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James Chadwick’s Atomic Theory and Its Lasting Impact Explained
Today, the concept of neutron and its function is common knowledge. Its existence was first hypothesized by Rutherford in 1920, and later proved by James Chadwick in 1932. This ScienceStruck post explains how the discovery came about and the revolutionary impact it had on the understanding of the atomic structure.
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Missed Opportunity!
In 1932, James Chadwick’s experiment, which led to the discovery of neutrons, was inspired by the work of Irène and Frédéric Joliot-Curie. Had they interpreted their findings accurately, they would have been the first to discover neutrons!
Sir James Chadwick (20 October 1891 – 24 July 1974) was an English physicist, most noted for his discovery of neutrons in 1932. He received a Nobel Prize in the field of physics in 1935 for this significant discovery. In his lifetime, he worked closely with outstanding scientists like Ernest Rutherford and Johannes “Hans” Wilhelm Geiger, both of whom have made various substantial and vital contributions to the field of radiation physics.
With the accidental discovery of radiation and radioactive materials in 1896 by Henri Becquerel, a new path emerged in the study of materials and their compositions. Experimentation led scientists to discover that a radioactive substance when subjected to a magnetic field, emits three types of energy rays (radiations). These rays were identified and named as alpha (α), beta (β), and gamma (γ) rays by Rutherford, based on their charge and mass. Further experiments conducted by Rutherford in collaboration with Ernest Marsden and Hans Geiger, led them to the discovery of the atomic nucleus in 1911. This discovery was instrumental in realizing that the atom was not a solid spherical structure.
Rutherford’s Atomic Model
Based on his findings about the atomic nucleus, from his famous gold-foil experiment, Rutherford put forth an atomic model that postulated:
◈ The atom is a hollow structure with its mass and positive charge concentrated into a tiny and dense core at the center, and the negatively charged lighter electrons orbiting the core like the planetary structure. Hence, this model is also called the planetary model.
◈ The electrons do not affect the pattern and trajectory of alpha particles.
◈ The atomic mass correlates with the charge of the atomic core or the atomic nucleus.
This model, however, had a very obvious limitation. Since the electron was a constantly accelerating particle of a negative charge, it would be attracted to the positive charge of the nucleus, thus causing the atom to become unstable and implode. To overcome this, in 1921, Rutherford with the help of Niels Bohr put forth a theory, hypothesizing the existence of a neutral-charged particle that had the same mass of a proton. The particle would be a composite of an electron and a proton and would be called a “neutron”. This theory, however, was not readily accepted by the scientific community due to lack of proof.
James Chadwick’s Contribution to the Atomic Theory
In 1930, Walther Bothe and Herbert Becker conducted experiments involving bombarding the element Beryllium with alpha particles emitted from radioactive polonium. They observed that the bombardment led to the emission of a neutral radiation from the Polonium, which was highly penetrative. They mistakenly believed this radiation to be a high energy form of gamma radiations. Hence, when Irène and Frédéric Joliot-Curie performed similar experiments in 1932, involving the emission of photons from paraffin and other such hydrogen-containing compounds when bombarded with this neutral radiation, they too believed that the radiation was a high-energy gamma radiation, and published results to that effect.
However, when Chadwick read the paper, he realized that a photon could not possibly be dislodged by a mere alpha particle. He, henceforth, conducted similar experiments of his own utilizing a linear amplifier, a refined polonium source, and an ionization chamber. He treated a number of substances and elements to this radiation and measured the recoil atom’s energy. He finally concluded that the radiation was, in fact, composed of neutral particles that had the same mass as protons. He called these neutral particles as neutrons. A paper to this effect was published by him in 1932, and shortly thereafter, other papers replicating the find were published by scientists like Norman Feather and Philip Dee.
Modified Atomic Theory
The existence and discovery of neutrons revolutionized the understanding of the atomic structure. It proved the validity of Rutherford’s atomic model and explained its stability. The postulates added to the atomic theory were:
◈ The nucleus of an atom consists of subatomic particles called nucleons.
◈ These nucleons are of two types: protons and neutrons.
◈ The neutrons are neutrally charged particles with the mass equal to that of a proton.
◈ A neutron is composed of an electron and proton couple.
◈ The collective mass of the protons and neutrons provides the atomic mass of an element.
German physicist Werner Heisenberg later, through his own experiments, proved that the neutrons were, in fact, a new particle and not a electron-proton composite.
This theory along with the discovery of neutrons was later instrumental in the invention of the atomic bomb. The atomic theory was later improved on by Niels Bohr to account for emission and absorption energy spectra observed in atoms, and a new model called the Niels Bohr model was hypothesized. This model is the presently accepted model to explain the atomic structure.
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Discovery of Neutron: Chadwick’s Experiment
- Updated by Scienly
- On September 16, 2024
In this chapter, we will understand the discovery of neutron discovered by James Chadwick in 1932. Up to 1932, it was considered that an atom is composed of only electrons and protons. Since electrons have negligible mass, scientists considered that the entire mass of the atom was due to the number of protons present in the nucleus.
