Formula unit

In chemistry, a formula unit is the smallest unit [1] of any Ionic compound or covalent network solid or metal (not for molecular substances). [2]. 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.[3] The formula unit is used as an independent entity for stoichiometric calculations. [4][5] Examples of formula units, include ionic compounds such as NaCl and K2O and covalent networks such as SiO2 and C (as diamond or graphite).[6]

In most cases the formula representing a formula unit, will also be an empirical formula e.g. calcium carbonate CaCO3 or sodium chloride NaCl but this isn't always the case. For example the ionic compound Potassium persulfate has formula unit K2S2O8 which isn't an empirical formula. And the ionic compound Hg2N2O6 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 KSO4 and HgNO3 respectively. [7]

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.[8]

References
  1. Peter Atkins, Julio de Paula, Ronald Friedman, Physical Chemistry: Quanta, Matter, and Change, 2nd Edition
  2. https://www.quora.com/Does-the-term-Formula-Unit-apply-to-giant-covalent-networks-composed-of-one-type-of-atom-like-Diamond-or-Graphene-which-have-formula-C-And-so-would-the-term-apply-to-some-covalent-networks-and-not-others/answer/Daniel-James-Berger
  3. "Chapter 4 – Covalent Bonds and Molecular Compounds". wou.edu. Retrieved 2023-10-08.
  4. "Formula Units vs Empirical Formula - CHEMISTRY COMMUNITY". lavelle.chem.ucla.edu. Retrieved 2023-09-10.
  5. "2.2: Chemical Formulas". Chemistry LibreTexts. 2015-01-27. Retrieved 2023-09-10.
  6. Steven S. Zumdahl; Susan A. Zumdahl (2000), Chemistry (5 ed.), Houghton Mifflin, pp. 470-6, ISBN 0-618-03591-5
  7. "Is Formula Unit usually an empirical formula but not always? - Chemical Forums". chemicalforums.com. Retrieved 2023-10-06.
  8. Smyth, Joseph R. and Tamsin C. McCormick, 1995, Crystallographic Data for Minerals, American Geophysical Union


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