What does the oxidation states of each element in a compound mean?

[gsingh2011]

If I have SO₄^2-, I can figure out that the oxidation state of each O ion is 2- and the sulfur ion must then be 6+. What does this number mean? What does it tell you about the electrons in that molecule?

 

[espen180]

Oxidation number is not a physical quantity. It is a tool used to keep track of electrons in reactions. If the oxidation number is increased during a reaction, the atom has electrons taken away from it and vice versa.

Example:
Ox. numbers:　+III　　　+II　　　　　　0　　+III
Reaction:　　　　Au³⁺ + 3Fe²⁺ → Au + 3Fe⁺³

In the process of this reaction, a gold ion (Au³⁺) steals one electron each from three iron ions (Fe⁺²). This is shown in the change of their oxidation numbers.

 

[gsingh2011]

So the oxidation number of an element at a point in time has no meaning, it is only the change in the number that has meaning?

 

[espen180]

You could put it that way, but you don't have to give so much thought to what the numbers represent so long as you can get the neccesary information from their utilization.

When I learned about them first, it was as a tool for balancing reactions, since some reactions have to be balanced with respect not to molecules o ions, but with respect to electrons, like the one I posted above. In that sense you are right, the actual alue has no physical meaning. It's kind of like potential energy. The value in question can be anything, but it's change during precesses is well defined.

 

[Borek]

some reactions have to be balanced with respect not to molecules o ions, but with respect to electrons

Actually every correctly balanced reaction should be balanced both in terms of atoms (to reflect mass conservation) and charges (to reflect charge conservation).