## Oxidation number

Kellina Tran 2I
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Joined: Sat Jul 22, 2017 3:00 am

### Oxidation number

I know determining oxidation numbers was covered in 14A, but can someone remind me on how to do so when balancing redox reactions?

Chem_Mod
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### Re: Oxidation number

You need to know how to determine oxidation numbers to see how many electrons need to be transferred.

Whereas with formal charge, where we assume that the electrons in a bond are shared equally between the two atoms, we assume that the more electronegative atom holds both electrons in the bond.

The general rule is to assume that oxygen has an oxidation number of -2, and hydrogen +1, and calculate the other atoms accordingly considering the molecule's charge. For example, for $Cr_{2}O_{7}^{2-}$, we assume each oxygen has an oxidation number of -2, so in total we have -14. The whole molecule is -2, so both Cr must contribute +12. Therefore, each Cr has an oxidation number of +6.

Neither formal charge nor oxidation numbers describe exactly how the electrons are behaving, but they are useful for their respective contexts.