| If you have a pure element, the oxidation state is 0. |
For example, K is a pure element in its 0 state. |
| If you have an isolated ion that consists of one atom that has either
gained or lost electrons, the oxidation state is equal to the charge on that ion. |
Na+ is in a +1 state. Chloride ion (Cl-) has a -1
state for the chlorine. |
| In a compound more figuring has to be done. Remember that all of the
oxidation states have to add up to zero. First find the most electronegative element and
give it the negative state that it wants. That usually works. Then figure out what the
other element has to be to bring the total to zero. |
In Cl2O the oxygen gets to be -2, then the chlorine must be +1
in order for the oxidation states of all the atoms to add up to zero. In Fe2O3
the oxygen gets to be -2, then the iron must be +3 in order for the oxidation states of
all the atoms to add up to zero. |
| If there is more than one other element, start with the ones you can count
on being +1 because they are in group IA or +2 because they are in group IIA. Then figure
out the others. |
In HNO3 the oxygen gets to be -2, and we can count on the
hydrogen to be +1, then the nitrogen must be +5 in order for the oxidation states of all
the atoms to add up to zero. |
| If you are dealing with a polyatomic ion by itself, the only change is
that all of the oxidation states have to add up to the charge on the ion. |
In PO43- the oxygen gets to be -2, then the
phosphorus has to be +5 so that the oxidation states of all the atoms adds up to -3. |
| If you recognize a polyatomic ion in the formula, it is often helpful to
note it and its charge because that can help you figure out the oxidation state of the
element it is combined with. Then continue with the rest as before. |
In FeSO4 the sulfate ion has a -2 charge, therefore the iron
has to be +2 in this compound. The oxygen gets to be -2, then the sulfur has to be +6 in
order for the oxidation states of all the atoms to add up to zero. CoCO3,
because carbonate is -2 the cobalt has to be +2. The oxygen is -2 and the carbon is +4. |
