One or more protons are present in thenucleusof everyatom; they are a necessary part of the nucleus. The number of protons in the nucleus is the defining property of an element, and is referred to as theatomic number(represented by the symbolZ). Since eachelementhas a unique number of protons, each element has its own unique atomic number.

The wordprotonis Greek for "first", and this name was given to the hydrogen nucleus byErnest Rutherfordin 1920. In previous years, Rutherford had discovered that thehydrogennucleus (known to be the lightest nucleus) could be extracted from the nuclei ofnitrogenby atomic collisions.[3]Protons were therefore a candidate to be afundamental particle, and hence a building block of nitrogen and all other heavier atomic nuclei.

Although protons were originally considered fundamental orelementary particles, in the modernStandard Modelofparticle physics, protons are classified ashadrons, likeneutrons, the othernucleon. Protons are composite particles composed of threevalence quarks: twoup quarksof charge +2/3eand onedown quarkof charge –1/3e. Therest massesof quarks contribute only about 1% of a proton's mass.[4]The remainder of a proton's mass is due toquantum chromodynamics binding energy, which includes thekinetic energyof the quarks and the energy of thegluonfields that bind the quarks together. Because protons are not fundamental particles, they possess a measurable size; the root mean squarecharge radiusof a proton is about 0.84–0.87fm(or0.84×10−15to0.87×10−15m).[5][6]In 2019, two different studies, using different techniques, have found the radius of the proton to be 0.833 fm, with an uncertainty of ±0.010 fm.[7][8]

At sufficiently low temperatures, free protons will bind toelectrons. However, the character of such bound protons does not change, and they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, until it is captured by theelectron cloudof an atom. The result is a protonated atom, which is achemical compoundof hydrogen. In vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutralhydrogen atom, which is chemically afree radical. Such "free hydrogen atoms" tend to react chemically with many other types of atoms at sufficiently low energies. When free hydrogen atoms react with each other, they form neutral hydrogen molecules (H2), which are the most common molecular component ofmolecular cloudsininterstellar space.