Proton

Lore & Background

The concept of a hydrogen-like particle as a constituent of other atoms was developed over a long period. Later that year J. J. Thomson determined a value for the electric charge and showed canal rays included material with charge-to-mass ratio consistent with the hydrogen ion.

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.