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A subatomic particle with a positive charge present in the nuclei of all atoms. The number of protons in the nucleus determines the properties of a chemical element and is referred to as the atomic number. The proton alone forms the nucleus of the hydrogen atom. Positively charged protons generate an electrostatic force that attracts electrons and holds the atom together. Also, because of their charge, the trajectories of protons can be influenced by the magnetic field. A proton located outside the atomic nucleus is stable. Protons are used in accelerators for particle physics research or as a treatment in proton therapy. In nature, they are, for example, part of cosmic rays.