RESEARCH STARTER

Structure Of The Atomic Nucleus

The atomic nucleus is a central component of an atom, comprised primarily of protons and neutrons, which are collectively known as nucleons. Protons carry a positive charge, while neutrons are neutral, and both are densely packed in a tiny volume that contains most of the atom's mass. Surrounding the nucleus is a cloud of electrons, which possess a negative charge that balances the positive charge of the nucleus. The interactions within the nucleus are governed by quantum physics, which describes the dual nature of atomic particles as both wave-like and particle-like entities.

Historically, the understanding of the atomic nucleus has evolved significantly since the late 19th century, beginning with J.J. Thomson's discovery of electrons and leading to Ernest Rutherford's identification of the nucleus. The development of particle accelerators has allowed physicists to probe deeper into the structure of nucleons, leading to the discovery of quarks and hadrons, the latter being complex particles that make up protons and neutrons. Theoretical frameworks such as the nuclear shell model describe how nucleons are arranged and interact within the nucleus, leading to concepts like binding energy and magic numbers that signify particularly stable configurations.

Research into the atomic nucleus is not only pivotal for understanding matter at a fundamental level but also has implications for cosmology and fundamental physics, including theories related to the origins of the universe and the potential instability of matter over time. As scientists continue to explore the complexities of the atomic nucleus, they also seek a unified theory that encompasses the fundamental forces and particles of the universe.

Full Article

Type of physical science: Nuclear physics

Field of study: Nuclei

The atomic nucleus consists of protons and neutrons held together by the strong force. These nucleons are themselves made of quarks bound by gluons.

Overview

The first real science to address the subject of atomic structure occurred on April 29, 1897, when Joseph John Thomson announced that he had discovered exceedingly small subatomic particles, which he called corpuscles. Later, corpuscles were renamed electrons. In 1911, New Zealand physicist Ernest Rutherford, a colleague of Thomson, discovered that a very dense central core, known as the nucleus, was at the center of the atom. Rutherford later showed that nuclei contain positively charged particles, which by 1920 he had named protons.

The electron is much lighter in mass than the whole atom. Yet, the nucleus, which comprises such an overwhelming part of the mass (it is thousands of times heavier than the electron), is only a tiny part of the atom's total volume. The electron carries a net negative charge, which is counterbalanced by the nucleus's net positive charge.

Rutherford constructed a device that used a radioactive α particle emitter as a rudimentary particle accelerator and aimed the particles at a gold screen target with a fluorescent screen just beyond. If a few of the massive α particles struck the nucleus of the atoms in the gold foil, the nucleus' nature would be revealed by the degree of deflection of the particles from the center of the fluorescent screen. It was discovered that the nucleus is extremely dense, very small, and surrounded by an orbiting cloud of electrons. This landmark discovery drew a clear line between classical physics and the newly emerging quantum physics.

Danish physicist Niels Bohr pondered Rutherford's model and found that the electrons could not possibly orbit the nucleus in the classical sense; the atom would be unstable and fly apart. A year after Rutherford made his announcement, Bohr radically changed physics when he announced that classical laws gave way to a peculiar set of quantum laws at atomic sizes.

Quantum physics makes a clear distinction between the macroscopic world larger than the atom and the microscopic world existing inside the electron shell in the interior of the atom. Atoms exhibit both wave-like and particle-like properties. In fact, all the atomic constituents exhibit this dual nature. Quantum physics defines these particles as fields that obey the laws of quantum physics and relativity. Thus, subatomic particles are actually defined by intense fields at a point.

In 1932, English physicist James Chadwick discovered that the nucleus contained another particle that he called the neutron, a neutrally charged particle. By the beginning of World War II, the atom was thought to consist of a nucleus of protons and neutrons embedded within a shell of electrons. In 1935, Japanese physicist Hideki Yukawa deduced that there must be an enormously powerful force holding together the quanta (protons and neutrons) inside the nucleus. He correctly noted that if the force binding the electrons had an associated quantum (photons), then the force binding the nuclear particles also should have an associated quantum particle, which he called the meson. With that conjecture, Yukawa initiated the discoveries of an endless profusion of particles.

