An Exact Value for Avogadro's Number

Abstract

Avogadro’s number, NA, is the fundamental physical constant that links the macroscopic physical world of objects that we can see and feel with the submicroscopic, invisible world of atoms. In theory, NA specifies the exact number of atoms in a palm-sized specimen of a physical element such as carbon or silicon. The name honors the Italian math-ematical physicist Amedeo Avoga-dro (1776–1856), who proposed that equal volumes of all gases at the same temperature and pressure contain the same number of molecules. Long af-ter Avogadro’s death, the concept of the mole was introduced, and it was experimentally observed that one mole (the molecular weight in grams) of any substance contains the same number of molecules. This number is Avogadro’s number, although he knew nothing of moles or the epony-mous number itself. Today, Avogadro’s number is for-mally defined to be the number of car-bon-12 atoms in 12 grams of unbound carbon-12 in its rest-energy electronic state. The current state of the art esti-mates the value of NA, not based on experiments using carbon-12, but by using x-ray diffraction in crystal sili-con lattices in the shape of a sphere or by a watt-balance method. According to the National Institute of Standards and Technology (NIST), the current ac-cepted value for NA is:

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