Final answer:
Roman numerals are used in the names of compounds with transition metals to indicate the metal cation's charge, but for those with a fixed charge like Ag+, Zn2+, and Cd2+, the charge can be omitted.
Step-by-step explanation:
When naming ionic compounds that contain transition metals, Roman numerals are used to indicate the charge of the metal cation. This is because many transition metals can form more than one type of cation with different charges. For example, iron can form Fe2+ and Fe3+.
As a result, the names of the compounds FeCl2 and FeCl3 are written as iron(II) chloride and iron(III) chloride to distinguish between the different ionic charges. However, for transition metals with a fixed charge like silver (Ag+), zinc (Zn2+), and cadmium (Cd2+), the charge is often omitted or can be included in the name as Roman numerals. The charge is omitted because these metals only form one type of cation and therefore there is no ambiguity.