In his elegant and revolutionary theories, Albert Einstein demonstrated unequivocally that matter and energy, under the right conditions, were interchangeable. In the quantum state, a binding force (such as the nucleus) can be transformed into a quantum particle and back again. In the stable form, atoms maintain an equilibrium where their quantum particle constituents are stable and well defined as their electrons and nucleus of protons and neutrons. As the atom is destabilized or blown apart in a particle accelerator, quantum forces that hold the atom together in a stable form may come apart and be identified as quantum particles. According to Yukawa, if the proton and neutron are separated from the nucleus, the quantum force binding them together would be seen as a particle he called the meson, later renamed the pion. In 1947, the existence of the pion, as the particle associated with the quantum force holding the proton and neutron together, was confirmed in cosmic-ray experiments.

When the powerful particle accelerators were brought online in the 1950s and 1960s, the short list of three nuclear particles grew dramatically. These machines probed deeper and deeper into the atomic nucleus, accelerating protons to very high velocities and slamming them into one another. The energy of the collision broke the protons into their constituent particles, which physicists photographed and identified. These scientists were trying to determine whether the protons and neutrons were composed of smaller particles. The number and variety of hadrons appeared infinite, which stunned and disappointed physicists who were hoping for a simplification of matter. The Italian American physicist Enrico Fermi commented that if he had known about hadrons, he would have gone into zoology instead of physics.

By 1960, physicists had determined that the nucleus was composed of protons and neutrons bound together by pions. As the protons and neutrons were broken apart, they appeared to be composed of an infinite variety of hadrons. In 1961, American physicist Murray Gell-Mann discovered a pattern in the glut of hadrons. He indexed and cataloged them according to some of their most common characteristics, such as their spin, charge, and mass. He called his classification scheme the eightfold way. This eventually led to the postulation that hadrons were made up of simpler quantum particles, which he called quarks. According to Gell-Mann, an infinite variety of hadrons could be created in the breakup of a proton. Hadrons actually were composite particles, including protons and neutrons, composed of quarks.

Physicists use models to describe nuclear interactions. Shell models describe the nature of the proton and neutron inside the nucleus, collectively called a nucleon. The nuclear shell model is analogous to the electronic shell models of atoms, except that the nuclear shell model describes stable nuclear configurations as they are bound together by the strong nuclear force.

The nuclear shell models predict stable configurations of nucleons inside the nucleus. The shell models describe how each nucleon moves about in the nucleus. They predict that each nucleon may be described in terms of their respective states and that these states of nucleons form shells or layers within the nucleus.

In a given nucleus, nucleons tend to assemble together in the most stable state possible.

In this process, the nuclear particles exist in a state of lowest possible internal energy while giving off external energy. Thus, the internal energy is negative, and most nuclei tend to attain the lowest or most negative possible energy. The reciprocal of this negative measure is called the nucleus' "binding energy." Thus, a very heavy nucleus with many nucleons bound together would have much more binding energy than a light nucleus with fewer nucleons. Observation of these properties has demonstrated that the internal or binding energy is almost directly proportional to the number of nucleons. Since the binding energy is proportional to the number of nucleons, the binding energy itself has been measured at about eight million electronvolts per nucleon.

As the nucleons in a given nucleus fill up these shells or layers within the nucleus, particularly stable orbital configurations are attained. These orbital configurations are defined in terms of how many nucleons occupy a given shell or layer. The number of neutrons and protons which correspond to particularly stable structures in closed shells are called "magic numbers."

The magic numbers for nuclei are 2, 8, 20, 28, 50, 82, and 126. These numbers correspond to especially stable closed-shell configurations.

