Oxidation number

[kennedyp]

When balancing a redox reaction, how do we find the oxidation number? The book doesn't really say how, unless this was something that I was supposed to have learned before??

[AArmellini_1I]

This goes back to Chem14A. You can find an elements oxidation level by looking at the periodic table. For examples, group 1 elements like to lose one electron so they are +1, and halogens like to gain one electron so they are typically -1. But when you are dealing with a compound with multiple elements it gets a bit more complicated. In this situation you have to consider electronegativity. For example, O has one of the most electronegative elements so it is almost always 2-, meaning if you are dealing with a compound you compare the overall charge of the compound to the charge of the O molecules are use the difference in charge to identify the oxidation levels of the metals, etc. Khan Academy has some great vidoes on this topic if you need a bit more detailed explanation!

[Charlyn Ghoubrial 2I]

The most ones you need to know for this class are:
Any element standing by itself (unattached to another element) is always 0
O2 (when attached) is almost always 2- (H2O2 is an exception: O2 is -1)
H is always +1
And from there you can figure out the rest from looking at equations
The organic chemistry tutor explained this topic really well and has a bunch of other topics explained really good also

[Sara Richmond 2K]

I've attached an image of a rule chart that I use. But basically you set up an algebraic equation for each compound and solve for the elements whose oxidation number you do not know.

[Kayli Choy 2F]

Additionally, when oxidation number of the reactant increases (becomes more positive), that is the substance that is being oxidized. When the oxidation number of the reactant decreases (becomes more negative), that is the substance being reduced.

[Rohit Ghosh 4F]

You can think of the oxidation number as the charge that an atom would have it were to be in a completely ionic bond with the species that is currently covalently bonded to. For example in H2O, the oxygen would grab an electron from each hydrogen and have an oxidation state of -2, because of the extra two electrons. The hydrogens would then have a state of +1.