Problem: What is the oxidation number of phosphorus in  PO43−?

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We’re being asked to determine the oxidation state of phosphorus in PO43– (phosphate ion).


The rules for oxidation states are as follows:

A. General Rules:

1. For an atom in its elemental form (Zn, Cl2, C(graphite), etc.)    O.S. = 0

2. For an ion (Li+, Al3+, etc.)                                                        O.S. = charge


B. Specific Rules:

1. Group 1A              O.S. = +1

2. Group 2A              O.S. = +2

3. Hydrogen              O.S. = +1, with nonmetals

                                  O.S. = –1 with metals and boron

4. Fluorine                 O.S. = –1

5. Oxygen                  O.S. = –1 in peroxides (X2O2, X = Group 1A)

                                  O.S. = –1/2 in superoxides (XO2, X = Group 1A)

                                  O.S. = –2 in other compounds

6. Group 7A               O.S. = –1 (except when bonded to O)


Since PO43– is neither a peroxide or superoxide, the oxidation state of each oxygen atom is –2


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What is the oxidation number of phosphorus in  PO43−?