En física, el protón (del griego πρῶτον, prōton 'primero') es una partícula subatómica con una carga eléctrica elemental positiva 1 (1,6 × 10-19 C), igual en valor absoluto y de signo contrario a la del electrón, y una masa 1836 veces superior a la de un electrón. Experimentalmente, se observa el protón como estable, con un límite inferior en su vida media de unos 1035 años, aunque algunas teorías predicen que el protón puede desintegrarse en otras partículas. Originalmente se pensó que el protón era una partícula elemental pero desde la década de 1970 existe una evidencia sólida de que es una partícula compuesta. Para la cromodinámica cuántica el protón es una partícula formada por la unión estable de tres quarks. El protón y el neutrón, en conjunto, se conocen como nucleones, ya que conforman el núcleo de los átomos. En un átomo, el número de protones en el núcleo determina las propiedades químicas del átomo y qué elemento químico es. El núcleo del isótopo más común del átomo de hidrógeno (también el átomo estable más simple posible) está formado por un único protón. Al tener igual carga, los protones se repelen entre sí. Sin embargo, pueden estar agrupados por la acción de la fuerza nuclear fuerte, que a ciertas distancias es superior a la repulsión de la fuerza electromagnética. No obstante, cuando el átomo es grande (como los átomos de Uranio), la repulsión electromagnética puede desintegrarlo progresivamente.

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A proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collectively referred to as "nucleons". One or more protons are present in the nucleus of every atom. 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 the atomic number (represented by the symbol Z). Since each element has a unique number of protons, each element has its own unique atomic number. The word proton is Greek for "first", and this name was given to the hydrogen nucleus by Ernest Rutherford in 1920. In previous years Rutherford had discovered that the hydrogen nucleus (known to be the lightest nucleus) could be extracted from the nuclei of nitrogen by collision. Protons were therefore a candidate to be a fundamental particle and a building block of nitrogen and all other heavier atomic nuclei. In the modern Standard Model of particle physics, protons are hadrons, and like neutrons, the other nucleon (particle present in atomic nuclei), are composed of three quarks. Although protons were originally considered fundamental or elementary particles, they are now known to be composed of three valence quarks: two up quarks and one down quark. The rest masses of quarks contribute only about 1% of a proton's mass, however. The remainder of a proton's mass is due to quantum chromodynamics binding energy, which includes the kinetic energy of the quarks and the energy of the gluon fields that bind the quarks together. Because protons are not fundamental particles, they possess a physical size, though not a definite one; the root mean square charge radius of a proton is about 0.84–0.87 fm or 0.84×10−15 to 0.87×10−15 m. At sufficiently low temperatures, free protons will bind to electrons. 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 the electron cloud of an atom. The result is a protonated atom, which is a chemical compound of hydrogen. In vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral hydrogen atom, which is chemically a free 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 of molecular clouds in interstellar space. Such molecules of hydrogen on Earth may then serve (among many other uses) as a convenient source of protons for accelerators (as used in proton therapy) and other hadron particle physics experiments that require protons to accelerate, with the most powerful and noted example being the Large Hadron Collider.

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