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Sum of the atomic weights of all atoms in a chemical formula. The term is generally applied to a substance that consists of ions (seeionic bond) rather than individual molecules (and thus does not have a molecular weight). An example of such a substance is sodium chloride (table salt). Such a substance's chemical formula describes the simplest ratio of the number of atoms of the constituent elements. See alsostoichiometry.
This entry comes from Encyclopædia Britannica Concise. For the full entry on formula weight, visit Britannica.com.