NA = 6.02214076 × 10²³ mol⁻¹
Avogadro's number (NA = 6.02214076 × 10²³) defines the number of particles in one mole of any substance. It was redefined exactly in 2019 by the SI. One mole of water contains 6.022 × 10²³ water molecules and weighs 18.015 grams. One mole of carbon atoms weighs 12.011 grams and contains the same 6.022 × 10²³ atoms. To put this number in perspective: if you counted one atom per second, it would take 19 quadrillion years to count one mole — about 1.4 million times the age of the universe. Named after Amedeo Avogadro, though the actual number was first estimated by Josef Loschmidt in 1865.