Oxidation States Ref

Oxidation States

Remember that elements can have various oxidation states depending on the number of electrons that they have compared to the number of protons that are in the nucleus.

	If the number of electrons is the same as the number of protons, then we say that the atom is neutral and that it has an oxidation state of 0.
	If the element has reacted in a way to have gained electrons, then it has a negative oxidation state, because the gained electrons have negative charges.
	If the element has reacted in a way that has caused it to lose electrons, then it has a positive oxidation state, because it has lost negative charges.

Guidelines

Here are some guidelines that you can use to figure oxidation states of chemicals.

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.

Practice

Now practice by determining the oxidation states of the elements in these chemicals (which are also listed in exercise 11 in your workbook). Check your answers below.

Au3+

N2

FeCl3

N2O3

NO3-

PCl5

OF2

H2SO4

HNO2

MnO4-

Answers

Au3+	N2	FeCl3	N2O3	NO3-	PCl5	OF2	H2SO4	HNO2	MnO4-
Au is +3	N is 0	Cl is -1 Fe is +3	O is -2 N is +3	O is -2 N is +5	Cl is -1 P is +5	F is -1 O is +2	O is -2 H is +1 S is +6	O is -2 H is +1 N is +3	O is -2 Mn is +7