Nucleons | Definition, Discovery, Types, Properties


A nucleon can be either a proton or a neutron. Its role is considered as a component of an atomic nucleus. The number of nucleons in a nucleus describes the mass number of an isotope.

Definition of Nucleons

  • A nucleon is one of the subatomic particles of the nucleus of an atom. Each atomic nucleus can contain one or more Nucleons, and one or more electrons surround these nucleons. 
  • Nucleons occupy a very small space within the nucleus. Each atom is made up of nucleons divided into particles viz: electrons, protons, and neutrons that orbit the nucleus. 
  • Protons (positively charged) and neutrons (uncharged) behave identically under the influence of the short-range nuclear force, both in the way they are bound in nuclei and in the way they are scattered by each other. This strong interaction is independent of electric charge. Unstable subatomic particles heavier than nucleons (hyperons and baryon resonances) have a nucleon among their final decay products; the nucleon is thus the baryon ground state. The antinucleons include the antiproton and the antineutron.

Definition of Antinucleons


Both nucleons have corresponding antiparticles: the antiproton and the antineutron, which have the same mass and opposite charge as the proton and neutron, respectively, and they interact in the same way.

In particular, antinucleons can bind into an “antinucleus.” So far, scientists have created antideuterium and antihelium-3 nuclei.

Discovery of Nucleons

Discovery of Nucleons

After Ernest Rutherford (1871–1937) discovered the atomic nucleus in 1911, he proposed the name proton for the very lightest of all nuclei: the nucleus of the ordinary hydrogen atom. Proto– is Greek for first.

In 1932, when James Chadwick (1891–1974) discovered another particle in the nucleus that was very similar to the positive proton except that it was electrically neutral, it was natural for him to call it a neutron. It was then equally natural to call nuclear particles nucleons, especially when nuclear theory began to treat the proton and the neutron as two different states of the same fundamental particle.

Types of Nucleons

Types of Nucleons

Primarily, there are two types of nucleons viz: protons and neutrons. 

  1. A proton carries a positive electric charge.
  2. A neutron has a neutral electric charge, which means it bears no electric charge. 

These two particles reside in the atom’s nucleus and generate a positive charge because the neutron has no charge at all.

We must note that protons and neutrons are the only best-known components of atomic nuclei so far. These two particles can be found independently, not being part of the larger nucleus (residing inside it). 

Binding Energy of a Nucleon

Binding Energy of a Nucleon

As we know, a nucleus consists of neutrons and protons; however, the mass of the nucleus is less than the sum of individual masses of the proton and the neutron. The difference lies in the measurement of binding energy per nucleon that tightly holds nucleons.

Einstein’s equation can determine the binding energy. The relationship is as follows:

Nuclear binding energy = ∆mc²


∆m = mass of a nucleon

c = speed of light, i.e., 3.8 x 10⁸ m/s

Since Δm for alpha particles is 0.0304 u (unit), this gives us the binding energy of a nucleon as 28.3 MeV.

Nucleon Interactions in the Nucleus

Nucleon Interactions in the Nucleus

Protons repel each other because they have electric charges, but all nucleons attract each other due to the strong interaction. The strong interaction is more powerful than electric attraction or repulsion, but it acts over a very short range.

When nucleons attract one another, they bind via the strong nuclear force. As in chemical bond formation between electrons, the binding of nucleons also releases energy called nuclear binding energy.

One consequence of nuclear binding is that the sum of the masses of the protons and neutrons used to make an atomic nucleus is greater than the mass of the resulting nucleus. This is called the mass defect. Also, breaking a proton or neutron free of the nucleus requires an input of energy.

Atom diagrams typically depict protons and neutrons as separate spheres randomly crammed together to form a nucleus. In reality, nucleons are partially delocalized. In fact, particle physicists consider protons and neutrons in the nucleus to be two nucleon states rather than separate entities. The two states form an isospin doublet. Neutrons can be converted into protons, and protons can be converted into neutrons.

Properties of Nucleons

Properties of Nucleons

There are some properties of nucleons:

  1. We use the word nucleon when we want to refer to either a proton or a neutron without distinguishing between them. Since they have about the same mass, protons and neutrons act as if they were identical particles that differ only in their electric charge.
  2. The proton carries a charge of +1 (in units of the electron charge), and the neutron is neutral (zero charges). Protons and neutrons are themselves made up of quarks, as this schematic picture indicates. The only difference between a proton and a neutron in the quark model is that an up quark has been replaced with a down quark. Quarks are held together by the strong force, or equivalently, by gluons, which mediate the strong force at the quark level.     
  3. The spin of the nucleon is 1/2, which means that they are fermions and, like electrons, are subject to the Pauli exclusion principle: no more than one nucleon, e.g., in an atomic nucleus, may occupy the same quantum state.
  4. The isospin and spin quantum numbers of the nucleon have two states each, resulting in four combinations in total. An alpha particle is composed of four nucleons occupying all four combinations. Namely, it has two protons (having opposite spin) and two neutrons (also having opposite spin), and its net nuclear spin is zero. By Pauli exclusion, larger nuclei constituent nucleons are compelled to have relative motion, which may also contribute to nuclear spin via the orbital quantum number. They spread out into nuclear shells analogous to electron shells known from chemistry

Stability of Nucleons

Stability of Nucleons

A neutron in a free state is an unstable particle with a half-life of around ten minutes. It undergoes β− decay (radioactive decay) by turning into a proton while emitting an electron and an electron antineutrino. A proton by itself is thought to be stable, or at least its lifetime is too long to measure.                    

                              Inside a nucleus, on the other hand, combined protons and neutrons (nucleons) can be stable or unstable depending on the nuclide or nuclear species.

Inside some nuclides, a neutron can turn into a proton (producing other particles) as described above; the reverse can happen inside other nuclides, where a proton turns into a neutron (producing other particles) through β+ decay or electron capture. And inside still other nuclides, both protons, and neutrons are stable and do not change form.

Importance of Nucleons

Importance of Nucleons

Because of this close packing, the internal structure of protons and neutrons (collectively called nucleons) plays an important role in nuclear physics: it influences and can, in turn, be modified by the distribution, motion, and interactions of nucleons inside nuclear matter.

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