Stable subatomic particle (one of the baryon s) 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 neutron s. 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 accelerator s to produce and study nuclear reactions. They are the chief constituent of primary cosmic ray s and are among the products of radioactive decay (see radioactivity ) and nuclear reactions.