In Mn3(PO4)2, what is the oxidation state of manganese?

• A. +3
• B. +2
• C. 0
• D. -2

Answer: (B)

Balancing charges in a neutral compound is the key. In Mn3(PO4)2 there are two phosphate ions, each with a -3 charge, giving a total negative charge of -6. The three manganese atoms must then contribute +6 in total to neutralize the compound. Dividing +6 among three Mn atoms gives an average oxidation state of +2, and in this simple salt each manganese is typically +2. If manganese were +3, the total would be +9 and couldn’t balance the -6 from phosphate; if it were 0 or -2, the charges wouldn’t cancel to zero either. So the manganese oxidation state is +2.