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Nucleon – Definition, Properties and Nuclear Stability

Mar 15, 2023

You might already know protons, neutrons, and electrons, but do you know what nucleons are? Protons are the subatomic particles that have a positive charge, while neutrons are neutrally charged subatomic particles. What are nucleons? Nucleons are the subatomic particles found inside the nucleus of an atom. The mass number of an isotope of an element is determined by the number of nucleons in its nucleus. This article aims to explain in detail the nucleons and acquaint you with their common properties. 

What is a Nucleon?

What is a nucleon? Here is the nucleon definition. A nucleon is a particle that lies inside the nucleus of an atom. An atom can have one or more than one nucleons in its nucleus, which are surrounded by one or multiple electrons. These nucleons occupy a tiny space inside the nucleus.


Every atom comprises nucleons that are further categorized into protons, neutrons, and electrons that revolve around the nucleus. You can think of it as a mini solar system, with the electrons revolving around a central star (here, the nucleus) made up of nucleons.

The atomic nuclei contain protons and neutrons, which are very well known by many students. They can even exist on their own, not being a part of the larger nucleus. For example, the nucleus of a hydrogen-1 atom contains only one proton, and it is the most commonly found isotope of hydrogen. 


Binding Energy Per Nucleon

There are both protons and neutrons in the nucleus, but their mass is less than the sum of their individual masses. The difference is due to the binding energy per nucleon that holds them together. The following relationship given by Einstein can determine it:

Nuclear binding energy =Δmc2


For alpha particles, Δm = 0.0304 u. Its binding energy is 28.3 MeV.

Nucleon Properties

According to the nucleon definition, a neutron is independently an unstable particle. It occurs in nuclear reactions and is employed for scientific analysis. There are three quarks in both the neutron and proton. The proton consists of one down quark (the second lightest of all quarks after the up quark) and two up quarks ( a type of elementary particle and the lightest quark). A neutron consists of two down quarks and one up quark. 


As nucleons cannot exist independently, they are an integral part of the atomic nucleus. They are composite particles made up of three quarks that have a strong interaction binding them together. The nucleons are held with a strong force by the nucleus of the atom. If the force is broken, a lot of power is generated.

This power is called nuclear energy and is employed in the making of nuclear bombs. The nucleons of certain radioactive materials like uranium are very dangerous as they can spread alpha radiation in a matter of a few seconds. 


There are other types of unstable subatomic particles that have an increased mass than that of nucleons. They are called baryons and hyperons and form nucleons as the final products of their decay. There are two types of antinucleons— antineutrons and antiprotons. 

Nucleon Number

What is a nucleon number according to the nucleon definition? As discussed earlier, the atomic mass number of an atom is determined by the total number of protons and neutrons it is made up of. It is denoted by the symbol A. The atomic mass number plus the number of neutrons gives its nucleon number.


The formula, therefore, is N+Z=A. Another term to mention here is the neutron excess which is the difference between the neutron number and the atomic number. The formula for neutron excess is:

D = N – Z = A – 2Z.


Every chemical element’s isotope has a different nucleon number. It is written as a superscript to the left of the symbol of an element or after its name. Suppose carbon 12 is the most common isotope of carbon. Amu is the unit of mass measure, where one atomic mass unit equals 1.67 x 10-23 grams. The commonest isotope of carbon, carbon-12, has been assigned an atomic mass unit of 12. 

1u = 1.67 x 10-23 grams.

Unified Atomic Mass Unit:

It can be defined as the weight of a neutron or a proton (a nucleon) and is numerically equal to 1g/mol.

As a carbon atom is used as a standard, its atomic mass is equal to exactly 12 amu. Usually, the isotopic mass is within 0.1 u of the mass number and differs for all the isotopes. For instance, the isotope of copper 63 Cu’s mass number is 63. In the nuclear ground state, its isotopic mass is 62.91367 u.

Mass Defect

Mass defect is the difference between the isotopic mass and mass number of an element, and there are two reasons responsible for it. 

The proton is slightly lighter than a neutron, which increases the mass of nuclei that have more neutrons as compared to protons relative to the scale of the atomic mass unit on the basis of 12C having protons and neutrons in equal numbers. 

The nuclei also have different nuclear binding energies, with the nucleus with greater binding energy having lower total energy and a decreased mass. According to the mass-energy relationship of Einstein, E= mc2. 63Cu has a lesser atomic mass than 63.

The rest of an atomic nucleus was measurably found to be smaller than the combined value of the individual rest masses of the protons, electrons, and neutrons that constitute it. In the closed system, nucleon mass is not considered unchangeable. 

Conservation of Nucleon Number in Nuclear Reactions

In general, the nucleon number is not conserved in nuclear reactions. Many conservation laws are applied in analyzing nuclear reactions. They are dependent on the classical laws of angular momentum, charge, momentum, and energy. There are some additional laws of conservation not anticipated by classical physics, like the baryon number, electric charge, and lepton number. 

Conservation of Baryon Number

Physicists define the Baryon number instead of the conservation of nuclear numbers. The baryon number is a conserved quantum number in all particle reactions. An example of the generalization of nucleon numbers is the baryon number. It is conserved in non-relativistic nuclear decays and reactions.

According to the law of conservation of baryon number, the combination of the baryon number of all incoming particles equals the combination of all the particles that are produced as a result of the reaction.

Number of Nucleons and Nuclear Stability

Nuclear Chart:

It is a chart presenting the nuclear structure and decay properties of all nuclides known through a user-friendly interface. 

Segre Chart:

A function of all the known nuclides is shown in this chart as a function of their neutron and atomic numbers. This chart shows that there are more neutrons than protons for nuclides that are higher than 20. The function of these extra neutrons is to stabilize heavier nuclei by acting like nuclear glue. 

The stability of a nucleus depends on the neutrons as they attract each other as well as other protons. This helps to alleviate the electrical repulsion between the protons. Resultantly, as there is an increase in the number of protons, we need an increasing ratio of neutrons to protons to get a stable nucleus.

If the number of neutrons for protons is too many or too less, the nucleus that results is unstable and undergoes radioactive decay. Various radioactive decay pathways are followed for decay by unstable isotopes, especially alpha or beta decay and electron capture.

There are many other known types of decay like neutron emission or spontaneous fission. Gamma radiation may be subsequently emitted in all of these decay pathways. Pure alpha decay and beta decay are not commonly seen. 


Hope that the article helps you have a basic understanding of what a nucleon is, what its properties are, and what nucleon number and nucleon mass is. As it is a very important concept in Physics, having knowledge of it is important. 

Frequently Asked Questions

1. What are antinucleons?

Antinucleons comprise the antineutron and the antiproton. These are a type of sub-atomic particle made up of quarks and, therefore, react through an agency of a strong force.

2. Can nucleons move?

A nucleus can move inside the nucleus under the influence of certain forces in a complex way. A nucleus model called the shell model depicts the nucleons as revolving in orbits around each other in a way similar to that of electrons that move in an orbit around the nucleus in the Bohr atom. 

3. What determines the stability of an isotope?

The stability of an isotope is identified by the concept of nuclear stability, which is required to determine the ratio of neutrons to protons to get to know about the stability of an isotope. The ratio of neutron/proton can be used to determine an isotope’s stability. Additionally, there is a Segre chart that can be used to understand this concept. As discussed above, it depicts a plot of the known nuclides as a function of their neutron and atomic numbers. 



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