Definition of formula unit

1) A formula unit in chemistry refers to the simplest ratio of ions represented in an ionic compound. It indicates the number of atoms of each element in a compound, providing a snapshot of the compound's composition.


The term "formula unit" is most relevant in the field of inorganic chemistry, specifically when studying ionic compounds and crystal lattices.

Related Terms

  • Molecule
  • Ionic Compound
  • Covalent Compound
  • Empirical Formula
  • Molecular Formula


For example, in the ionic compound sodium chloride (NaCl), one formula unit consists of one sodium ion (Na+) and one chloride ion (Cl-). In calcium chloride (CaCl2), one formula unit includes one calcium ion (Ca2+) and two chloride ions (Cl-).

Chemical Structures/Formula

Formula units are represented as chemical formulae. For instance, the formula unit for sodium chloride is NaCl, and for calcium chloride, it's CaCl2.

Complexity Level

Basic. The concept of a formula unit is fundamental to understanding chemical composition and is typically introduced early in chemistry education.


The term "formula unit" is derived from the concept of a chemical formula, which has been used since the 19th century to represent the composition of chemical compounds.

  1. Silberberg, M. S., & Amateis, P. (2018). Chemistry: The Molecular Nature of Matter and Change. McGraw-Hill Education.
  2. Petrucci, R. H., Harwood, W. S., & Herring, F. G. (2002). General Chemistry: Principles and Modern Applications. Prentice Hall.