Applications

High-energy particle physics requires increasingly powerful and larger instruments to look deeper into the atom. Particle accelerators are the ultimate microscopes; yet, they require huge energies and even large land areas to contain the enormous tracks down which the particles are accelerated. Current particle accelerators cover hundreds of meters. The newest accelerators will span tens of kilometers. Peering ever deeper into the atomic core requires not only huge machines costing billions of dollars, but also extraordinary energies. Breaking the bonds between the nuclear quanta requires that the colliding protons be traveling at enormous velocities.

Physicists have found that using the current from a common household outlet can enable them to look into the atom and distinguish the electron from the atomic nucleus. To look inside the nucleus requires far more energy, and to look inside the proton and neutron requires the energy of a small city.

Disassembling the atom ultimately will reveal whether all laws of the physical universe are characterized in a single set of laws that have been called the unified field theory. Physicists have been working to discover a concise law that describes all matter, forces, and gravity. Such a law is impossible until the exact nature of all matter and the forces that bind that matter are understood completely. The structure of the atomic nucleus also has applications in cosmology.

The Big Bang theory states that the universe expanded from an extremely hot, dense early state about 13.8 billion years ago from which matter condensed out of the energy of a primordial fireball. Investigation into the atomic interior relates to how that matter condensed and what universal constants fell out of such a condensation.

Other questions relating to the atomic nucleus are of vital importance to theoretical physics. Many theories relating to the nature of matter state that matter itself is ultimately unstable, that atoms should eventually disintegrate into energy. According to one theory, the proton is unstable and eventually decays. When the proton decays, the atom spontaneously disintegrates. Though inconclusive, scientists continue to test these theories. Yet, even the radically new ideas such as those embodied in string theory also call for proton decay.

A new mathematics is needed to incorporate some of the discoveries being made in theoretical and experimental physics. Sir Isaac Newton needed to invent calculus to describe his concept of gravity, and Albert Einstein transformed Riemannian geometry and tensor calculus to describe relativity. The new mathematics that will ultimately describe the new physics will have a ripple-down effect on not only physics but also other sciences and technologies as well.

Context

The social imperative to explore the universe has not always been linked to profit, a consideration which tends to drive many other technological efforts. There are a few direct benefits from particle physics, yet some of the most profound discoveries in theoretical physics result from the use of particle accelerators. From the earliest philosophical questions about the nature of matter to the latest discoveries in string theory, scientists have found that matter is both tangible and ephemeral, with both particle-like and quantum characteristics. The structure of the atomic nucleus has depicted a world of quantum reality which challenges logic and requires a new philosophy of nature.

Principal terms

BINDING ENERGY: the amount of external energy emitted in the binding process of the nucleons

EIGHTFOLD WAY: the classification scheme applied to hadrons, which led to the discovery of quarks

HADRON: a composite particle, such as a proton or neutron, made of quarks

MAGIC NUMBERS: the number associated with the quantity of nuclear particles required to form a stable shell or layer of protons or neutrons in an atomic nucleus

MESON: the quanta associated with the force binding protons and neutrons together in the nucleus; later called the pion

NUCLEON: the collective name for the proton and neutron inside the atomic nucleus bound together by the strong nuclear force

QUANTA: the designation of the constituents of atoms which are actually states of a field that may be described either as wave-like energy or discrete particles

QUARK: the quanta of which protons and neutrons are composed

SHELL MODELS: nuclear models which describe the basic nuclear properties of a nucleus; ordering the protons and neutrons inside the nucleus in their most stable configurations


Bibliography

Crease, Robert P., and Charles C. Mann. The Second Creation. Macmillan, 1986.

“DOE Explains...Nuclei.” Department of Energy, www.energy.gov/science/doe-explainsnuclei. Accessed 19 Apr. 2026.

“DOE Explains...Quarks and Gluons.” Department of Energy, www.energy.gov/science/doe-explainsquarks-and-gluons. Accessed 19 Apr. 2026.

Hawking, Stephen W. A Brief History of Time: From the Big Bang to Black Holes. Bantam, 1988.

Jolos, R. V., and E. A. Kolganova. “Structure of the Atomic Nucleus: Yesterday and Today.” Physics of Particles and Nuclei, vol. 55, no. 5, Oct. 2024, pp. 1209–21, doi:10.1134/S1063779624700916. Accessed 19 Apr. 2026.

