Cambridge IGCSE Chemistry · Syllabus 0620 · Stoichiometry
Formula unit
What is Formula unit?
A formula unit is the simplest whole-number ratio of ions in an ionic compound, written so that the total positive charge exactly cancels the total negative charge; magnesium chloride, MgCl2, is one formula unit containing one magnesium ion and two chloride ions.
This definition is part of the Stoichiometry chapter in Cambridge IGCSE Chemistry.