Oxidation Numbers

[Emily_Bennett_3C]

When a question asks "indicate the initial and final oxidation numbers", how do you go about finding those numbers?

[kara_kremer_2N]

All elements have a zero oxidation charge

All molecules have the charge specified (some you may have to memorize)

Group 1 elements are always +1

Group 2 elements are usually +2

Chalcogens are usually -2

Halogens are usually -1

The charges of the ions have to equal the charge of the total molecule

[Marie_Rhoads_2C]

Some other good things to know about oxidation numbers is that O is almost always a -2 charge when it's with other elements in a compound like SeO₄^2-. For this one the O charges will add up to -8, so the Se must have an oxidation number of +6 to make the overall charge -2. Also, elements in their natural state will have an oxidation number of 0. This still applies for diatomic molecules, so Br₂ has an oxidation number of 0.