What is a Nucleon ?

"What is a nucleon ?" is a question that can be answered at various levels of detail and complexity, depending on the context.

This page describes nucleons at the the level required for high school chemistry (GCSE to A-Level in the UK).

Definitions of a Nucleon:

  1. A nucleon is the collective name for two important subatomic particles: neutrons and protons.
  2. A nucleon is one of either of the two types of subatomic particles (neutrons and protons) which are located in the nucleus of atoms.

Key Facts about Nucleons

  • Nucleons are subatomic particles located in the nucleus of atoms.
  • Nucleons have a relative mass of 1.
  • The total number of nucleons in the nucleus of an atom is called the Mass Number of that atom.
    Another name for the Mass Number is the Nucleon Number.
  • The charge of a nucleon depends on which type of nucleon it is: Protons have a charge of +1. Neutrons have a charge of 0 (zero ; no charge ; "charge neutral").

Questions about Nucleons

  • What does the number of nucleons in an atom say / determine about that atom ?
    The number of protons in an atom determines which element that particular atom is an atom of, and hence its properties - especially its chemical properties. The physical properties of the atom are also influenced by the number of neutrons in the atom, i.e. the particular isotope that atom is an example of.)
  • What holds nucleons (and so the nuclei of atoms) together ?
    The force, which may also be called an "attraction" or "interaction" between two or more nucleons is called the internucleon interaction, internucleon interactions or nuclear force. It is an extremely powerful interaction which scientists explain by saying that it is due to "the strong interaction", a term used in particle physics to refer to one of the four fundamental interactions of nature. This is a very interesting topic beyond the scope of most school chemistry courses.
  • What are nucleons made of ?
    What are their constituent parts ?

    Until about fifty years ago (1960s) nucleons were thought to be elementary particles.
    However, they are now known to be composite particles which means that they are the result of even smaller particles attached together. Each nucleon is composed of three quarks bound together by the "strong interaction".
  • How are nucleons classified (in terms of particle physics) ?
    This more advanced information is not usually required at school-level chemistry.
    The proton and neutron are both baryons and both fermions (not bosons).
    According to particle physics, protons and neutrons form an isospin doublet (I = 1⁄2), which explains why their masses are so similar: Neutrons are approx. 0.1% heavier than protons.
  • What is the antiparticle of a nucleon ?
    There is no such particle as a generic 'antinucleon'.
    However, both types of nucleons (neutrons and protons) have corresponding antiparticles. They are called the antineutron and the antiproton respectively. The antineutron and antiproton have the same mass and the opposite charge as the neutron and proton respectively, so an antineutron has a charge of 0 (zero) and an antiproton has a charge of -1.

Why do I need to know about Nucleons ?

Although it is useful to understand the word 'nucleon', 'nucleons' is not usually a topic in school chemistry.

Instead, consider why it is useful to know about and understand protons and neutrons. They are also important in the study of particle physics, nuclear physics (and chemistry), radiation and radioactivity.

See also the pages about What is an atom ?, What is a Proton and What is a neutron ?

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