity of electron
- e = charge on electron
- B = intensity of applied magnetic field
By performing a series of several experiments, Thomson proved that whatever be the gas is taken in the discharge tube and whatever be the material of the electrodes, the value of e / m is always the identical. Thus, electrons are common universal constituents of all atoms. J. J. Thomson provided the following formula to calculate the charge / mass ratio:
e/m = E / rB2
The numerical value of e/m obtained by Thomson is 1.758803 x 1011 C kg-1.
Determination of Charge on Electron
In 1909, an American physicist Robert A. Millikan measured the electric charge on an electron by his famous experiment known as oil drop experiment. The charge on an electron was found to be -1.602 x 10-19 coulombs.