In
chemistry, a formula unit is the
empirical formula of any
ionic or
covalent network solid compound used as an independent entity for
stoichiometric
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...
calculations. It is the lowest whole number ratio of
ions represented in an ionic compound. Examples include ionic and and covalent networks such as and
C (as
diamond
Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...
or
graphite).
[Steven S. Zumdahl; Susan A. Zumdahl (2000), ''Chemistry'' (5 ed.), Houghton Mifflin, pp. 470-6, ]
Ionic compounds do not exist as individual
molecules
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioc ...
; a formula unit thus indicates the lowest reduced ratio of ions in the compound.
In
mineralogy
Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proce ...
, 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.
[Smyth, Joseph R. and Tamsin C. McCormick, 1995, ''Crystallographic Data for Minerals,'' American Geophysical Union ]
References
{{DEFAULTSORT:Formula Unit
Chemical formulas