Nucleus – Nucleons

All matter is composed of individual entities, called the elements. These elements differ in their fundamental physical and chemical properties—the atom. Initially, the atom was thought to be the “smallest” and “indivisible” particle of matter, but it is now known to possess a substructure and can be “divided” into smaller components.

Each atom consists of a small central core called the nucleus, where most of the atomic mass is located. The nucleus is surrounded by a “cloud” of electrons moving in orbits around it. The atomic radius (radius of the electronic orbits) is approximately 10−10 m, while the nucleus has a much smaller radius, about 10−15 m.

The nucleus

The properties of atoms depend on the constitution of their nuclei and the number and organisation of the orbital electrons.

The nucleus contains two fundamental particles: protons and neutrons. The protons possess a positive charge, while the neutrons have no charge. As an electron has a negative unit charge = 1.602 × 10−19 C and the proton has the same unit charge, the number of protons in the nucleus equals the number of electrons outside the nucleus of an electrically neutral atom. 

What are nucleons?

In physics, the components of the atomic nucleus, namely the protons and neutrons, are known as nucleons.

Hence, the nucleon is considered as a subatomic particle of the atomic nucleus. Each atomic nucleus can have one or more Nucleons. The nucleon comprises protons, which carry a positive net charge and the neutrons are electrically neutral. The mass is only about 0.13% less than the mass of the neutron. Hence, they can be viewed as two states of the same nucleon and form an isospin doublet, I = ½.

Discovery of nucleon 

Ernest Rutherford (1871–1937) discovered a nucleus in 1911. He proposed the name proton, from the Greek word Proto, for the lightest of all nuclei – the nucleus of the ordinary hydrogen atom.

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

Nucleon Number

An atom is specified by the formula AXZ,

where, X is the chemical symbol for the element

A is the nucleon number or mass number

and Z is the atomic number.

The nucleon number is also referred to as the mass number. It denotes the number of protons and neutrons, i.e., nucleons in the nucleus of an atom. The atomic number is denoted as the number of protons in the nucleus or electrons revolving around the nucleus. When an atom is represented in such a manner, it is also called a nuclide.

For example,

4He2 represents atoms or nuclei or nuclides of helium, in which 4 is the nucleon number and 2 is the atomic number.

The binding energy of a nucleon

The nucleus consists of protons and neutrons. However, the total mass of the nucleus is less than the sum of individual masses of the proton and the neutron. It is referred to as the mass defect of the nucleus. This difference lies in the measurement of binding energy per nucleon that tightly holds nucleons.

Binding energy is defined as the energy that has a nucleus together.

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

Nuclear binding energy = ∆mc²

Where, ∆m = mass of a nucleon

c = speed of light = 3.8 x 10⁸ m/s

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

Properties of nucleon

1. The nucleon comprises either a proton or a neutron without distinguishing between them. As they have about the same mass, protons and neutrons act as if they were identical to each other and differ only in their electric charge.
2. The total charge of a nucleon is +1 due to the proton, which carries a charge of +1 in units of the electron charge and the neutron, which is neutral, i.e., zero charges.
3. The spin of the nucleon is ½. This implies that they are fermions and like electrons, they are also subjected to Pauli’s exclusion principle. According to this, no more than one nucleon in the nucleus may occupy the same quantum state.
4. The nucleon’s isospin and spin quantum numbers have two states each, resulting in four combinations in total.

For example, consider an alpha particle composed of four nucleons occupying all four combinations. It consists of two protons and two neutrons in opposite spin, making its net nuclear spin-zero.

Anti-nucleon

Anti-nucleon is the corresponding antiparticle of the nucleon. It is formed by the corresponding antiparticles of protons and neutrons: the antiproton and the antineutron. They have the same mass and opposite charge as the proton and neutron, respectively and interact in the same way.
Conclusion 

The protons and the neutrons of an atomic nucleus constitute neutrons. They carry a net charge = zero and an isospin doublet = ½. The number of nucleons in a nucleus is known as its nucleon number or mass number, denoted along with the atomic number in a nuclide. Anti-nucleon is the corresponding antiparticle of a nucleon. Binding energy, defined as the energy that holds a nucleus together, results from the nucleus’s mass defect.