Nickel is found in two oxidation states: +2, nickel(II); and +3, nickel(III). Nickel(II) is more common. Nickel in its +2 oxidation state is green. Nickel(II) chloride is a common +2 oxidation state compound. Nickel(II) oxide is normally dark green, but sometimes it is gray. This is because some of the nickel is in the +3 oxidation state (nickel(III). Nickel(III) compounds are oxidizing agents. They also are grayish. Nickel compounds can be green, blue, gray, or black.
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