Who Discovered Electrons, Protons and Neutrons?

Most people around the world give credit to below four scientists as those – who discovered electrons, protons, neutrons, and the nucleus.

• J.J. Thomson, an English discovered electrons in the year 1897,
• Eugen Goldstein, a German discovered the presence of positive particles in the year 1898,
• Ernest Rutherford, a New-Zealand born British discovered nucleus  in 1911 and protons in 1917, and
• James Chadwick, an English discovered neutrons in the year 1932.

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1.0 DISCOVERY OF ELECTRONS

Who discovered electrons?

For a long period in history, scientists were of opinion that atoms could not be broken further. But things changed after the discovery made by an English scientist named J.J. Thomson in the year 1897.  In his quest to study properties of cathode rays, he discovered that atoms contain a negatively charged subatomic particles – ‘electrons’. Thus, they are divisible.

Soon, after J.J. Thomson’s discovery of electrons, the race began among other scientists to uncover the basic structure of an atom. This led to the discovery of the nucleus and other subatomic particles – protons and neutrons.

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1.1 Cathode Rays

Scientists in the early nineteenth century were aware of electricity and the effect of electric potential. The electric potential is a driving force that results in the flow of current through a substance due to the difference in concentration of charges at two ends of it. The magnitude of the electric current flowing through a substance is directly proportional to the electric potential applied across it. They proved that the electricity or the electric current can flow through any substance – solid, liquid or gas if enough driving force or electric potential exists. After carrying out a series of experiments with – solids, liquids, and gases, scientists took one step further to drive electricity through a vacuum. However, most of them failed as none of them were able to create a perfect vacuum.

William Crooke, the British Scientist, in the year 1875 successfully created a near to perfect vacuum (0.01 mm of mercury) in a glass tube sealed at both ends with metal plates. He named it the discharge tube. William observed that when he applies a high electric potential across the discharge tube, then current flows from the negative terminal to the positive terminal in the form of rays. He called these ‘cathode rays’. Although, he could not establish what electric current or cathode rays comprise and what is actually moving from the negative terminal to the positive terminal.

Finally, in 1897 J. J Thomson discovered electrons while studying characteristics of cathode rays. He discovered that cathode rays consist of negatively charged subatomic particles (now called electrons), present in all atoms of the elements.

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1.2 Thomson’s Experiment and Result

Discovery of electrons

In his experiment, J. J. Thomson applied an electric field in the path of cathode rays in the discharge tube. He observed that cathode rays were deflected towards the positive plate of the electric field. This showed that cathode rays consist of negatively charged particles.

Next, he applied a magnetic field in the path of cathode rays. He noted that cathode rays again deflects in a direction in which moving negative charge would be deflected. This again proved that cathode rays contained negatively charged particles called electrons.

The below video beautifully demonstrate the effect of a magnetic field on cathode rays. Courtesy kosasihiskandarsjah

https://www.youtube.com/watch?v=XU8nMKkzbT8

Johnson further noted that the amount of deviation in cathode rays is directly proportional to the strength of the magnetic or electric field applied. Using this principle he measured the mass to the charge ratio (m/q) of the particles present in the cathode rays. He also noted that this ratio of mass to the charge (m/q) is constant and does not get affected by the type of gas in the discharge tube. This again proved that cathode rays consist of particles of the same type. Although Johnson discovered electrons he failed in measuring either its mass (m) or the charge (q) separately.

The exact mass (m) of the electrons was discovered later on in the year 1911, when another scientist Robert Andrews Millikan, an American successfully calculated the minimum charge (q) which can be carried by any particle.

1.3 Symbol & Properties of Electron

An electron is denoted by the symbol _-1e⁰. The superscript 0 represents its mass and the subscript -1 represent it’s one unit negative electrical charge. The mass of an electron is 1/1837 of the mass of a hydrogen atom (9.108 x 10^-28 g) and its charge is one unit negative charge i.e. 1.602 x 10^-19 Coulombs

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2.0 DISCOVERY OF PROTONS

Who discovered protons?

After the discovery of electrons, Eugen Goldstein in the year 1898, came to the conclusion that since all atoms are electrically neutral, thus there must be positively charged particles present in them. He discovered positively charged particles while carrying out an experiment using a discharge tube, although with slight variation.

