Why the invariant atomic-scale unit, entity, is essential for understanding stoichiometry without ‘Avogadro anxiety’

Abstract

The Avogadro constant is generally considered to be the relevant physical invariant for stoichiometry, intimately associated with the SI base unit for the amount of a specified substance. Unfortunately, it has dimensions of reciprocal amount rather than of amount itself, and this appears to be the source of the widespread confusion permeating this subject. It would make sense, instead, to use its reciprocal—an amount consisting of a single representative entity—as the fundamental invariant for amount. Major simplifications in comprehension then result. In particular, the mole would be redefined in a straightforward way as a designated exact number of elementary entities, making stoichiometric calculations much more easily comprehended. Unfortunately, acceptance by the CIPM of recent CCU recommendations for redefining the mole and kilogram (while keeping the inexactly known carbon-12-based dalton definition) would inject even more confusion into this subject, because the proposed definitions violate a basic compatibility condition relating the mole, kilogram and dalton, stemming from the fundamental concept of the mole itself. However, there are a number of alternative mole-concept-compatible and easily comprehended definitions worthy of consideration, where, in each case, a compatible dalton must be an exact sub-multiple of the (redefined) kilogram. These alternatives should receive much wider discussion before final decisions are made in the major restructuring of the international system of units.