Determining Oxidation Number

[Naples]

Determine the oxidation number of the underlined atom.
1. P4O6
2. Cr2O7^2-
3. Fe2O3
4. NiCl3

Could someone please explain how to do this.
Any help is greatly appreciated.

[Russ]

I'll do one as an example.

1. P4O6

The overall charge is 0, thus the sum of the charges has to be 0. There are 6 O atoms, each of which have a charge (oxidation number) of -2. Thus, the total charge is -12.
Therefore, the phosphorous atoms need to have a total charge of 12 in order to balance this. There are 4 atoms, which means the ON is +3

[Sonic]

I'll do number 2 as an example, Russ' suggestions can be applied directly to the others

2. The overall charge is -2 so the sum must be -2. Oxygen nearly always has an oxidation state of -2:

The total charge on the oxygen is -14 so it follows the dichromate must have a total of +12 which leads to +6 for the Cr.

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Common Values of oxidation numbers


H = +1, except in halides when it's -1
O = -2, except in peroxides when it is -1
Halogens = -1
Group I metals = +1