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Stable subatomic particle (one of the baryons) with a unit of positive electric charge and a mass 1,836 times that of the electron. Protons are found in the atomic nucleus along with neutrons. For every nucleus of a given element, the number of protons is always the same; this number is the element's atomic number. A single proton is the nucleus of an atom of ordinary hydrogen; as such, it is identical to the hydrogen ion (H+). Protons have antimatter counterparts (antiprotons), with the same mass but a negative charge. Protons are used as projectiles in particle accelerators to produce and study nuclear reactions. They are the chief constituent of primary cosmic rays and are among the products of radioactive decay (seeradioactivity) and nuclear reactions.
This entry comes from Encyclopædia Britannica Concise. For the full entry on proton, visit Britannica.com.