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Avogadro's number (Avogadro constant, abbreviated NA) is a large number aimed at representing the number of nucleons (protons and neutrons) in a gram of substance. No single number can do this with perfect precision because the mass of atoms and molecules do not precisely match (the sum of) their mass numbers. Avogadro's number is chosen to be a realistic representative, now having the standard definition of 6.02214076 × 1023. A mole (mol) of a substance is defined as that many molecules. The number is convenient for chemists, who can relate grams to elements and molecular formulae, and can be convenient in calculations for allowing smaller numbers to be used than particle counts.
Earlier in its history, Avogadro's number and the mole were pinned to the standard gram along with some selected element isotope. However, though the number is useful in chemistry, a definition based upon nucleons-per-gram is not stable (such a value varies by element, isotope, molecular structure, pressure, temperature) and an exact number is considered more useful: a rounded (to the nearest multiple of 1015) value well-within the range of such varying physical values has been adopted for inclusion in SI.