 Now that you know how to name and write the formulae for ionic compounds, we'll look at a subset of these, the salts of the transition metals, and the post-transition metals, and the lanthanides and actinides. Until now it's been assumed that, given a name or a formula, you would be able to work out the charges on the ions involved by just looking up where they are on the periodic table. However, most of the transition metals and post-transition metals and so on are not so predictable. They can form ions with different charges. For instance, iron, iron, the metal, can form Fe2 plus and Fe3 plus ions. So let's say I tell you that I have some iron oxide and I want to know its formulae. Well the oxide ion is no problem. You could look up oxygen on the periodic table and being in group 6 or 16, depending on which periodic table you have, you would know that it has 6 valence electrons and forms ions with a charge of 2 minus. However, you wouldn't know which iron ion to use. Is this iron oxide with the Fe2 plus ions or iron oxide with the Fe3 plus ions? Both of them exist. So there's a piece of information missing. If I tell you the name of a salt which involves the transition metal, I have to tell you which ion of that transition metal is involved. I do this by using a Roman numeral after the metal's name. So iron oxide is an incomplete name. I have to specify either iron 2 oxide or iron 3 oxide. The Roman numeral tells you the charge on the metal ion. So iron 2 oxide contains Fe2 plus ions and iron 3 oxide contains Fe3 plus ions. And now you can work out the formulae. What if it's the other way around? I give you a formula and you have to name it. This first one looks like gold hydroxide. And if gold weren't a transition metal, that would be the end of it. However, gold can form ions of plus 1 or plus 3. So we need to specify which one it is. So use what you know. Hydroxide ions have a charge of minus 1. And there are three of them in this formula. So that's a total negative charge of 3 minus 3. There is only one gold ion present. So its whole charge must be balanced by the minus 3 of the 3 hydroxide ions. Hence, this is gold 3 plus. We therefore name the compound gold 3 hydroxide. How about this one? VC. It barely even looks like a chemical formula. However, V is the symbol for vanadium and C is the symbol for carbon. If carbon is in this compound, it must be one of the rare times when carbon forms an ion. You know this is ionic because it's a metal and a non-metal. And because the carbon appears second in the formula, it must be an anion. Hence, the basic name is vanadium carbide. Since we know the carbon is an anion, its charge must be minus 4. And since the formula shows that for every carbide ion, there is one vanadium ion, the charge on the vanadium ion must be plus 4. So the complete version of the name is vanadium 4 carbide.