How can you tell how many valence electrons go with the transition elements?

I am trying to figure out how to draw Lewis structures of compounds with metals. For example, Cr2O3. But I cannot figure out how to determine bond order, or number of electron pairs on each atom.

Simple Lewis structures of compounds with metals can not be drawn for a number of reasons but primarily:

1. Compounds with transition and lanthanide metals do not obey the octet rule. If a transition metal is present in a molecule, they often obey 16 electron or 18 electron rules, but these have many exceptions as well.

2. Many metal containing compounds (especially oxides, minerals etc) are "non-molecular" meaning they do not form discrete molecules like water or methane. Rather they form extended networks of atoms bonding in lattices. The example you give is a non-molecular solid.

3. You can still estimate the number of valence electrons in your example. Your chromium is in the (III) oxidation state if you know how to count d electron shells. (the answer is 3 unpaired electrons)
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