Pagels, Heinz R. The Cosmic Code: Quantum Physics as the Language of Nature. Bantam, 1982.

Pagels, Heinz R. Perfect Symmetry: The Search for the Beginning of Time. Bantam, 1985.

“Planck Reveals an Almost Perfect Universe.” European Space Agency, 21 Mar. 2013, www.esa.int/Science_Exploration/Space_Science/Planck/Planck_reveals_an_almost_perfect_Universe. Accessed 19 Apr. 2026.

Sutton, Christine. The Particle Connection. Simon & Schuster, 1984.

Full Article

Type of physical science: Nuclear physics

Field of study: Nuclei

The atomic nucleus consists of protons and neutrons held together by the strong force. These nucleons are themselves made of quarks bound by gluons.

Overview

The first real science to address the subject of atomic structure occurred on April 29, 1897, when Joseph John Thomson announced that he had discovered exceedingly small subatomic particles, which he called corpuscles. Later, corpuscles were renamed electrons. In 1911, New Zealand physicist Ernest Rutherford, a colleague of Thomson, discovered that a very dense central core, known as the nucleus, was at the center of the atom. Rutherford later showed that nuclei contain positively charged particles, which by 1920 he had named protons.

The electron is much lighter in mass than the whole atom. Yet, the nucleus, which comprises such an overwhelming part of the mass (it is thousands of times heavier than the electron), is only a tiny part of the atom's total volume. The electron carries a net negative charge, which is counterbalanced by the nucleus's net positive charge.

Rutherford constructed a device that used a radioactive α particle emitter as a rudimentary particle accelerator and aimed the particles at a gold screen target with a fluorescent screen just beyond. If a few of the massive α particles struck the nucleus of the atoms in the gold foil, the nucleus' nature would be revealed by the degree of deflection of the particles from the center of the fluorescent screen. It was discovered that the nucleus is extremely dense, very small, and surrounded by an orbiting cloud of electrons. This landmark discovery drew a clear line between classical physics and the newly emerging quantum physics.

Danish physicist Niels Bohr pondered Rutherford's model and found that the electrons could not possibly orbit the nucleus in the classical sense; the atom would be unstable and fly apart. A year after Rutherford made his announcement, Bohr radically changed physics when he announced that classical laws gave way to a peculiar set of quantum laws at atomic sizes.

Quantum physics makes a clear distinction between the macroscopic world larger than the atom and the microscopic world existing inside the electron shell in the interior of the atom. Atoms exhibit both wave-like and particle-like properties. In fact, all the atomic constituents exhibit this dual nature. Quantum physics defines these particles as fields that obey the laws of quantum physics and relativity. Thus, subatomic particles are actually defined by intense fields at a point.

In 1932, English physicist James Chadwick discovered that the nucleus contained another particle that he called the neutron, a neutrally charged particle. By the beginning of World War II, the atom was thought to consist of a nucleus of protons and neutrons embedded within a shell of electrons. In 1935, Japanese physicist Hideki Yukawa deduced that there must be an enormously powerful force holding together the quanta (protons and neutrons) inside the nucleus. He correctly noted that if the force binding the electrons had an associated quantum (photons), then the force binding the nuclear particles also should have an associated quantum particle, which he called the meson. With that conjecture, Yukawa initiated the discoveries of an endless profusion of particles.

In his elegant and revolutionary theories, Albert Einstein demonstrated unequivocally that matter and energy, under the right conditions, were interchangeable. In the quantum state, a binding force (such as the nucleus) can be transformed into a quantum particle and back again. In the stable form, atoms maintain an equilibrium where their quantum particle constituents are stable and well defined as their electrons and nucleus of protons and neutrons. As the atom is destabilized or blown apart in a particle accelerator, quantum forces that hold the atom together in a stable form may come apart and be identified as quantum particles. According to Yukawa, if the proton and neutron are separated from the nucleus, the quantum force binding them together would be seen as a particle he called the meson, later renamed the pion. In 1947, the existence of the pion, as the particle associated with the quantum force holding the proton and neutron together, was confirmed in cosmic-ray experiments.

