The proton is a stable subatomic particle with a positive elementary electric charge, found in the nucleus of every atom. Along with neutrons, protons make up the atomic nucleus, with the number of protons in an atom's nucleus defining the atom's atomic number (), and consequently, the chemical element to which it belongs. For instance, all hydrogen atoms contain one proton, while all helium atoms contain two.
Protons carry a positive electric charge equal in magnitude to the elementary charge of an electron, approximately coulombs. This positive charge is crucial for the electromagnetic interaction that binds electrons to the nucleus, forming a stable atom.
The mass of a proton is approximately kilograms, which is about 1,836 times greater than the mass of an electron. In atomic mass units (amu), a proton's mass is approximately amu, making it a significant contributor to the total mass of an atom.
Protons are fermions, meaning they have a half-integer spin. Specifically, a proton has a spin of , a quantum mechanical property analogous to angular momentum. This property is fundamental to the Pauli exclusion principle and the behavior of particles within the nucleus.
Protons are not elementary particles in the same sense as electrons. Instead, they are composite particles, meaning they are made up of smaller, more fundamental constituents called quarks. A proton consists of two up quarks (each with a charge of ) and one down quark (with a charge of ), resulting in a net charge of .
Protons belong to a class of particles called hadrons, which are particles composed of quarks held together by the strong nuclear force. More specifically, they are baryons, a type of hadron composed of three quarks. The strong nuclear force, mediated by gluons, is responsible for binding the quarks together within the proton and for binding protons and neutrons together to form the atomic nucleus.
The balance between the strong nuclear force, which attracts protons and neutrons, and the electromagnetic force, which repels positively charged protons, dictates the stability of atomic nuclei. The presence and arrangement of protons are critical for nuclear binding energy and determine which isotopes are stable or undergo radioactive decay.
Protons are also involved in various nuclear reactions, including nuclear fusion (where protons combine to form heavier nuclei, releasing immense energy, as seen in stars) and certain types of radioactive decay, such as beta-plus decay (positron emission), where a proton transforms into a neutron, emitting a positron and a neutrino. Understanding the properties and behavior of protons is thus essential for comprehending the fundamental processes of nuclear physics and energy production of energy within reactors.