# Formula unit

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{{Short description|Chemical formula of a unit of an ionic compound or covalent network solid}}

In [chemistry](/source/chemistry), a '''formula unit''' is the smallest unit of a non-molecular substance, such as an [ionic compound](/source/Salt_(chemistry)), [covalent network solid](/source/covalent_network_solid), or [metal](/source/metal).<ref>Peter Atkins, Julio de Paula, Ronald Friedman, Physical Chemistry: Quanta, Matter, and Change, 2nd Edition </ref>  It can also refer to the [chemical formula](/source/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](/source/molecule) or molecular formula are applied to molecules.<ref>{{Cite web | title= Chapter 4 – Covalent Bonds and Molecular Compounds | url=https://wou.edu/chemistry/courses/online-chemistry-textbooks/ch150-preparatory-chemistry/ch150-chapter-4-covalent-bonds-molecular-compounds/ | access-date=2023-10-08 | website=wou.edu}}</ref> The formula unit is used as an independent entity for [stoichiometric](/source/stoichiometric) calculations.<ref>{{Cite web |date=2015-01-27 |title=2.2: Chemical Formulas |url=https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_(Averill)/02%3A_Molecules_Ions_and_Chemical_Formulas/2.02%3A_Chemical_Formulas |access-date=2023-09-10 |website=Chemistry LibreTexts |language=en}}</ref> Examples of formula units, include ionic compounds such as {{chem2|[NaCl](/source/Sodium_chloride)}} and {{chem2|[K2O](/source/Potassium_oxide)}} and covalent networks such as {{chem2|[SiO2](/source/Silicon_dioxide)}} and [C](/source/Carbon) (as [diamond](/source/diamond) or [graphite](/source/graphite)).<ref name=Zumdahl>Steven S. Zumdahl; Susan A. Zumdahl (2000), ''Chemistry'' (5 ed.), Houghton Mifflin, pp. 470-6, {{ISBN|0-618-03591-5}}</ref>

In most cases the formula representing a formula unit will also be an [empirical formula](/source/empirical_formula), such as [calcium carbonate](/source/calcium_carbonate) ({{chem2|CaCO3}}) or [sodium chloride](/source/sodium_chloride) ({{chem2|NaCl}}), but it is not always the case. For example, the ionic compounds [potassium persulfate](/source/potassium_persulfate) ({{chem2|K2S2O8}}),   [mercury(I) nitrate](/source/mercury(I)_nitrate) {{chem2|Hg2(NO3)2}}, and [sodium peroxide](/source/sodium_peroxide) {{chem2|Na2O2}},  have empirical formulas of {{chem2|KSO4}},  {{chem2|HgNO3}}, and {{chem2|NaO}}, respectively, being presented in the simplest whole number ratios.{{citation needed|date=October 2025}}

In [mineralogy](/source/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](/source/Crystallography) axes are used in defining the [unit cell](/source/unit_cell).<ref>Smyth, Joseph R. and Tamsin C. McCormick, 1995, ''Crystallographic Data for Minerals,'' American Geophysical Union </ref>

==References==
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{{Mole concepts}}

Category:Chemical formulas

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