= Formula unit
{wiki}
A <formula unit> is a specified compositional accounting entity for a <chemical substance> that need not consist of discrete <molecules>. For ordinary crystalline $\mathrm{NaCl}$, one <formula unit> counts one <sodium> <ion> $\mathrm{Na}^+$ and one <chlorine> <ion> $\mathrm{Cl}^-$; it does not identify an isolated pair within the crystal. https://iupac.org/wp-content/uploads/2019/05/IUPAC-GB3-2012-2ndPrinting-PDFsearchable.pdf#page=65[Cohen et al., IUPAC Green Book, 3rd ed., §2.10.1(iii)(b), p. 51] defines a <formula unit> as a specified group of <chemical atoms>, rather than a unit of measurement. An <amount of substance> of one <mole> of $\mathrm{NaCl}$ <formula units> therefore corresponds to one <mole> of each kind of <ion>.
Back to article page
