Rules for assigning oxidation numbers

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1.) In Free Elements (elements w/o a charge)..

..each atom has an oxidation number of zero(0).
Ex. H₂, F, S₃

2.) Monoatomic ions (elements w/ a charge)..

..oxidation number is the same as the charge.
Ex. Li⁺ ON=1 Sr²⁻ ON=-2

3.) The ON for Oxygen..

..is USUALLY -2.
Common exception is in Hydrogen Peroxide (H₂O₂) and other peroxides (O₂²⁻) in which the ON is -1

4.) The ON for Hydrogen..

..is usually +1 except when it is bondend to metals in binary compounds, in which case it is -1
Ex. (H₂O) ON= +1
LiH ON=-2

5.) Flourine has an ON of..

-1 Other halogens can have an ON of -1 or various positive ON depending on what they are paired with.

6.) In a neutral molecule..
In a polyatomic ion..

..the sum of the all ON have to equal zero (0).
..the sum of all the ON's of all elements must equal the net charge of the ion.

Oxidation numbers..

..dont have to integers
Ex. O₂⁻ ON=-1/2

Metals have positive oxidation states

Group 1A metals ALWAYS have an oxidation state of +1
Group 2A metals ALWAYS have an oxidation state of +2

Non Metals: F, H, O, Group 7A, Group 6A, Group 5A

F: -1
H: +1
O: -2
7A: -1
6A: -2
5A: -3

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