| Astrophysics (Index) | About |
Avogadro's number is a large number intended to represent the number of nucleons (protons and neutrons) in a gram of substance and the mole (mol) was defined to be that many molecules, a useful number to chemists, who can relate grams to elements and molecular formulas. The Avogadro constant (abbreviated NA) is an SI-defined constant representing the same thing, defined to be 6.02214076 × 1023, and an SI standard mole is defined based on it.
No single number can consistently relate mass to such particle numbers because the mass of atoms and molecules do not precisely match (the sum of) their mass numbers, and this difference varies by element, isotope, molecular structure, pressure, and temperature. Avogadro's number was based upon a standard gram and a particular isotope of a particular element. The Avogadro constant is currently chosen to be a realistic representative, well within the range of this variation, and though it only approximates the true relation of particle numbers to mass, that would be true of any specific number, and such an approximation is still very useful in chemistry.