Chemistry: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 K
2O and covalent networks such as SiO
2 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 CaCO
3 or sodium chloride NaCl but this isn't always the case. For example the ionic compound Potassium persulfate has formula unit K
2S
2O
8 which isn't an empirical formula. And the ionic compound Hg
2N
2O
6 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 KSO
4 and HgNO
3 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
- ↑ Peter Atkins, Julio de Paula, Ronald Friedman, Physical Chemistry: Quanta, Matter, and Change, 2nd Edition
- ↑ 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
- ↑ "Chapter 4 – Covalent Bonds and Molecular Compounds". https://wou.edu/chemistry/courses/online-chemistry-textbooks/ch150-preparatory-chemistry/ch150-chapter-4-covalent-bonds-molecular-compounds/.
- ↑ "Formula Units vs Empirical Formula - CHEMISTRY COMMUNITY". https://lavelle.chem.ucla.edu/forum/viewtopic.php?t=62876.
- ↑ "2.2: Chemical Formulas" (in en). 2015-01-27. 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.
- ↑ Steven S. Zumdahl; Susan A. Zumdahl (2000), Chemistry (5 ed.), Houghton Mifflin, pp. 470-6, ISBN:0-618-03591-5
- ↑ "Is Formula Unit usually an empirical formula but not always? - Chemical Forums". https://www.chemicalforums.com/index.php?topic=113340.
- ↑ Smyth, Joseph R. and Tamsin C. McCormick, 1995, Crystallographic Data for Minerals, American Geophysical Union
 |  |