When the powerful particle accelerators were brought online in the 1950s and 1960s, the short list of three nuclear particles grew dramatically. These machines probed deeper and deeper into the atomic nucleus, accelerating protons to very high velocities and slamming them into one another. The energy of the collision broke the protons into their constituent particles, which physicists photographed and identified. These scientists were trying to determine whether the protons and neutrons were composed of smaller particles. The number and variety of hadrons appeared infinite, which stunned and disappointed physicists who were hoping for a simplification of matter. The Italian American physicist Enrico Fermi commented that if he had known about hadrons, he would have gone into zoology instead of physics.

By 1960, physicists had determined that the nucleus was composed of protons and neutrons bound together by pions. As the protons and neutrons were broken apart, they appeared to be composed of an infinite variety of hadrons. In 1961, American physicist Murray Gell-Mann discovered a pattern in the glut of hadrons. He indexed and cataloged them according to some of their most common characteristics, such as their spin, charge, and mass. He called his classification scheme the eightfold way. This eventually led to the postulation that hadrons were made up of simpler quantum particles, which he called quarks. According to Gell-Mann, an infinite variety of hadrons could be created in the breakup of a proton. Hadrons actually were composite particles, including protons and neutrons, composed of quarks.

Physicists use models to describe nuclear interactions. Shell models describe the nature of the proton and neutron inside the nucleus, collectively called a nucleon. The nuclear shell model is analogous to the electronic shell models of atoms, except that the nuclear shell model describes stable nuclear configurations as they are bound together by the strong nuclear force.

The nuclear shell models predict stable configurations of nucleons inside the nucleus. The shell models describe how each nucleon moves about in the nucleus. They predict that each nucleon may be described in terms of their respective states and that these states of nucleons form shells or layers within the nucleus.

In a given nucleus, nucleons tend to assemble together in the most stable state possible.

In this process, the nuclear particles exist in a state of lowest possible internal energy while giving off external energy. Thus, the internal energy is negative, and most nuclei tend to attain the lowest or most negative possible energy. The reciprocal of this negative measure is called the nucleus' "binding energy." Thus, a very heavy nucleus with many nucleons bound together would have much more binding energy than a light nucleus with fewer nucleons. Observation of these properties has demonstrated that the internal or binding energy is almost directly proportional to the number of nucleons. Since the binding energy is proportional to the number of nucleons, the binding energy itself has been measured at about eight million electronvolts per nucleon.

As the nucleons in a given nucleus fill up these shells or layers within the nucleus, particularly stable orbital configurations are attained. These orbital configurations are defined in terms of how many nucleons occupy a given shell or layer. The number of neutrons and protons which correspond to particularly stable structures in closed shells are called "magic numbers."

The magic numbers for nuclei are 2, 8, 20, 28, 50, 82, and 126. These numbers correspond to especially stable closed-shell configurations.

Applications

High-energy particle physics requires increasingly powerful and larger instruments to look deeper into the atom. Particle accelerators are the ultimate microscopes; yet, they require huge energies and even large land areas to contain the enormous tracks down which the particles are accelerated. Current particle accelerators cover hundreds of meters. The newest accelerators will span tens of kilometers. Peering ever deeper into the atomic core requires not only huge machines costing billions of dollars, but also extraordinary energies. Breaking the bonds between the nuclear quanta requires that the colliding protons be traveling at enormous velocities.

Physicists have found that using the current from a common household outlet can enable them to look into the atom and distinguish the electron from the atomic nucleus. To look inside the nucleus requires far more energy, and to look inside the proton and neutron requires the energy of a small city.

Disassembling the atom ultimately will reveal whether all laws of the physical universe are characterized in a single set of laws that have been called the unified field theory. Physicists have been working to discover a concise law that describes all matter, forces, and gravity. Such a law is impossible until the exact nature of all matter and the forces that bind that matter are understood completely. The structure of the atomic nucleus also has applications in cosmology.

