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A proton is a subatomic particle with a mass of 1.672 621 637 × 10−27 kg,[1] a charge equal to the elementary charge of 1.602 176 487 × 10−19 coulomb[2] and a spin of 1/2. Protons are one of the three fermions which make up most of the matter we deal with every day. They, along with neutrons, form the nucleus of every atom. They were briefly believed to be a fundamental particle but the rapid discovery of other particles in the middle of the twentieth century disproved this theory. They are now known to be composed of two up quarks having charge +2/3e each, and one down quark, having charge -1/3e, resulting in a net charge of +1e. Protons are the lightest and most stable member of the baryon family.


Protons, along with neutrons and electrons, are the three building blocks of atoms, and are directly responsible for nearly 50% of the matter humans encounter on a daily basis.


The proton was first proposed by Ernest Rutherford who discovered that there was a very dense positively charged center to an atom. For a while protons and neutrons were supposed to be the most fundamental particles (instead of atoms), but in 1968 experiments held at the Stanford Linear Accelerator Center showed that they both consisted of the still smaller particles now known as quarks.[3]


According to the standard model, protons are composed of three quarks: two up quarks and one down quark. These quarks combine to give it its charge and spin. The +1 charge of the proton results from the combined charges of the two up quarks (+2/3 each) and one down quark (-1/3). The three quarks are held together by the strong force; the energy of these bonds is responsible for much of a proton's mass. Because a proton consists of three quarks bound together, it is a baryon. The neutron, also a baryon, consists of two down quarks and one up quark, and thus has no net charge.

The proton g-factor is:[4]

corresponding to a proton magnetic moment of about 2.79 nuclear magnetons (μN):[5]

So far, a theoretical calculation of the magnetic moment of the proton in terms of quarks exchanging gluons is a work in progress, with the present estimate as 2.73 nuclear magnetons.[6]


When a proton gains one electron, one atom of the element hydrogen (1H) is formed. Every chemical element is defined by the number of protons present in each atom of that element. Thus, the first six chemical elements, hydrogen, helium, lithium, boron, nitrogen and carbon have 1, 2, 3, 4, 5 and 6 protons, respectively, as well as electrons and neutrons. The number of neutrons present in a chemical element determines which isotope form the element is. Thus, a proton can be combined with one electron to form hydrogen (1H), and the addition of one or two neutrons to the hydrogen atom forms the isotopes deuterium (2H) or tritium (3H), respectively. In nuclear chemistry reactions, high energy protons, travelling at near the speed of light, can be smashed into a target element to form new chemical elements, which may or may not be stable. It was the observation of decay products from such experiments in high energy colliders, such as the new Large Hadron Collider, that promulgated and verified quarks and the standard model.


  1. Proton mass. The NIST reference on constants, units and uncertainty. National Institute of Standards and Technology. Retrieved on 2011-03-28.
  2. Elementary charge. The NIST reference on constants, units and uncertainty. National Institute of Standards and Technology. Retrieved on 2011-03-28.
  3. Greene, B. The Elegant Universe, 1999.
  4. Proton g factor. Fundamental physical constants. NIST. Retrieved on 2011-03-28.
  5. Nuclear magneton. Fundamental physical constants. NIST. Retrieved on 2011-03-28.
  6. See, for example, Brian Martin (2009). “Table 3.5”, Nuclear and Particle Physics: An Introduction, 2nd ed. Wiley, p. 104. ISBN 0470742747.  and Steven D. Bass (2008). “Chapter 1: Introduction”, The spin structure of the proton. World Scientific, pp. 1 ff. ISBN 9812709460.