Determining Oxidation Numbers

[Emily Jacobo 1C]

Does anyone have any tips or methods of how they remember how to find the oxidation number? I sometimes have trouble (most recently in the Sapling homework #9) finding the oxidation number.

[Kaylee Messick 3J]

I remember it as the charge of the atom/molecule that will make the charge for the entire compound equal to what was stated in the problem. So I would add all the known charges and find the difference between that number and the entire charge of the compound. Hope this helps!

[Gina Spagarino 3G]

I liked the example that the electrons "go back" to where they "came from," or thinking of the compound as if it were made of ions and not bonded covalently... so in a coordinate covalent bond, both electrons "go back" to the ligand for each bond

[SophiaBarden 2E]

When trying to find the charge of a metal in a complex that you are unfamiliar with, it is best to calculate the formal charge of each atom respectively. Then do the equation [overall charge] - [sum of all formal charges] = the oxidation number (charge) of the atom you need.

For example, finding the charge on a metal ion in a complex with an overall charge of -2, and a sum of formal charges of -4 ;
[-2] - [-4] = +2 <-- so the metal has a charge of +2 and an oxidation number of (II)

[Hana Sigsbee 3B]

I remember that we obviously need to charge to be equal to the charge given in the problem so I add all the known charges and then I know the oxidation number of the unknown will make it reach the correct charge from there. For example, if the total charge is known to be +1 and with the given charges we have a -1 charge then we know the oxidation number will be +2.

[Natalie 3k]

I have some trouble with this too, but normally I just think of it like an equation! Each atom has its own charge, and the molecule as a whole has a charge, and the sum of the charges of the atoms needs to equal the charge of the molecule. Normally all the atoms except the one you're looking for has a clear or known charge which makes it easier to find the one you don't know.

[Aydin Karatas 1F]

Always start by identifying the charge of the atoms/molecules directly bound to the metal. Then compare that to the overall charge of the larger complex, because the charge of the metal (oxidation number) plus the charge of the other atoms/molecules should equal the overall charge. In the case where a coordination compound is bound to an external ion, you need to look at the charge of the external ion as well, such as in [Co(NH₃)₆]Cl₃ (we know the oxidation number of Co is III because, even though NH₃ is neutral, Cl₃ provides a 3- charge.

Hope this helps.