I've been beating my head against this for a while, so hopefully someone can help me out here. I really don't understand what determines the common oxidation numbers for some transition metals.
Looking at Cobalt, the 3d subshell is partially filled (
), and the 4s subshell is completely filled (
). So I guess I can understand the oxidation number of +2; the 4th shell is incomplete, so you lose the two electrons from the highest (4s) energy subshell. But why +3? You lose the two 4s electrons, but then you lose 1 electron from 3d? Why do you stop at 1 electron from 3d? Why is it more likely to lose 1 than to lose 0, or 4 (to leave 3d half-filled which I thought was more stable)?
Am I missing something obvious here? Is it just one of those weird rules of transition metals that just some combinations of incomplete subshells are more stable than others?