Each proton has a mass equal to 1.676 * 10 -24 grams. This mass was taken as a basic unit of mass for measuring atomic and molecular masses.
However, Ernest Rutherford in 1920 found that, except for the hydrogen atom, the atomic masses of other atoms could not be explained in the terms of the numbers of electrons and protons only. For example, he observed that there were two protons inside the nucleus of a helium atom because the atomic number of helium is 2.
But, the actual mass of the helium atom was found to be twice the mass of two protons. In order to account for the remaining mass in the helium atoms, Rutherford in 1920 predicted that there must be the presence of two electrically neutral particles, each having the mass nearly equal to that of a proton inside the nucleus of an atom.
This prediction paved the way for the later discovery of the neutron by James Chadwick in 1932. Neutrons, as Rutherford suggested, are neutral particles with a mass nearly equal to that of protons. They exist along with protons in the nucleus, contributing to the overall mass of atoms.
Chadwick’s Experiment – Discovery of Neutron
Now it had predicted that the nucleus contains not only protons but also another type of particle named neutron. James Chadwick in 1932 discovered the third fundamental particles called neutrons inside the nuclei of atoms.
Sir Chadwick bombarded a thin foil of beryllium or boron with a stream of alpha particles. He observed that highly penetrating rays were produced, which were not affected by electric and magnetic fields. These rays, or say radiations, consisted of electrically neutral particles, which were called neutrons. The nuclear reaction is as follows:
4 Be 9 + 2 He 4 – – – – – – – – > 6 C 12 + 0 n 1
Here, n is neutron. 4 Be 9 represents beryllium atom, while 2 He 4 represents an alpha particle.
Important Characteristics of Neutrons
There are the following characteristics of neutrons. They are as:
- Neutron is the third fundamental subatomic particle present in the nucleus of an atom.
- This subatomic particle carries no charge or zero electric charge, meaning it is an electrically neutral particle.
- The mass of the neutron is determined to be 1.675 x 10 -24 g, 1.675 x 10 -27 kg, or 1 amu, which is nearly equal to the mass of the proton or hydrogen atom.
- The e / m value of a neutron is zero.
Note : Except for hydrogen atom ( 1 H 1 ), neutrons are present in all atoms of other elements along with the fundamental particles called electrons and protons.
Subatomic Particles
In this section, we have listed the important properties of the three principal fundamental particles of the atom, namely the electron, proton, and neutron in a table.
Structure of Atom
The atom consists of three most important fundamental subatomic particles called electrons, protons, and neutrons. The protons and neutrons are found in the central part, named the nucleus of an atom. Electrons revolve around the nucleus in an imaginary circular path called orbit.
Almost all the ordinary chemical properties of matter are examined in terms of atoms composed of electrons, protons and neutrons. Therefore, we will assume that atom contains only these three principal subatomic particles.
Read also: Discovery of Proton
Other Subatomic Particles
In addition to electrons, protons and neutrons, many other subatomic particles such as mesons , positrons , neutrinos and antiprotons have been discovered. A great deal of recent research has discovered more subatomic particles, such as quarks , pions , and gluons , which play important roles in the deeper understanding of atomic structure. These discoveries have made the picture of the atom more complex, especially in fields like particle physics.
However, the three fundamental subatomic particles of the atom (electron, proton, and neutron) remain sufficient for most of the needs of chemistry. Chemists primarily focus on how atoms interact and bond with each other. This basic model still effectively explains chemical reactions and properties.
FAQs on Discovery of Neutron
1. what is a neutron.
A neutron is a subatomic particle found in the nucleus of an atom. It has no electric charge, meaning that it is an electrically neutral. Its mass is slightly heavier than the mass of a proton.
2. Who was known for the discovery of the neutron?
British physicist James Chadwick was known for the discovery of the neutron. He discovered it in 1932. He and Niels Henrik David Bohr were the students of Sir Ernest Rutherford. Chadwick was born on 20 Oct 1891 at the place of Bollington, Cheshire, England. He died at Cambridge, England.
3. What was awards given to James Chadwick for the discovery of neutron?
For the discovery of neutron, James Chadwick was awarded the Hughes Medal of the Royal Society in the year 1932 as well as the Nobel prize for physics in the year 1935.
4. What role do neutrons play in an atom?
Neutrons, along with protons, make up the nucleus of an atom. They contribute atomic mass as well as play a crucial role in the stability of the nucleus.
5. How is the mass of a neutron compared to a proton?
The mass of a neutron is nearly the same as that of a proton, but it is slightly heavier. Both have a mass approximate to 1 atomic mass unit (amu).
6. Do all atoms have neutrons?
Except for the hydrogen atom, all atoms of other elements have neutrons. Hydrogen atom contains only one proton and one electron.
7. What is the difference between a neutron and a proton?
The main difference between them is that neutrons have no electric charge, while protons carry a positive charge. Both have nearly similar masses and are located in the nucleus of an atom.
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