Some of the people around the world mistakenly consider Eugen Goldstein as the one who discovered protons. But, it is not correct. He discovered anode rays, which comprise of positively charged particles which may or may not consists of protons.

Ernest Rutherford discovered protons in the year 1917.

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2.1 Eugen Goldstein’s Experiment and Result

Discovery of positively charged particles

Goldstein performed an experiment with a discharge tube fitted with a perforated cathode and a layer of Zinc Sulphide applied on the wall of the glass tube behind the cathode.

On applying a high electric potential across the discharge tube he noted the presence of another ray along with cathode rays. These rays were traveling as waves in the opposite direction i.e. from anode to the cathode. Some of these rays pass through holes in the cathode and result in red glow after striking the glass wall covered with Zinc Sulphide. He named them ‘anode rays’.

The rays or waves emitted from the anode behave just the opposite to the cathode ray in all respects when subjected to an electric and magnetic field. This proved that anode rays contained positively charged particles. Goldstein successfully measured the mass to the charge (m/q) ratio of the particles comprising anode rays. He also noted that this ratio of mass to the charge (m/q) varies with the type of gas inside the discharge tube. Thus, it proved that anode rays comprise particles of varying types.

Later on, Ernest Rutherford in the year 1911 conducted an alpha particle scattering experiment to uncover the basic structure of an atom. He discovered that all the positive charged particles join together to form a ‘nucleus’. This nucleus is present at the center of an atom and all electrons revolve around it.

In the year 1917, he discovered that these positively charged particles are similar to those as H+ ions and named them ‘protons’.

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2.2 Symbol & Properties of Porton

A proton is denoted as ₊₁p¹, where the superscript ‘1’ represents 1 amu (atomic mass unit) mass and the subscript ‘+1’ represent it’s one unit positive charge. The mass of a proton is equal to the mass of an atom of hydrogen, i.e. 1.672 x 10^-24 g. The positive charge on a proton is equal to the negative charge on an electron, i.e. 1.602 x 10^-19 coulombs.

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3.0 THOMSON MODEL OF AN ATOM – PLUM PUDDING MODEL

In 1904,  J. J. Thomson worked out the first model of an atom – the Plum Pudding Model.

According to this model, an atom is a positively charged sphere in which electrons are embedded just like dry fruits are distributed in a pudding. Therefore it is known as the Plum Pudding Model.

Since the total positive charged of the atom was equal to the total negative charge of its electrons, it followed that an atom would become negatively charged if it gained electrons and positively charged if it lost electrons. However, this model failed to explain many experimental observations about atoms.

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4.0 DISCOVERY OF THE NUCLEUS

In 1911, Earnest Rutherford a scientist from New Zealand conducted an experiment in order to find the arrangement of electrons and proton in an atom. His experiment led to the discovery of a small, positively charged nucleus at the center of an atom.

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4.1 Rutherford Alpha Particle Scattering Experiment

Alpha particles are positively charged particles comprising of two numbers of protons and two numbers of neutrons. In other words, they are helium atoms without two numbers of electrons. Some atoms of heavy elements with a large neutron to proton ratio in their nuclei emit alpha particles to bring parent nuclei to a more stable configuration.

Nucleus was discovered by Rutherford in an experiment widely known as the alpha particle scattering experiment. In his experiment, Rutherford surrounded a thin sheet of gold (0.00004 cm thickness) with a screen made of Zinc Sulphide. The screen allowed passage for a thin beam of alpha particle emitted from the radioactive source. He bombarded this thin sheet of gold with alpha particles in an evacuated chamber. Alpha particles after reflecting or passing through the gold sheet struck the Zinc Sulphide screen and produced red glow at point of contact. He noted locations where alpha particles struck the screen and concluded as follows:

1. Most of the alpha particles passed straight through the foil without any deflection from their path.
2. A small fraction of them was deflected from their original path by small angles.
3. Only a few particles bounced back.

