What is a Neutron ?

"What is a neutron ?" is a question that can be answered at various levels of detail and complexity, depending on the context. This page describes neutrons at the the level required for high school chemistry (GCSE to A-Level in the UK).

Definition of a Neutron:

A neutron is a subatomic particle found in the nucleus of atoms that differs from the other subatomic particles (called protons) in the nucleus of atoms because neutrons have no (zero) charge whereas each proton has a positive charge of +1.

The number of neutrons in an atom determines which isotope of the element (which is determined by the number of protons in the atom), that particular atom is an example of.

The number of neutrons in an atom determines not only the isotope but also, therefore, its physical properties e.g. in general the greater the mass of its individual particles (due to more neutrons in their nuclei) the higher the melting and boiling points of a material tend to be.

Neutrons that do not form part of the nucleus of an atom are called 'free neutrons' and are unstable but important for the understanding of aspects of nuclear power production and experimental particle physics.

Key Facts about Neutrons

  • Neutrons are located in the nucleus of atoms, together with the protons in the atom.
  • Neutrons have zero charge.
    Another way to state the same thing is to say that neutrons are 'charge neutral'.
  • Neutrons have a relative mass of 1, which is the same as the relative mass of protons.
  • The number of neutrons in an atom may be calculated from the atomic number of that atom (element) and the mass number of that atom (isotope).

    Number of Neutrons (in an atom) = Mass Number (A) - Atomic Number (Z)
  • Protons and neutrons are collectively referred to as nucleons.
  • Neutrons are responsible for about half the weight of conventional matter by volume.
  • Neutrons play important roles in nuclear power production.

Questions about Neutrons

  • When was the neutron first discovered ? and Who discovered the neutron ?
    The neutron was discovered in 1932 by James Chadwick who was working at Cambridge University, England.
    Chadwick had been working with Ernest Rutherford who had found evidence for the proton in 1919 and then (in 1920) suggested that a particle such as the neutron might exist because the disparity between the atomic number of an atom and its atomic mass could be explained by the existence of a neutrally charged particle within the atomic nucleus.
    In 1930 Viktor Ambartsumian and Dmitri Ivanenko, who were working in the USSR showed that, contrary to the mainstream scienticific view of the day, the nucleus of an atom does not consist of protons and electrons, and that some neutral particles must be present in the nuclei of atoms (in addition to protons). Then Walther Bothe and Herbert Becker, working in Germany in 1931, found that if the very energetic alpha particles emitted from the radioactive element polonium struck light (that is, having relatively low atomic mass) elements, specifically beryllium, boron, or lithium, an unusually penetrating radiation was produced. Initially this new and interesting form of radiation was thought to be gamma radiation even though it was more penetrating than any previously reported gamma rays. The next development in the path to the discovery of the neutron was reported in 1932 by Irène Joliot-Curie and Frédéric Joliot who were working in Paris. They found that if this unknown radiation (then thought by many to be a form of gamma radiation) interacted with paraffin or any other hydrogen-containing compound, it ejected extremely high-energy protons. Detailed analysis of data from their experiements made it increasingly difficult to explain the 'new' radiation as gamma rays. Finally, in 1932, James Chadwick conducted a series of experiments that demonstrated that the radiation in question could not be gamma rays. Instead, he suggested that it consisted of uncharged particles of approximately the mass of the proton, then performed further experiments that proved his theory correct. The uncharged particles whose existance was established by Chadwick's experiements were called 'neutrons'.
  • Where does the word 'neutron' come from ?
    The word 'neutron' can be considered in two parts, neutral and -on.

    The Latin (language) is often used to explain the derivation (also known as 'etymology') of the word 'neutron':

    The Latin word 'ne'

    Latin word 'neuter' (meaning 'neither')

    Latin word 'neutralis'

    The (Old) French word 'neutral'

    English word 'neutral', which has several meanings in different contexts, e.g. politically neutral, neither acid or alkali, and (in this context) neither positively nor negatively charged, i.e. charge neutral.

    The English suffix '-on' is consistent with the naming of the other sub-atomic particles already accepted at the time of discovery of the neutron, namely the proton and electron.

  • What are neutrons made / composed of ?
    (This is more advanced information that may not be required at school-level chemistry.)
    Neutrons consist of three fundamental particles, two down quarks and one up quark.
    A neutron is therefore classified as a baryon.
  • Is there any matter that doesn't include neutrons ?
    The only example of stable matter that does not include neutrons is the hydrogen atom.
  • What is neutronium ? Is it a neutron ?
    Even though 'free' neutrons (i.e., neutrons not located in and so part of the nucleus of an atom) are unstable and are not a chemical element, the 'free neutron' is sometimes included in tables of nuclides. Where included it is generally considered to have an atomic number of zero and a mass number of one. In this context a 'free neutron' is sometimes referred to as 'neutronium', which is a term (word) proposed by Andreas von Antropoff in 1926 - which was before the discovery of the neutron and intended to refer to the conjectured 'element of atomic number zero' (Z=0) that he placed at the top of the periodic table.
  • What is the opposite of a neutron ?
    (This is beyond the scope of most school chemistry courses.)
    The antineutron is the antiparticle of the neutron discovered in 1956, a year after the antiproton was discovered. Antineutrons form part of more advanced science topics and, in common with all antimatter, they explode on contact with 'normal matter'.

Why do I need to know about Neutrons ?

It is important to know about neutrons (for school level, e.g. GCSE Chemistry) because neutrons are important for several key topics necessary for the understanding of basic chemistry. These topics include the structure of atoms, isotopes and relative atomic mass.

See also the pages about What is an atom ?, What is a proton ? and What is an electron ?

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