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The atomic number or proton number (symbol Z) of a chemical element is the number of protons found in the nucleus of an atom. The atomic number uniquely identifies a chemical element.
The atomic number is identical to the charge number of the nucleus. In an uncharged atom, the atomic number is also equal to the number of electrons.
Atoms with the same atomic number Z but different neutron numbers N, and hence different atomic masses, are known as isotopes.