Avogadro constant
/ av-uh-GAH-droh /
The Avogadro constant is the size of the chemist's counting bundle — the number of particles in one mole. It is a colossal figure, about 602 thousand billion billion, written 6.02214076 × 10²³. It exists for one humble reason: atoms are so tiny that you need this many of them before you have a heap big enough to weigh and handle.
Precisely, the Avogadro constant, written N_A, is the number of elementary entities — atoms, molecules, ions — in exactly one mole of a substance. Since the 2019 redefinition of SI units, it is fixed by definition at exactly 6.02214076 × 10²³ per mole, rather than measured. It links the invisible world of individual particles to the gram-scale quantities we weigh in the lab.
Its value matters because it sets the scale of the atomic world. Knowing N_A, you can work out the mass of a single atom, the number of molecules in a breath, or how many ions are in a pinch of salt. It is the constant that makes the leap from one atom to a spoonful — between the microscopic and the human scale — a matter of simple arithmetic.
If you could count atoms at one per second, counting through a single mole would take longer than a million times the present age of the universe — that is how large the Avogadro constant is.
An almost unimaginable count, packed into a single spoonful of matter.
Strictly, the Avogadro number is the pure count 6.02214076 × 10²³, while the Avogadro constant carries the unit per mole (mol⁻¹); in casual use the two names are often swapped.