Question

When do we have to use roman numerals in the name of a compound?

Answer 1

Roman numerals are used in naming ionic compounds when the metal cation forms more than one ion. The metals that form more than one ion are the transition metals, although not all of them do this.

Explanation:
For example, copper can form $C{u}^{+}$ ions and $C{u}^{2+}$ ions. If they combine with chlorine, we can have $CuCl$ and $CuC{l}_2$. The first compound is composed of copper $1+$ ions bonded to choride $1-$ ions. It's called copper(I) chloride. The second compound is composed of copper $2+$ ions bonded to chloride $1-$ ions. It's called copper(II) chloride.