In considering redox reactions, you must have some sense of the oxidation number (ON) of the compound. The oxidation number is defined as the effective charge on an atom in a compound, calculated according to a prescribed set of rules. An increase in oxidation number corresponds to oxidation, and a decrease to reduction. The oxidation number of a compound has some analogy to the pH and pK measurements found in acids and bases - the oxidation number suggests the strength or tendency of the compound to be oxidized or reduced, to serve as an oxidizing agent or reducing agent. The rules are shown below. Go through them in the order given until you have an oxidation number assigned.
For atoms in their elemental form, the oxidation number is 0
For ions, the oxidation number is equal to their charge
For single hydrogen, the number is usually +1 but in some cases it is -1
For oxygen, the number is usually -2
The sum of the oxidation number (ONs) of all the atoms in the molecule or ion is equal to its total charge. As a side note, the term "oxidation", with its obvious root from the word "oxygen", assumes that oxygen has an oxidation number of -2. Using this as a benchmark, oxidation numbers were assigned to all other elements. For example, if we look at H2O, and assign the value of -2 to the oxygen atom, the hydrogens must each have an oxidation number of +1 by default, since water is a neutral molecule. As an example, what is the oxidation number of sulfur in sulfur dioxide (SO2)? Given that each oxygen atom has a -2 charge, and knowing that the molecule is neutral, the oxidation number for sulfur must be +4. What about for a sulfate ion (SO4 with a total charge of -2)? Again, the charge of all the oxygen atoms is 4 x -2 = -8. Sulfur must then have an oxidation number of +6, since +6 + (-8) = -2, the total charge on the ion. Since the sulfur in sulfate has a higher oxidation number than in sulfur dioxide, it is said to be more highly oxidized. http://www.shodor.org/unchem/advanced/redox/
See Also
12.21 - Fibonacci Whole Numbers v Irrational Decimal near Equivalents
atomic number
Figure 3.00 - Infinite Number of Atomoles or Alphanon filling all Space
Indig Numbers
Ionization
Oxygen
proton number
Redox Reaction
Reduction
Reduction potential
Valence
wave number