The account system is not necessarily straight forward since in molecular componds, elements share electrons not have ions (i.e. taken an electron, anion or lost an electron, cation) transferred. Inthe molecular compound, an "apparent charge" is used where the more electronegative element is assumed to have the electrons located on it and the least electronegative element not.
As stated above, in a chemical reaction, one element (in a compound or alone) is gaining electron(s) from in the reactant to the product and another is losing an electron(s).Remember, it is the element not the compound that is gaining/losing electrons. So you must first determine the oxidation numbers of all the elements in the reactants and products. Then you can see which elements is losing gaining electrons. There are special name that given to each and other relevant information.
Oxidized element - element that is losing electrons (oxidation number is increasing)
Reducing element - element that is gaining electrons (oxidation number is decreasing)
Oxidizing agent - the substance that is "helping" the oxidized element. (the species that does not have the odixizing element in it, the other chemical formula).
Reducing agent - the substance that is "helping" the reducing element. (the species that does not have the reducing element in it, the other chemical formula).
You can remember what element is oxidized or reduced by the following phrase
LEO the Lion goes GER
The other major point in Redox reactions is that # of electrons gained = # of electrons lost. In other words, there is no Electron Bank to withdraw from or deposit into in the universe.
Balancing Redox Reactions
The following are the two methods to balance Redox Reactions:
a. Balancing Redox reaction by Half-reaction method (book does great job, so learn from book directly)
b. Balancing Redox Reaction by Oxidation Numbers