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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.
atomic number (Z) a number expressive of the number of protons in an atomic nucleus, or the positive charge of the nucleus expressed in terms of the electronic charge. Avogadro's number (N) (NA) the number of molecules in one mole of a substance: 6.023 Ã 1023. Called also Avogadro's constant.