Cambridge IGCSE Chemistry · Syllabus 0620 · Chemical Reactions
Roman Numeral in a Chemical Name
What is Roman Numeral in a Chemical Name?
A Roman numeral written in brackets immediately after the name of an element indicates the oxidation number of that element in the compound. In iron(III) chloride the iron has oxidation number plus three. It refers only to the named element, not to the whole compound or ion, and it is not a count of atoms.
This definition is part of the Chemical Reactions chapter in Cambridge IGCSE Chemistry.