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Hadrons are categorized into two broad families: baryons, made of an odd number of quarks (usually three) and mesons, made of an even number of quarks (usually two: one quark and one antiquark). [1] Protons and neutrons (which make the majority of the mass of an atom) are examples of baryons; pions are an example of a meson.
Although protons were originally considered to be elementary particles, in the modern Standard Model of particle physics, protons are known to be composite particles, containing three valence quarks, and together with neutrons are now classified as hadrons. Protons are composed of two up quarks of charge + 2 / 3 e each, and one down ...
An atom consists of a small, heavy nucleus surrounded by a relatively large, light cloud of electrons. An atomic nucleus consists of 1 or more protons and 0 or more neutrons. Protons and neutrons are, in turn, made of quarks. Each type of atom corresponds to a specific chemical element. To date, 118 elements have been discovered or created.
The best known baryons are protons and neutrons, which make up most of the mass of the visible matter in the universe, whereas electrons, the other major component of atoms, are leptons. Each baryon has a corresponding antiparticle, known as an antibaryon, in which quarks are replaced by their corresponding antiquarks.
Owing to a phenomenon known as color confinement, quarks are never found in isolation; they can be found only within hadrons, which include baryons (such as protons and neutrons) and mesons, or in quark–gluon plasmas. [2] [3] [nb 1] For this reason, much of what is known about quarks has been drawn from observations of hadrons.
Subatomic particles such as protons or neutrons, which contain two or more elementary particles, are known as composite particles. Ordinary matter is composed of atoms, themselves once thought to be indivisible elementary particles. The name atom comes from the Ancient Greek word ἄτομος which means indivisible or uncuttable.
The atom helium-3 (3 He) consists of two protons, one neutron, and two electrons. The deuterium atom consists of one proton, one neutron, and one electron. The number of bosons within a composite particle made up of simple particles bound with a potential has no effect on whether it is a boson or a fermion.
On a scale less than about 0.8 fm (roughly the radius of a nucleon), the force is carried by gluons and holds quarks together to form protons, neutrons, and other hadrons. On a larger scale, up to about 3 fm, the force is carried by mesons and binds nucleons (protons and neutrons) together to form the nucleus of an atom. [2]