Inchemistry, aformula unit is the smallest unit of a non-molecular substance, such as anionic compound,covalent network solid, ormetal.[1] It can also refer to thechemical 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 termsmolecule or molecular formula are applied to molecules.[2] The formula unit is used as an independent entity forstoichiometric calculations.[3] Examples of formula units, include ionic compounds such asNaCl andK2O and covalent networks such asSiO2 andC (asdiamond orgraphite).[4]
In most cases the formula representing a formula unit will also be anempirical formula, such ascalcium carbonate (CaCO3) orsodium chloride (NaCl), but it is not always the case. For example, the ionic compoundspotassium persulfate (K2S2O8),mercury(I) nitrateHg2(NO3)2, andsodium peroxideNa2O2, have empirical formulas ofKSO4,HgNO3, andNaO, respectively, being presented in the simplest whole number ratios.[citation needed]
Inmineralogy, 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 thecrystallographic axes are used in defining theunit cell.[5]
