Discovery of Subatomic Particles: You must be well aware of Dalton’s Atomic Theory. Modern atomic theory is based on Dalton’s concepts. However, one of his essential assumptions was later revealed to be incorrect. Atoms, according to Dalton, are the smallest units of matter—tiny, hard spheres that can’t be broken down any further. This belief remained until physics experiments revealed that the atom was made up of even smaller particles. In this article, we discuss the discovery of these smaller particles which are known as subatomic particles in detail.

Subatomic Particles

A subatomic particle is one that is smaller than an atom and is invisible to the human eye. Electrons, protons, and neutrons are the three subatomic particles. Neutrons have no charge, while electrons have a negative charge and protons have a positive charge. One of the four fundamental forces holding the particles within the atom are: gravity, electromagnetic force, strong force, and weak force.

Discovery of Electrons

Gases are poor conductors of electricity, noted by British scientist William Crooks. However, at a low pressure of \(0.001\,{\text{mm}}\,{\text{Hg}}\) and a high voltage of \(10000\,{\text{V}},\) the gas becomes a conductor, and current begins to flow through it.

J.J. Thomson, a physicist from the late \(1800\,{\text{s}},\) was one of the first to propose a theory of relativity in the late \(1800\,{\text{s}}.\) Thomson began experimenting with cathode ray tubes.

Experiment to Discover Electorns

Cathode ray tubes are vacuum-sealed glass tubes that have most of the air removed. At one end of the tube, a high voltage is placed across two electrodes, causing a beam of particles to flow from the cathode (the negatively charged electrode) to the anode (the positively charged electrode). Because the particle beam, or “cathode ray,” starts at the cathode, the tubes are termed cathode ray tubes. The ray can 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 placed two oppositely charged electric plates around the cathode ray to examine its characteristics. The cathode ray was redirected from the negatively charged electric plate to the positively charged plate. This indicated that the cathode ray was made up of negatively charged particles.

Thomson also placed two magnets on either side of the tube and observed that the cathode ray was diverted by the magnetic field. Thomson was able to determine the mass-to-charge ratio of cathode ray particles as a result of these studies, which led to a remarkable discovery: each particle’s mass was much, much lower than that of any known atom.

Thomson repeated his experiments with several metals as electrode materials and discovered that the characteristics of the cathode ray were constant regardless of the cathode material they originated from.

The observations of the cathode ray experiment are as follows-

• 1. Cathode rays originate at the cathode and move towards the anode.
• 2. Phosphorescent materials are the only way to see these rays.
• 3. On a soda glass screen put in the tube, they produce a greenish-yellow glow.
• 4. When placed in an electric field, they are inflected by a positive field and deflected by a negative field, and travel in a straight line in the absence of electrical or magnetic fields.
• 5. When a beam of cathode rays strikes a hard metallic object like tungsten, \({\text{X}}\)-rays are produced.
• 6. They are capable of penetrating matter.

These observations lead to the conclusion that electrons are the fundamental component of all atoms.

One of the earliest subatomic particles of an atom was discovered in this way.

Characteristics of Electrons

• 1. The mass of an electron is \(1/1837,\) the mass of a hydrogen atom or \(9.108 \times {10^{ – 31}}\,{\text{kg}}.\)
• 2. An electron carries a unit negative charge and has a magnitude of \(1.602 \times {10^{ – 19}}\) coulombs.
• 3. The electron is extremely small; its radius is less than \(1. \times {10^{ – 15}}\,{\text{m}}.\)

Discovery of Protons

Although Goldstein identified a positively charged particle, Ernest Rutherford is credited with discovering protons.

Goldstein Experiment

In \(1886,\) E Goldstein discovered an entirely different type of rays in the discharge tube. He observed that if a perforated disc cathode (as used by J.J Thomson), and the gas pressure was not too low, coloured rays emerged on the opposite side of the anode. These rays passed through the holes of the cathode, travelled in the form of a stream or canal and produced a glow on the other end of the discharge tube. Goldstein called them canal rays or anode rays. These rays were found to consist of positively charged particles. Hence, they are also called positive rays.

After the discovery of canal rays, attempts were made to discover the lightest particle which carries a unit positive charge. Therefore, the canal rays produced by using various gases were analysed and it was found that the lightest positive particle having a unit positive charge could be obtained by using hydrogen gas in the discharge tube.

Experiment to Discover Protons

A glass tube with two metallic electrodes, referred to as the cathode and anode, was used. The tube was filled with gas, and the pressure of the gas was controlled by a vacuum pump attached to the tube. A high-voltage source was used to link the electrodes. The pressure was very low, around \(0.001\,{\text{mm}}\,{\text{Hg}},\) and the voltage was very high, over \(10000\,{\text{V}}.\) Anode rays were produced.

