What is a Proton ?

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

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

Definitions of a Nucleon:

  1. A proton is a subatomic particle found in the nucleus of atoms that differs from the other subatomic particles (called 'neutrons') in the nucleus of most atoms because each proton has a positive charge of +1 (as opposed to neutrons, which have no charge).
  2. A proton may also be defined as the hydrogen ion, H+.
    While it is true to say that the hydrogen ion (H+) is a proton, the definition above is more general and is usually the answer expected in first-level courses in school chemistry. That is why some texts may say that a proton has a "second identity" as the hydrogen ion, H+.

Key Facts about Protons

  • Protons are located in the nucleus of atoms, together with any neutrons also present in the atom.
  • Protons have a positive (+1) charge, which balances the negative (-1) charge of the electrons that surround the nuclei of atoms.
  • Protons have a relative mass of 1, which is the same as the relative mass of neutrons.
  • The number of protons in an atom is called the 'Atomic Number' of that atom and is sometimes referred to as 'Z'. This is important because it defines the type of atom, i.e. which element it is an atom of.
  • Protons and neutrons are collectively referred to as nucleons.

Questions about Protons

  • When was the proton first discovered ? and
    Who discovered the proton ?
    It is difficult to state an exact date for the discovery of the proton because this resulted from a series of experiments that took place over a long period of time. In 1815, William Prout suggested that all atoms were composed of hydrogen atoms, a theory based on simple consideration of early values of atomic weights - but later disproved by measurements. In 1886 Eugen Goldstein discovered 'canal rays' (also known as 'anode rays') which he showed to be positively charged particles (ions) produced from gases - but those from different gases had different values of charge-to-mass ratio (e/m), so they could not be identified with a single particle. Much research was taking place and the scientists of the day had realised that there must be positively charged particles in or associated with atoms following the discovery of the (negatively charged) electron by J. J. Thomson in 1897.
    In 1911 Ernest Rutherford discovered the atomic nucleus then Antonius van den Broek suggested that the place of each element in the periodic table (i.e. its atomic number) is equal to its nuclear charge - that is, the charge of the nucleus, (remember that the charge of a complete atom is zero because the charge of the electrons balances that of the protons). This was confirmed experimentally by Henry Moseley in 1913 using X-ray spectra. In 1918-19 Rutherford fired alpha particles (which are effectively helium nuclei - so, helium atoms but without their electrons) into nitrogen gas. His detectors indicated the characteristic signature of hydrogen nuclei being produced. Rutherford concluded that the hydrogen nuclei could have only have come from the nitrogen gas - which led to the theory that the nucleus of a hydrogen atom was an elementary particle, which was later called a 'proton'. It was also postulated that such protons existed in the nuclei of all atoms. Hence Rutherford is generally said to have discovered the proton in 1919.
  • What are protons composed of ?
    (This is more advanced information that may not be required at school-level chemistry.)
    Protons consist of three fundamental particles, two up quarks and one down quark.
  • Is there any matter that doesn't include protons ?
    Yes, but only in special circumstances that aren't usually included in school chemistry.
    Examples include neutron stars and powerful particle accelerators.
  • What is the opposite of a proton ?
    This question is not likely to be asked as part of school chemistry tests. If you think this question has been asked on your school chemistry exam look carefully ... perhaps you are being asked "Which subatomic particle has the opposite charge to that of a proton?" The answer to that question is, of course, an electron.
    However, for those really interested in the 'opposite' of a proton, try researching 'antiproton'.
    The antiproton was discovered in 1955. Unlike a proton (which has a charge of +1), an antiproton has a charge of -1. It is different from an electron because although an electron also has a charge of -1, electrons have a tiny relative mass whereas antiprotons have a relative mass of 1, which is the same as the relative mass of protons. Antiprotons form part of more advanced science topics and, in common with all antimatter, they explode on contact with 'normal matter'.
  • How are protons used in experimental nuclear physics ?
    This is a big question to which there a long answers! In short, protons are sometimes accelerated to high speeds (up to significant proportions of the speed of light). Such 'ballistic' protons have led to many interesting discoveries in particle physics / chemistry. One reason why protons are so important for such research is that, unlike neutrons, protons are stable outside of an atomic nucleus, which makes it possible to use them in certain experiments.

Why do I need to know about Protons ?

It is important to know about protons (for school level, e.g. GCSE Chemistry) because protons 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 neutron ? and What is an electron ?

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