Step by step demonstration of Alpha particles scattering experiment. Video courtesy blenderIITB

On the basis of the above observations, Rutherford made the following conclusions:

1. Most of the space in an atom was empty because alpha particles went straight.
2. The deflection of a small fraction of alpha particles occurs due to the presence of a heavy positively charged mass in the atom.
3. The positively charged mass is very small and is centrally located because only a few particles bounced back. He named this positively charged mass discovered by him – ‘nucleus’ of an atom.

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4.2 Rutherford Atomic Model

Based on his experiment, he suggested a model for the structure of the atom. According to this model, an atom consists of mainly two parts:

Part-1: The centrally located nucleus

1. The nucleus is a centrally located positively charged mass.
2. As electrons have negligible mass thus, the entire mass of the atom is present inside the nucleus. It is the densest part of the atom.
3. If compared to the size of the atom as a whole the size of the nucleus is very small.

If we consider a circular stadium as an atom, then its nucleus is no more than a cricket ball placed at the center of the stadium.

Part-2: The outer circular orbits

1. Electrons revolve around in circular orbits (shells) in the space available around the nucleus.
2. An atom is electrically neutral i.e. the number of protons and the number of electrons present in an atom is equal.
   

   Rutherford’s Atomic Model

Thus, a model proposed by Rutherford is somewhat similar to that of the solar system. Just as in solar system, the sun is at the center and the planets revolve around it, in an atom the electrons revolve around the centrally located nucleus containing protons.

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5.0 DISCOVERY OF THE NEUTRONS

James Chadwick in the year 1932, discovered neutrons. He studied the atomic model and discovered that the nucleus of the atom consists of particles that have no charge but mass almost equal to that of the protons. He named particles discovered by him ‘neutrons’.

Until 1930, scientists were of the opinion that atoms consist of two types of subatomic particles – protons and electrons. The atomic model proposed by Rutherford answered most of the questions with respect to the structure of an atom. However, some of them still remained unanswered.

1. As particles bearing similar charge repel each other, then how can all positively charged protons stay together inside a nucleus?
2. Why the mass of the nucleus of an atom is greater than the total mass of protons present inside it?
3. Why some atoms of the same element bear different mass?

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5.1 Experiments Conducted Before James Chadwick

In 1930, two scientists Walther Bothe and Herbert Becker conducted an experiment in which they struck beryllium atoms with alpha particles. It resulted in the emission of high energy radiation. This radiation had a very high penetrating power and does not ionize the gas through which it passes. They concluded this radiation to be gamma rays. Two years later, Curie and Joliot showed that when you bombard a target, which is rich in hydrogen with this radiation, it ejects high energy protons. As gamma rays do not carry the amount of energy and momentum required to displace protons. Thus, the result of Curie and Joliat contradicts the conclusion made by Bothe and Becker.

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5.2 James Chadwick Experiment and Result

In the year 1932, James Chadwick carried out a series of experiments on paraffin wax with Bothe and Becker radiation and discovered neutrons. He used paraffin wax because it is a hydrocarbon which is rich in hydrogen and so in protons. Chadwick measured the energy of the protons and stated –  gamma rays do not carry the amount of energy and momentum required to displace protons thus, radiation coming from beryllium is not just electromagnetic in nature and must consist of particles. He named particles discovered by him neutrons as they had no charge and their mass was almost equal to that of protons.

 

James Chadwick answered all the above questions when he discovered ‘neutrons’ and presented his atomic model. The silent features of his model are –

1. The third type of sub-atomic particles – ‘neutrons’ are present in the nucleus and had neither a positive nor a negative charge.
2. As each proton has a positive charge on them, thus they tend to repel each other. Neutrons help in keeping protons together.
3. Mass of the nucleus is more due to the presence of neutrons. Neutrons are of the same size as protons.
4. Atoms of the same element may differ in the number of neutrons leading to the formation of isotopes.

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5.3 Symbol & Properties of Neutron

A neutron is denoted as ₀n¹, where the superscript 1 represents 1 amu  (atomic mass unit) mass and the subscript ‘0’ represent electrically a neutron is neutral, i.e. it has no charge. The mass of a neutron is slightly more than that of a proton, i.e. 1.676 x 10^-24 g compared to 1.672 x 10^-24 g.

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