Proton

Lore & Background

A proton is a stable subatomic particle carrying a positive electric charge of +1 elementary charge. Its mass is slightly less than that of a neutron and roughly 1836 times that of an electron. Protons and neutrons, each with a mass near one dalton, are collectively called nucleons. One or more protons reside in the nucleus of every atom, providing the electrostatic central force that binds atomic electrons. The number of protons in a nucleus defines an element, known as its atomic number, which determines the element’s identity and chemical properties. The word “proton” is Greek for “first,” and the name was given to the hydrogen nucleus by Ernest Rutherford in 1920, following his discovery that hydrogen nuclei could be extracted from nitrogen atoms via collisions. Though once considered elementary, protons are now understood in the Standard Model as composite particles—hadrons containing three valence quarks: two up quarks (each charge +⅔ e) and one down quark (charge −⅓ e). The rest masses of these quarks account for only about 1% of the proton’s mass; the remainder comes from quantum chromodynamics binding energy, including quark kinetic energy and gluon field energy. The proton’s charge radius is around 0.8 femtometers, though measurements yield slightly different values. At sufficiently low temperatures, free protons bind electrons in any matter they traverse. Free protons are used in accelerators for proton therapy and particle physics experiments, notably the Large Hadron Collider. Protons are spin-½ fermions, classified as baryons, and are held together by the strong force mediated by gluons, with a positive charge distribution that decays approximately exponentially. The nucleus of ordinary hydrogen is a single proton; deuterium and tritium nuclei contain one proton bound to one or two neutrons, respectively, while all other atomic nuclei consist of two or more protons with various neutrons.

Reader's Guide

The proton is fundamental to the structure of matter. As the nucleus of the most common hydrogen isotope, and a constituent of all other atomic nuclei, the proton defines each element through its atomic number. Although originally considered elementary, the modern Standard Model shows protons are composite particles containing three valence quarks held together by gluons via the strong force. The proton's stability is a key assumption in physics; while spontaneous decay has never been observed, some grand unified theories predict lifetimes between 10^31 and 10^36 years, with experimental lower bounds exceeding 10^34 years for certain decay modes. Free protons occur naturally in cosmic rays, plasmas, and thunderstorms, and are used in accelerators for proton therapy and particle physics experiments, such as the Large Hadron Collider. The proton's role in binding electrons and forming nuclei underpins chemistry and the structure of the universe.

Did You Know?

• Protons are composed of two up quarks of charge +2/3 e each and one down quark of charge -1/3 e.
• The rest masses of quarks contribute only about 1% of a proton's mass; the remainder is due to quantum chromodynamics binding energy.
• Free protons make up 90% of cosmic rays, which propagate through the interstellar medium.