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

For example, copper can form $\text{Cu"^(+)}$ ions and $\text{Cu"^(2+)}$ ions. If they combine with chlorine, we can have $\text{CuCl}$ and $\text{CuCl"_2}$. 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.