The Big Bang theory states that the universe expanded from an extremely hot, dense early state about 13.8 billion years ago from which matter condensed out of the energy of a primordial fireball. Investigation into the atomic interior relates to how that matter condensed and what universal constants fell out of such a condensation.

Other questions relating to the atomic nucleus are of vital importance to theoretical physics. Many theories relating to the nature of matter state that matter itself is ultimately unstable, that atoms should eventually disintegrate into energy. According to one theory, the proton is unstable and eventually decays. When the proton decays, the atom spontaneously disintegrates. Though inconclusive, scientists continue to test these theories. Yet, even the radically new ideas such as those embodied in string theory also call for proton decay.

A new mathematics is needed to incorporate some of the discoveries being made in theoretical and experimental physics. Sir Isaac Newton needed to invent calculus to describe his concept of gravity, and Albert Einstein transformed Riemannian geometry and tensor calculus to describe relativity. The new mathematics that will ultimately describe the new physics will have a ripple-down effect on not only physics but also other sciences and technologies as well.

Context

The social imperative to explore the universe has not always been linked to profit, a consideration which tends to drive many other technological efforts. There are a few direct benefits from particle physics, yet some of the most profound discoveries in theoretical physics result from the use of particle accelerators. From the earliest philosophical questions about the nature of matter to the latest discoveries in string theory, scientists have found that matter is both tangible and ephemeral, with both particle-like and quantum characteristics. The structure of the atomic nucleus has depicted a world of quantum reality which challenges logic and requires a new philosophy of nature.

Principal terms

BINDING ENERGY: the amount of external energy emitted in the binding process of the nucleons

EIGHTFOLD WAY: the classification scheme applied to hadrons, which led to the discovery of quarks

HADRON: a composite particle, such as a proton or neutron, made of quarks

MAGIC NUMBERS: the number associated with the quantity of nuclear particles required to form a stable shell or layer of protons or neutrons in an atomic nucleus

MESON: the quanta associated with the force binding protons and neutrons together in the nucleus; later called the pion

NUCLEON: the collective name for the proton and neutron inside the atomic nucleus bound together by the strong nuclear force

QUANTA: the designation of the constituents of atoms which are actually states of a field that may be described either as wave-like energy or discrete particles

QUARK: the quanta of which protons and neutrons are composed

SHELL MODELS: nuclear models which describe the basic nuclear properties of a nucleus; ordering the protons and neutrons inside the nucleus in their most stable configurations


Bibliography

Crease, Robert P., and Charles C. Mann. The Second Creation. Macmillan, 1986.

“DOE Explains...Nuclei.” Department of Energy, www.energy.gov/science/doe-explainsnuclei. Accessed 19 Apr. 2026.

“DOE Explains...Quarks and Gluons.” Department of Energy, www.energy.gov/science/doe-explainsquarks-and-gluons. Accessed 19 Apr. 2026.

Hawking, Stephen W. A Brief History of Time: From the Big Bang to Black Holes. Bantam, 1988.

Jolos, R. V., and E. A. Kolganova. “Structure of the Atomic Nucleus: Yesterday and Today.” Physics of Particles and Nuclei, vol. 55, no. 5, Oct. 2024, pp. 1209–21, doi:10.1134/S1063779624700916. Accessed 19 Apr. 2026.

Pagels, Heinz R. The Cosmic Code: Quantum Physics as the Language of Nature. Bantam, 1982.

Pagels, Heinz R. Perfect Symmetry: The Search for the Beginning of Time. Bantam, 1985.

“Planck Reveals an Almost Perfect Universe.” European Space Agency, 21 Mar. 2013, www.esa.int/Science_Exploration/Space_Science/Planck/Planck_reveals_an_almost_perfect_Universe. Accessed 19 Apr. 2026.

Sutton, Christine. The Particle Connection. Simon & Schuster, 1984.

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