Avogadro's number
/ av-uh-GAH-drohz NUM-ber /
Atoms are so small that any visible amount holds an unimaginable number of them. Avogadro's number is the size of the chemist's standard 'dozen' for atoms and molecules — a single, fixed count used to bundle the invisibly tiny into amounts we can weigh.
Avogadro's number is 6.022 × 10²³, the number of elementary entities (atoms, molecules, ions) in exactly one mole. It is the bridge between the count of individual particles and the macroscopic quantity called the mole.
Its value is now fixed by definition, which in turn defines the mole, anchoring all of stoichiometry to a single agreed number. The figure is staggeringly large precisely because individual particles are staggeringly small — a teaspoon of water already contains far more molecules than there are stars in the observable universe.
One mole of carbon weighs about 12 grams and contains 6.022 × 10²³ carbon atoms — the same count of particles you would find in a mole of any other substance.
One mole of anything contains Avogadro's number of particles.
Strictly, 'Avogadro constant' carries the unit per mole (mol⁻¹), while the bare 'Avogadro's number' is the dimensionless count. Casual usage treats them as the same value.