6.1.5 Oxidation Numbers in Compound Names
- Syllabus
- 9701–2028–2029
- Objective
- 6.1.5
- Level
- AS
Roman numerals in compound names show the oxidation number of a metal
A Roman numeral in a compound name gives the oxidation number of the named metal ion, especially when the element can form more than one charge.
Use the anion charge and overall neutrality to check the numeral. The numeral is not the number of atoms of that metal in the formula.
Iron(III) chloride contains Fe³⁺ and three Cl⁻ ions, so its formula is FeCl₃. Iron(II) chloride is FeCl₂.
Do not read “iron(III)” as three iron atoms. It specifies the oxidation state of each iron ion in the compound.
ConceptA-Level CAIE Chemistry AS