# Oxidation number

I can't understand the term oxidation number.According to my textbook, it is the charge assigned to an atom. But for what purpose we are doing this?Is it for identifying the oxident and reductant in a reaction?If yes, on what basis the rules (for assigning oxidation states) are implemented?
Also,my textbook doesn't provide the complete rules for balancing an equation(of redox reaction-Oxidation number meathod & ion electron meathod),so,i am very weak in balancing equations.I would like to get some HOT questions in balancing equations of redox reaction.

oxidation numbers are just a way of keeping track of the electron distribution. If electrons are added to an atom, the oxidation number decreases and vice versa.

Here is an example of balancing a redox reaction.

Balance the equation:

NO3-(aq) + H+(aq) + Ag(s) → NO(g) + H2O + Ag+

Balancing wrt. ions and atoms here is not enough. We have to balance wrt to electrons. We do this by isolatin the reduction and oxidation half-reactions and balancing these by atoms. Look up in your table the half reactions:

Red.: NO3- + 4H+ + 3e- → NO + 2H2O

Ox.: Ag → Ag+ + e-

From these we see that to account for the reduction half-reeaction to happen, three Ag atoms must be oxidized. From this we get our stochiometric coeffisients. The balanced reaction is

NO3 - (aq) + 4H+(aq) + 3Ag(s) → NO(g) + 2H2O + 3Ag+

Do you have any questions about the above?

Borek
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