The value of \({\text{e/m}}\) for the constituent particles of hydrogen gas was found to be maximum. Determination of the mass of these particles revealed that the mass of one particle was \(1837\) times the mass of an electron. This particle was called a proton.

Protons were also obtained in various nuclear reactions. Hence, it was thought that protons are subatomic particles and are present in all atoms.

Since the positive charge on a proton is exactly equal in magnitude to the negative charge of an electron, a neutral atom possesses the same number of protons as is the number of electrons present in it.

The observations of the anode ray experiment are as follows-

1. The anode rays follow a straight path.
2. They begin at the anode and work their way to the cathode.
3. When these rays are placed in an electric field, they are deflected by a positive field and attracted by a negative field.
4. When these rays collide with a metallic foil, they produce a heating effect.
5. They are microscopic material particles that cause mechanical effects.

Ernest Rutherford Experiment

In \(1909,\) Ernest Rutherford discovered protons in his gold foil experiment.

Gold-Foil Experiment

Rutherford bombarded an ultra-thin gold foil with alpha particles. The majority of alpha particles were undeflected, a few were deflected at a small angle, and just one out of every \(10000 – 20000\) particles bounced off from gold foil at a wide angle. As a result, he concluded that there must be a positively charged mass at the center of the atom to neutralize the negative charge of the atom. He named the positively charged particle as the proton around which the electron revolves.

By the discoveries, Rutherford discovered the protons.

Characteristics of Protons

• 1. The proton’s mass is the same as that of a hydrogen atom and that is \(1837\) times the mass of an electron which is \(1.672 \times {10^{ – 24}}\,{\text{g}}.\)
• 2. A proton carries a positive unit charge and has a magnitude of \(1.602 \times {10^{ – 19}}\) coulombs.
• 3. The proton is found in the nucleus, which is the centre component of the atom.

Discovery of Neutrons

Chadwick \(\left({1932} \right)\) bombarded a thin sheet of beryllium \(\left({{\text{Be}}} \right)\) with alpha particles. He noticed that an electrically neutral particle having a mass slightly greater than that of the protons was emitted. He named the particles as neutrons.

Characteristics of Neutrons

• 1. These particles are not deflected by an electric or magnetic field.
• 2. Its mass is equal to \({\rm{1}}{\rm{.676 \times 1}}{{\rm{0}}^{{\rm{ – 24}}}}{\rm{g}}\,{\rm{(1}}\,{\rm{amu)}}{\rm{.}}\)

Summary

In this article, we studied the following points-

• i. All atoms include tiny negatively charged subatomic particles or electrons, according to J.J. Thomson’s work with cathode ray tubes.
• ii. Eugene Goldstein, discovered evidence of this positively charged particle in \(1886.\) He discovered rays travelling in the opposite direction from the cathode rays using a cathode ray tube with holes in the cathode.
• iii. The gold foil experiment of Rutherford demonstrated that the atom is essentially empty space with a tiny, compact, positively charged nucleus.
• iv. The neutral charged particles- neutrons were discovered by Chadwick in \(1932.\)

FAQs on Discovery of Subatomic Particles

Frequently asked questions related to discovery of subatomic particles is listed as follows:

Q.1. Name the three subatomic particles of an atom. Give the difference between three subatomic particles.
Ans: Protons, electrons, and neutrons are the three subatomic particles that make up an atom. Protons are positively charged particles, electrons are negatively charged particles, and neutrons are electrically neutral particles, meaning they have no charge. The mass of each of the three subatomic particles is different.

Q.2. Who discovered subatomic particles?
Ans: The discovery of subatomic particles were done as follows-
i. The first subatomic particle electron was discovered in \(1897\) by J.J Thomson.
ii. In \(1909,\) Ernest Rutherford discovered protons in his gold foil experiment.
iii. Chadwick in \(1932\) discovered neutrons.

Q.3. Why are the subatomic particles important?
Ans: There are three subatomic particles-electrons, protons and neutrons. They are important because they are the building block of matter.

Q.4. What are the features of subatomic particles?
Ans: A subatomic particle is one that is smaller than an atom and is invisible to the human eye. Electrons, protons, and neutrons are the three subatomic particles. Neutrons have no charge, while electrons have a negative charge and protons have a positive charge. One of the four fundamental forces holding the particles within the atom are: gravity, electromagnetic force, strong force, and weak force.

Q.5. What are the subatomic particles made of?
Ans: Electrons are fundamental particles, and they belong to the first generation of the lepton particle family. Protons and neutrons are composite particles made of quarks.

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