# Calculating valence of oxides

Learning about Oxides. Basically when oxygen is combined with a metal.

$$\ce{FeO}$$

This is called "Iron Oxide (II)" according to my book.

Apparently, the II represents the valence. But how come? Look:

$\ce{Fe}$ has the configuration $1s^22s^22p^63s^23p^6\color{red}{4s^2}3d^6$. Since the outermost shell has $2$ electrons, the valence is $2$.

Now $\ce{O}$ has the configuration $1s^2\color{red}{2s^22p^4}$. Its outermost shell has $6$ electrons so oxygen has a valence of $6$ right?

Then how come $\ce{FeO}$ has valence $2$? Shouldn't the valence of $\ce{FeO}$ be like, the combination of the two valences? So $6 - 2 = 4$? (I read somewhere you had to subtract it).

Not only that. I am starting to think that my definition of valence is completely messed up. I just checked in here and apparently Oxygen has $-2$ valence? Why? And Iron has +2, +3, (+4), (+6)? What does all that even mean?

## 2 Answers

Ok, finding valencies of elements in simple compounds is very easy, but you have to know some thing to start:

You are treating compound as if it would be composed of discrete molecules

If you have compound/neutral molecule, sums of valencies of both types of atoms should be equal, as valency is number of bonds between atoms.

In case of FeO you need to know that O make usually two bonds (valence = 2) and as both valencies have to be equal Fe also has val = 2

In case of Fe2O3, 3*2=6, 6/2=3 so Fe is trivalent.

• $3$ oxygen atoms multiplied by the valence of Oxygen ($2$) results in $6$ and then it is divided by the number of Iron atoms resulting in $3$ - is that how it's done for simple compounds? Well great, I finally get it then. Thanks. – Voldemort Jun 18 '15 at 4:13
• For binary oxides it's that simple, but there are peroxides and superoxides that could be surprising – Mithoron Jun 18 '15 at 9:59

Valency is the combining power of an atom; so, when we talk of iron having a valency of 2, it can combine either with two atoms of valency of one; or with one atom of valency of two. Oxygen has a valency of two and so the formula for iron (II) oxide is FeO.

In the history of chemistry valency pre-dates the idea of electron configuration which looks like the source of your confusion.

Hope that is clearer.

Edit: you mentioned that oxygen has a valency of minus 2; that's not quite right. It has a valency of 2 but it can form an oxidation state of minus 2. You need to separate out the ideas of valency and oxidation state. Although the ideas are related, they are distinct.

• Why does Oxygen have valency $2$? It has $6$ electrons in its outer shell I think? Or am I calculating this all wrong? – Voldemort Jun 17 '15 at 18:31
• @voldemort one can imagine that oxygen gets 2 electr. to have 8 total - octet – Mithoron Jun 17 '15 at 22:52
• @Mithoron Then why doesn't Iron have a valency of $6$? Since its outer shell has only $2$ electrons, it could gain $6$ to reach $8$? – Voldemort Jun 17 '15 at 22:56
• @voldemort As you wrote iron can have +6 oxidation number and than also valence = 6 I'm afraid that you're trying to learn what they are on one of more difficult cases. – Mithoron Jun 17 '15 at 23:07
• @Mithoron ok so, it CAN have valence 6, but it normally has 2? How do you know which one is the most common / the lne I should use? – Voldemort Jun 17 '15 at 23:31