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What is the oxidation number?

What is the oxidation number of:

P in P4
P in PO4 -3
C in CO3 -2
Cr in Cr2O2

Solution This solution is FREE courtesy of BrainMass!

The oxidation state or oxidation number is defined as the sum of negative and positive charges in an atom, which indirectly indicates the number of electrons it has accepted or donated.

The following formula describes the element I2 accepting two electrons to gain an oxidation number of -1.
I2 + 2e- → 2I-

In P4, P has oxidation number of 0.

In PO4 -3, the total oxidation number is -3, therefore we apply the summation rule (O has oxidation number -2):
P + 4*(-2) = -3
P =5-3 = +5. This is the oxidation of P in PO4 -3

C in CO3 -2. We apply the summation rule:
C + 3*(-2) = -2
C = 6-2 =+4

Cr in Cr2O2. We apply the summation rule:
2*Cr + 2*(-2) = 0
2*Cr = 4
Cr= +2