Which metal is most likely to form more than one kind of positively charged ion?

Nickel is a transition metal, and transition metals commonly form ions with different positive charges. For example, nickel can form Ni²⁺ and Ni³⁺ ions. Sodium, barium, and magnesium usually form ions with one charge: Na⁺, Ba²⁺, and Mg²⁺, respectively.

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