Formula unit
In chemistry, a formula unit is the smallest unit of any ionic compound,covalent network solid, or metal (not for molecular substances). 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 NaCl and K2O and covalent networks such as SiO2 and C (as diamond or graphite).
In most cases the formula representing a formula unit will also be an empirical formula, such as calcium carbonate (CaCO3) or sodium chloride (NaCl), but isn't always the case. For example, the empirical formulas for the ionic compounds potassium persulfate and mercury(I) nitrate are KSO4 and Hg2(NO3)2, respectively, being presented in the smallest 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 crystallographic axes are used in defining the unit cell.