Avogadro’s constant is a large number used to discuss large quantities of atoms or molecules, usually when chemists talk about quantities they can actually see or measure out. The number itself (rounded at three decimal places) is 6.022 × 1023. It’s just a big number that relates an atomic or molecular mass to the mass of a collection of many atoms or molecules. Avogadro’s number of atoms of an element is called a mole of that element, and, similarly, Avogadro’s number of molecules of a compound is a mole of that compound. For example, the atomic mass of oxygen is about 16 grams per mole, and 6.022 × 1023 atoms (1 mole) of oxygen weigh(s) about 16 grams. The most recent (and accurate) definition of this constant was 6.02214078(18) × 1023, which was calculated by careful measurements of the mass and volume of 1-kilogram (about 2.2 lbs.) spheres of silicon-28, a particular isotope of silicon (see next chapter concerning isotopes).
