Fields, Thomas H. Physics Division, Argonne National Laboratory, Argonne, Illinois.
Last reviewed:March 2019
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- Proton, published November 2017:Download PDF Get Adobe Acrobat Reader
- Proton, published June 2014:Download PDF Get Adobe Acrobat Reader
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A positively charged composite particle found in the nucleus of every chemical element. Protons are subatomic particles composed of three elementary particles known as quarks, specifically two "up" quarks and one "down" quark (Fig. 1). The sum of the fractional charges of the quarks gives the proton a positive electric charge of one. Being made of three quarks, protons are categorized as baryons. Baryons belong to a broader particle family called hadrons which participate in all four fundamental interactions, or forces of nature: electromagnetism, the strong nuclear interaction, the weak nuclear interaction, and gravity. Protons are the lightest baryon in terms of mass and the only stable baryon known in the universe. Along with the second-lightest baryon, the neutrally charged neutron, protons comprise the nuclei of atoms. Each element in the periodic table is defined and distinguished by the unique number of protons in its nucleus, called the atomic number. An atom’s atomic number largely determines its chemical properties. See also: Atomic nucleus; Atomic number; Baryon; Chemistry; Electric charge; Electromagnetism; Elementary particle; Fundamental interactions; Gravity; Hadron; Periodic table; Quark
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