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In chemistry, a formula unit is the smallest unit of a non-molecular substance, such as an ionic compound, covalent network solid, or metal.Peter Atkins, Julio de Paula, Ronald Friedman, Physical Chemistry: Quanta, Matter, and Change, 2nd Edition It can also refer to the chemical formula for that unit. Those structures do not consist of discrete molecules, and so for them, the term formula unit is used. In contrast, the terms molecule or molecular formula are applied to molecules. The formula unit is used as an independent entity for stoichiometric calculations. Examples of formula units, include ionic compounds such as and and covalent networks such as and Carbon (as diamond or graphite).Steven S. Zumdahl; Susan A. Zumdahl (2000), Chemistry (5 ed.), Houghton Mifflin, pp. 470-6,
In most cases the formula representing a formula unit will also be an empirical formula, such as calcium carbonate () or sodium chloride (), but it is not always the case. For example, the ionic compounds potassium persulfate (), mercury(I) nitrate , and sodium peroxide , have empirical formulas of , , and , respectively, being presented in the simplest whole number ratios.
In mineralogy, as minerals are almost exclusively either ionic or network solids, the formula unit is used. The number of formula units (Z) and the dimensions of the Crystallography axes are used in defining the unit cell.Smyth, Joseph R. and Tamsin C. McCormick, 1995, Crystallographic Data for Minerals, American Geophysical Union
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