Avogadro constant

Avogadro's 1811 hypothesis said equal volumes of gas hold equal numbers of molecules; he never counted them. Loschmidt made the first real estimate in 1865, Perrin's Brownian-motion work in 1908 nailed it to a few per cent and won him a Nobel, and the modern route ran through counting atoms in a near-perfect silicon-28 sphere by X-ray crystal density.

Since 2019 the counting has stopped: one mole is defined as exactly 6.022 140 76 × 10²³ entities, so N_A is exact and the mole is no longer tied to 12 grams of carbon-12. The consequence is subtle but real — the molar mass of ¹²C is now 11.999 999 9958(36) g/mol rather than exactly 12, and the molar mass constant M_u carries a tiny uncertainty. For any practical purpose M(¹²C) = 12 g/mol still holds to nine digits. The number itself is beyond intuition: a mole of sand grains would bury the continental United States kilometres deep.