Oxidation Numbers

[thegenius]

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There appears not to be a general formula (like calculating formal charge) for calculating oxidation states of an atom in a molecule.

Kind of disturbing in my opinion, because there are all these little rules that apply. For instance

Oxygen is usually -2, but in peroxides it's -1, and in superoxides it's -1/2. Hydrogen is usually +1 when it's comparatively electropositive and can be -1 if it's comparatively electronegative (like in LiH).

Has anyone come up with a better system for calculating oxidation states? I guess it's not *completely* related to the periodic table. I wish it was.

 

[Heliums]

Not to be mean but if you are having problems determining oxidations numbers. . . you need to study my friend. No more parties for you

 

[MoosePilot]

This is a TPR technique, but it's pretty common, too.

These are in descending priority. If a higher rule conflicts with a lower rule, follow the higher rule.

1. Sum of oxidation states of all the atoms in a molecule must equal the charge of the molecule.

2. Group I = +1, Group II = +2

3. Fluorine = -1

4. Hydrogen = +1

5. Oxygen = -2

6. Halogens = -1, Group 16 (Oxygen family) = -2

 

[Assembler]

OK, well, just think of it like this:

Group 1 (Alkali metals): +1
Group 2 (Alkali earth metals): +2
Group 6 (Oxygen Family): -2
Group 7 (Halogens): -1

H is +1, but if some other metal is +1 already, then H can be -1.

I think the basics are all you really need to know. If they ask you to calculate the oxidation state of an individual atom of a molecule, as long as you know the other atoms' oxidation states, and the overall charge of the molecule, you should be good to go...

Edit: Then there's also junk like NO3- or what not, where oxygen would have a state of -2, so there's -6 total, then x + (-6) = -1, so x = -1 + 6 = +5 for N.

 

[thegenius]

Then there's junk like SO42-, NO3-/NO2-/NO-, ClO4-, etc. I think the basics are all you really need to know. If they ask you to calculate the oxidation state of an individual atom of a molecule, as long as you know the other atoms' oxidation states, and the overall charge of the molecule, you should be good to go...

It's that other crap that can be confusing - on a test recently I had As2S3. No rules about that. I made an educated guess and got it right - but it feels unreliable to me. What about something like Al3SI2P4, I just made that up. Can get confusing. But I like your chart (and the previous poster as well.)

 

[Assembler]

Hm, well, like for As2S3, I guess it would be 2(x)+3(-2)=0, so 2x-6=0, 2x=6, x=+3 (As=+3)

 

[thegenius]

Hm, well, like for As2S3, I guess it would be 2(x)+3(-2)=0, so 2x-6=0, 2x=6, x=+3 (As=+3)

Yup, that's right. I basically thought that S must be -2. Because it's close to O. 😀 Also, by electronal analysis and based on answer choices, it could have only been +3, (unless S had an oxidation state of -4, which seemed unlikely.)

 

[Assembler]

heh, yeah, then you seem to have it down. I don't think they'll throw a crazy one in there, like the one you mentioned (Al3SI2P4). You could assume I is -1 and S is -2, but what about Al and P? OK, wait, nvm, Al is commonly +3, and P is -3. OK, but those are like...well, I don't think anyone is going to bother to memorize those. Don't sweat it.

Let's see though, for kicks -> (3)(3)+(-2)+2(-1)+4(-3)=9-2-2-12=5-12=-7 (overall charge of molecule)

 

[thegenius]

heh, yeah, then you seem to have it down. I don't think they'll throw a crazy one in there, like the one you mentioned (Al3SI2P4). You could assume I is -1 and S is -2, but what about Al and P? OK, wait, nvm, Al is commonly +3, and P is -3. OK, but those are like...well, I don't think anyone is going to bother to memorize those. Don't sweat it.

Let's see though, for kicks -> (3)(3)+(-2)+2(-1)+4(-3)=9-2-2-12=5-12=-7 (overall charge of molecule)

LMAO!!! That is hilarious. I meant the molecule, in the first place, to be Aluminum3Silicon2Phosphorus4.

This is funny - I'm rating this thread with a thumbs up. Great thread except for the second poster who gave some, um, great advice.

 

[Assembler]

Whoops 🙂 😀

 

[PRamos]

My O chem review book shows a great way of getting oxidation states. For a molecular formula, it's basically the same thing listed above with the periodic column, but they show how to determine exceptions easily (using their electronegativity).

If the structure is drawn, an atom gets a +1 for every bond to a more electronegative atom and a -1 for every bond to a less electronegative atom. The oxidation state is a sum of those numbers plus its formal charge (if it has one).

I've never seen this method anywhere else and it works really well. For instance, if you draw out SO4exp2-, then you see that S makes six bonds to O, so it carries a +6 oxidation state.

For As2S3, S is more electronegative than As, so it gets a -2 for the two bonds it makes. As gets a +3 for being on the short end of electronegativity in all three of its bonds. Then again, you could say that it's like Al2O3, and assign similar oxidation states.

 

[DieselPetrolGrl]

Hm, well, like for As2S3, I guess it would be 2(x)+3(-2)=0, so 2x-6=0, 2x=6, x=+3 (As=+3)

i did that but i left it at -6 because i didnt know its charge per atom of As...is that suposed to be specified or is that always the case ( is it always the case that they mean per atom)?

 

[BisMuOxo]

i did that but i left it at -6 because i didnt know its charge per atom of As...is that suposed to be specified or is that always the case ( is it always the case that they mean per atom)?

Yes, they always mean per atom.