Oxidation numbers do not have a true physical significance in the real world, but they are tremendously useful as a book-keeping method when estimating oxidation states. In practice, students utilize oxidation numbers most frequently when balancing redox reactions (see the article "How do I balance a redox reaction?") and identifying which elements have have oxidized or reduced.

To assign oxidation numbers, there are a set of rules that people set forth in various forms. They can be listed out in an expansive form, citing all sorts of exceptions, or they can be listed in a prioritized list, so that earlier rules supercede any that follow them. I prefer the latter, since it makes them far easier to remember.

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RULES FOR ASSIGNING OXIDATION NUMBERS

Directions:
Apply the rules in order. If a later rule would conflict with an earlier rule, ignore it for that molecule or ion. For those who need the reminder, the "exceptions" are placed in [], along with the rules that make this so.

1. In a neutral atom or molecule, the oxidation numbers must add up to zero.

2. In an ion, the oxidation numbers must add up to the charge of the ion.

3. Atoms in elemental form always are assigned an oxidation number of 0.

4. Fluorine always takes a -1.

5. Group 1 metals always are +1. Group 2 metals always are +2.

6. Hydrogen is assigned +1. [will be -1 in metal hydrides. rule #1,#5]

7. Oxygen is -2. [-1 in peroxides, -1/2 in superoxides. rule #1,#5]

8. Halogens are -1. [+x when paired with oxygen or fluorine. rule #4,#7]

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It is always best to see a lesson in action, so here are a few examples using the rules.

CO2 - carbon dioxide
..rule #7, oxygen = -2
..rule #1, total = 0, so carbon must be +4 to balance two oxygens

C6H6 - benzene
..rule #6, hydrogen = +1
..rule #1, total = 0, so each carbon = -1

NH3 - ammonia
..rule #6, hydrogen = +1
..rule #1, total = 0, so nitrogen = -3

NO3 (-1) - nitrate ion
..rule #7, oxygen = -2
..rule #2, total = -1, so nitrogen = +5

CH2Cl2 - dichloro-methane (or methylene chloride)
..rule #6, hydrogen = +1
..rule #8, Cl = -1
..rule #1, total = 0, so carbon = 0

Cr2O7 (-2) - dichromate ion
..rule #7, oxygen = -2
..rule #2, total = -2, so chromium = +6

BrO (-1) - hypobromite ion
..rule #7, oxygen = -2
..rule #2, total = -1, so bromine = +1

C2F4 - tetrafluoro-ethene (precursor for Teflon)
..rule #4, F = -1
..rule #1, total = 0, so carbon = +2

S8 - sulfur
..rule #3, sulfur = 0

NaH - sodium hydride
..rule #5, Na = +1
..rule #1, total = 0, so hydrogen = -1

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