In an ionic compound, the formula mass (formerly formula weight ) is the sum of the atomic masses of the atoms involved in its structure . It finds its equivalent in the molecular mass of molecules . One can not speak of molecular mass in the case of ion crystals , since the entire crystal would represent one molecule and the mass would therefore depend on the size of the crystal. Therefore, the mass of ion crystals is described by the formula unit of the atoms involved.
- Hans-Dieter Jakubke, Ruth Karcher (Ed.): Lexicon of Chemistry , Spectrum Academic Publishing House, Heidelberg, 2001.