Redox Numbers

In order to identify Redox chemistry, chemists have developed a system by which we can think about the "number of electrons" a certain element "has".  This is an idea that simply helps sort out the oxidation and reduction process.

In every chemical species, we can assign an oxidation number (or oxidation state) to each element.  This can be thought of as the number of electrons that "belong" to that element compared to the number of valence electrons the element has.  It is important to note that this is an idea that helps us to think about and classify the chemistry.  Within a compound the electrons don't have any labels and don't belong to anyone one element.  This is merely an accounting trick for us to use as chemists.

Assigning oxidation numbers is very straight forward for atomic ions.  Take Na+ as an example.  Normally sodium has one valence electron.  As an ion it has zero valence electrons.  We could then state that the oxidation number (or oxidation state) of Na+ is plus one.  It is "missing" one electron.   Similarly the oxidation state of Cl- is negative one.  It has an "extra" electron.  So for any atomic ion, the oxidation state is simply the charge.  So in the compound NaCl, the oxidation number for Na is +1 and the oxidation number for Cl is -1.

What about polyatomic ions or compounds?   For these we need to consider the oxidation state for each element in the chemical species.  Here the oxidation number can be thought of as the charge that atom would have if the chemical species were broken up.  It is as if we are thinking about all compounds as being ionic compounds.

For this we have a series of rules.

  1. The sum of all the oxidation numbers in a chemical species must equal the charge on the chemical species.  Therefore if we are looking at a neutral molecule, when we add up all the oxidation numbers for all the atoms in the molecule, we should get zero.  If we have a polyatomic ion with a charge of -2, then the sum of the oxidation numbers of all the atoms should be -2.   This is the most important rule.  It will help us figure out the oxidation number of any elements that are not "assigned" a number.
  2. The oxidation number of an atom in its neutral elemental state is zero.   This is true of neutral monatomic species as well as polyatomic neutral elements. For example, what is the oxidation number of oxygen in O2?  Zero.
  3. The oxidation number for a monatomic ion is its charge. The sum of oxidation number is the charge. There is only one element. Thus the oxidation number is the charge.
  4. In compounds hydrogen is given the oxidation number of +1.  Unless it is bonded to a metal in which case it is -1.  (Note: H2 is not a compound.)  What is the oxidation state of H in H2O?  +1.  What is the oxidation state of H in CH4? +1.  In LiH?  -1.  In H2.  0.
  5. Oxygen is given the oxidation number of -2.  Unless it is bonded to another oxygen in a peroxided bond (oxygen oxygen single bond).  Then it is -1.   What is the oxidation number of O in H2O?  -2.  In hydrogen peroxide, H2O2? -1.
  6. The most electronegative element is assigned its charge as an ion.  What is the oxidation state of Mg and Br in MgBr2?  Br is -1,  Mg is +2.  This is essentially the rule for the monatomic ions.

From there you can work out any element in any compound.  General rule.  Assign H and O first.  The rest typically fall out form there.

What is the oxidation state of N in NH3?  H is +1, the whole compound is neutral, so N must be -3 since there are three hydrogen at +1 each.

Redox Numbers