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a positively-charged subatomic particle. One of 3 main types of subatomic particles (parts of atoms). Protons hang out with neutral neutrons in a dense central nucleus and are surrounded by a cloud of negatively-charged electrons. The number of protons an atom has determines what element it is (e.g. 6 protons means it’s carbon and 7 protons means it’s oxygen). The number of protons is also referred to as the “atomic number,” abbreviated Z. Elements are ordered by increasing atomic number in the periodic table of elements.

“Proton” is also often used to refer to a “free” H⁺. Hydrogen atoms only have 1 proton & 1 electron & they often leave that electron behind when they leave molecules, so all they’re left with is a H⁺, so we often call H⁺ a proton. This nomenclature is used a lot when discussing pH, which is a measurement of H⁺ concentration (where lower pH corresponds to higher H⁺ concentration).