# Question #c40e2

Roman numerals are used to indicate the oxidation state of metals which have more than one possible oxidation state.

#### Explanation:

Here's an example. Copper can form either $C {u}^{+}$ or $C {u}^{2 +}$ ions.

So there are actually two different compounds that have the name copper chloride. We need the roman numeral to know which is which. Remember that halogens form ions with a -1 charge. So a chloride ion is $C {l}^{1}$.

$C u C l$ has a $C {u}^{+}$ so its name is copper(I) chloride

$C u C {l}_{2}$ has a $C {u}^{2 +}$ so its name is copper(II) chloride

This video includes some more examples...

Hope this helps!