Formula unit
In chemistry, a formula unit is the smallest unit  of any Ionic compound or covalent network solid or metal (not for molecular substances). . And 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 C (as diamond or graphite). In most cases the formula representing a formula unit, will also be an empirical formula e.g. calcium carbonate  or sodium chloride  but this isn't always the case. For example the ionic compound Potassium persulfate has formula unit  which isn't an empirical formula. And the ionic compound  is not an empirical formula as you see those compounds have formula units with ratios that are not in simplest/reduced form. The empirical formulae for those ionic compounds would be  and  respectively. 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. References

Chemical formulas