Discovery of Electrons

The basic idea of the discovery of the elementary particles was generated by Dalton’s Atomic Theory. John Dalton in 1808 gave the first scientific theory about atoms, in which, he stated that atoms are the smallest particle of any matter. They are indivisible and indestructible. According to Dalton Atomic Theory:

• Matter consists of tiny indivisible particles called Atoms.
• Atoms can neither be created nor be destroyed.
• Atoms of the same element are alike in all respect but they differ from the atoms of the different elements.
• Atoms of an element combine in a simple whole-number ratio to form molecules.

The above statements are called the Postulates of Dalton’s Atomic Theory. But, later these postulates were proved wrong as the first indication of sub-atomic particles came due to the study of static electricity by Faraday which says that the flow of electricity is caused by due to the charged particles. 

Sir J. J. Thomson first proved the existence of negatively charged particles within an atom called electrons. Thus, the electron is the first sub-atomic particle to be discovered and this paved the way for the discovery of all other sub-atomic particles (i.e. proton and neutron) and also the actual structure of the atom.

In this article, we will discuss the discovery of electrons using the Cathode ray tube by William Crooke and later the actual discovery of electrons by J.J. Thomson.

J.J. Thomson and The Discovery of Electron

J.J. Thomson, a scientist, began working with cathode ray tubes in the early 1900s. Cathode ray tubes are vacuum-sealed glass tubes that have had most of the air removed. At one end of the tube, a high voltage is placed between two electrodes, causing a stream of particles to flow from the cathode (the negatively charged electrode) to the anode (the positively charged electrode) (the positively charged electrode). 

Because the particle beam, or cathode ray, starts at the cathode, the tubes are termed cathode ray tubes. The beam may be detected by painting phosphors on the tube’s far end, beyond the anode. When the cathode ray strikes the phosphors, they spark or emit light. Thomson surrounded the cathode ray with two oppositely charged electric plates to investigate the particles’ characteristics. The cathode ray was redirected from the negatively charged electric plate to the positively charged plate. The cathode ray was made up of negatively charged particles, according to this.

Thomson also installed two magnets on either side of the tube and noticed that the cathode ray was diverted by the magnetic field. Thomson used the findings of these tests to calculate the mass-to-charge ratio of cathode ray particles, which led to a surprising discovery: each particle’s mass was much, much lower than any known atom. Thomson continued his tests with several metals as electrode materials and discovered that the characteristics of the cathode ray were consistent regardless of the cathode material.

Thomson made the following findings based on the evidence:

• Negatively charged particles make up the cathode ray.
• Because the mass of each particle is ∼ 1/2000 that of a hydrogen atom, they must be part of the atom.
• Within the atoms of all elements, these subatomic particles may be found.

Thomson’s discoveries were initially contentious, but they were progressively accepted by scientists. His cathode ray particles were eventually given a more common name: electrons. The discovery of the electron contradicted Dalton’s atomic theory’s assumption that atoms were indivisible. An altogether new atomic model was required to account for the presence of electrons.

What are Cathode Ray and Cathode ray tube?

J. J. Thomson built a glass tube that was partly evacuated, meaning that a significant amount of air was pushed out. He then used two electrodes at either end of the tube to apply a large electrical voltage. He saw a stream of particles (ray) travelling from the negatively charged electrode (cathode) to the positively charged electrode (anode) (anode). The ray is referred to as a cathode ray, and the entire structure is referred to as a cathode ray tube.

What is an Electron?

The electron is a negatively charged, low-mass particle. As a result, passing close to other electrons or the positive nucleus of an atom might readily deflect it. The first basic particle identified was the electron. 

J.J. Thomson is credited with discovering the electron and its characteristics through tests conducted in a discharge tube. Subatomic particles with an elementary charge of -1 are known as electrons. The charge held by an electron is the same as the charge held by a proton (but has an opposite sign). 

• Charge on an electron: Although the magnitude of the charges possessed by protons and electrons are the same, an electron’s size and mass are significantly less than a proton’s (an electron’s mass is about 1/1836 that of a proton). A negatively charged particle is an electron. The magnitude of the negative charge is 1.602 × 10^-19 Coulomb. An electron has a mass of 1/1837 that of a proton.
• Mass of an electron: An electron has a mass of 9.10938356 × 10^-31 kilogram. When compared to the mass of the proton, the electron has a tiny mass.

Characteristics of Electrons

The atom is the smallest unit of matter’s composition. The nucleus is in the core of the atom, with one or more electrons orbiting around it. Protons and neutrons, together known as nucleons, make up the nucleus. Protons are electrically neutral particles with a mass of 1.00867 amu, while neutrons are positively charged particles with a mass of 1.00728 amu.

With a mass of 0.000549 amu, electrons are negatively charged particles. Protons and neutrons weigh roughly 1836 times as much as electrons. The number of electrons equals the number of protons, resulting in an element’s neutral atom. The chemical characteristics of an element are determined by its electronic arrangements, whereas the nuclear structure determines the atom’s stability and radioactive transition.

Sample Questions

Question 1: What is the condition inside the discharged tube used by J. J. Thomson in the discovery of the electrons or Cathode rays?

Solution:

The conditions inside the discharged tube used by J.J. Thomson in the discovery of electrons are:

1. It must have a very low pressure of 0.01 mm of Hg.
2. It should have a vacuum inside it.
3. Very high voltage must be applied across the terminals.

Question 2: What is the effect of the electric field on the cathode rays?

Solution:

Cathode rays are deflected from their own straight-line path under the influence of the electric field. They are attracted towards the positive field due to their own negative charge and repelled away from the negative field.

Question 3: State two properties of Cathode rays.

Solution:

The two properties of Cathode rays:

1. They are negatively charged and always travel in straight lines.
2. They can penetrate through matter and cause ionization of the particles of the gases through which they pass.
3. Cathode rays are deflected by electric and magnetic fields.

Question 4: How are X-rays produced?

Solution:

X-rays are produced when a beam of the cathode ray is made to fall on hard metallic substances like tungsten.

Question 5: How Faraday’s study of static electricity paved the way for the discovery of sub-atomic particles?

Solution:

Previously, Dalton proposed the atomic theory which says that the atoms are the most fundamental particles of matter and are indivisible but, Faraday while studying static electricity found that electricity flow through the matter due to the charged particles. This made many scientists believe that atoms are divisible into more fundamental particles. Thus, the study of static electricity by Faraday paved the way for the discovery of sub-atomic particles.