Finding Oxidation Numbers
You assign oxidation numbers by following a set of rules that were established because dealing with electron bookkeeping in every compound from scratch would take forever. The system works for most common chemistry problems, but it breaks down in transition metal clusters and organometallics where the simple rules give you fractions or contradictions. The basic hierarchy goes like this. Fluorine is always negative one. Oxygen is usually negative two except in peroxides where it is negative one, and when bonded to fluorine. Hydrogen is positive one when bonded to nonmetals, negative one in metal hydrides. Group 1 metals are positive one, Group 2 are positive two. Halogens are typically negative one unless bonded to oxygen or a more electronegative halogen.
How Do You Find The Oxidation Number in Practice
The actual procedure is simpler than people make it seem. Write the formula, assign the known values to the atoms that have fixed rules, then solve for the unknown using the constraint that the sum equals the overall charge. For something like potassium dichromate, K2Cr2O7, you put positive one for each potassium, negative two for each oxygen, then solve 2 plus 2x minus 14 equals zero, which gives you chromium at positive six. I spent three hours once trying to balance a reaction with manganese in an alkaline permanganate solution where the oxidation state assignment seemed straightforward until I realized the product was MnO2 at positive four, not the manganate ion. The reaction conditions changed what I thought the species actually was. That took me back to checking the pH dependence of manganese redox potentials rather than trusting my initial assignment. The tricky cases show up when you have sulfur in thiosulfate, S2O3 2 minus. The central sulfur is positive six and the terminal one is negative two, but the average comes out to positive two, which is what most introductory courses expect you to report. I used to mark students wrong for not giving the average, then realized I was the one being dogmatic about a convention that exists purely because it makes balancing equations easier.
Common Pitfalls and Where the Rules Fail
People routinely misassign oxidation numbers in compounds with heteroatoms bonded to each other. In peroxydisulfate, S2O8 2 minus, there is a peroxide bridge between the two sulfur centers, so two of the oxygens are negative one instead of negative two. If you treat all eight oxygens as negative two, you get sulfur at positive five, which is wrong. The correct value is positive six for each sulfur. Transition metals create the most consistent headaches. Iron in Fe3O4 is neither positive two nor positive three across the board, it is a mixed valence compound containing both Fe2O3 and FeO structural units. You can assign average oxidation states, which is standard practice, but the reality is two iron atoms at positive three and one at positive two. Organic compounds follow the same rules but require you to treat C-H bonds as giving carbon a negative contribution and C-X bonds as giving positive contributions. In ethanol, CH3CH2OH, the methyl carbon is negative three, the methylene carbon is negative one, and the carbon bearing the hydroxyl group is positive one. Most students just calculate an average for the whole molecule, which is technically defensible but misses the actual electron distribution.
Get the Full Details
When Oxidation Numbers Lie to You
The concept assumes ionic bonding as a model, so it performs poorly for covalent networks and metallic systems. In graphite, you could argue carbon is zero, but the delocalized electrons don't belong to any single atom. In metallic iron, the oxidation state is formally zero, but that tells you nothing about the electron density or reactivity. Bond orders and actual charges differ from oxidation numbers. Carbon monoxide has carbon at positive two and oxygen at negative two, but the actual charge distribution is closer to neutral because of the back-bonding from oxygen to carbon. The oxidation state formalism is a bookkeeping device, not a measurement of real electron density. If you need actual charge distributions, use Mulliken population analysis or natural bond orbital calculations. Oxidation numbers are useful for balancing equations and tracking electron transfer in redox reactions, but they are not quantum mechanical observables. I learned this the hard way when a student asked me why the oxidation state of iron in hemoglobin did not match the Mössbauer isomer shift data, and the answer was simply that they measure different things.
