This isn't a homework question, but seemed sufficiently low-level that it belonged in this part of the forum. When one speaks of "common" oxidation states, what is meant by "common?" Is this simply to say the states most commonly occurring in nature? This would be opposed to most commonly used in industry, or maybe just meaning using requiring common methods and energy to obtain (as opposed to, say, Brookhaven). "Most common in nature" would be intuitive, but as important as nomenclature is in chemistry, I'd rather ask an obvious question than make a false assumption. Thanks for your time.