What is a mole?

Abstract

The mole was the final base unit to be included in the International System of Units. This article explores: our understanding of the mole and its characteristics; its relationship with the base quantity, amount of substance, of which it is the base unit; and when alternative ways of expressing measurement results would be preferable. Importantly, since the SI is a practical system, this article also develops an understanding of when the concept of the mole is useful and meaningful. An important outcome is that the mole must consider elementary entities located in close enough vicinity such that they could, in theory, react or interact with each other stoichiometrically. Further, it is observed that the majority of practical uses of the mole relate to describing the composition of mixtures where, unusually, the concept of a mole of dissimilar elementary entities is often advantageous. It is also noted that where ratio and fractions are used it is essential to display the measurement units being used in full to clarify what quantities are being expressed.