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The definition of an atomic number is the number of protons in the nucleus of an atom of an element. Three for lithium and 20 for calcium are each an example of an atomic number.
In chemistry and physics, the atomic number of a chemical element (also known as its proton number) is the number of protons found in the nucleus of an atom of that element, and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element.