You may have noticed in my last post about tarnish I used roman numerals in some of the compound names (e.g., copper (II) oxide). Maybe you wondered why (humor me).
Metals, like copper, and non-metals, like oxygen, form ionic bonds. This means that if you were to pull apart the bond, the component atoms would be charged. In the case of copper (II) oxide, the copper has a +2 charge, and the oxygen has a –2 charge. Charged atoms are called “ions”.
Many elements can only ever form ions of one charge. For example, sodium and potassium ions are always +1. Certain metals, however, can form ions with different charges. Copper ions can be +1 or +2.
So, when you pull a jar of copper oxide off the shelf, how do you know if it’s CuO (one copper +2 atom and one oxygen –2 atom) or Cu2O (two copper +1 atoms and one oxygen –2 atom)? You know because the charge of copper is in the name. That’s how you notate metals that can have more than one charge.