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](http://chemistry.about.com/library/weekly/aa122002a.htm) and apparently Oxygen has $-2$ valence? Why? And Iron has +2, +3, (+4), (+6)? What does all that even mean?




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.


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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.


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