Now that we know the distinction between oxidation and reduction, it's important to set down
some ground rules to help us figure out what’s the particular oxidation state on a particular
element.
I break rules for assigning oxidation numbers, which we’re going to abbreviate ON, so oxidation
number is ON.
I break it down into two sets of rules.
We have our two general rules and then we have our more specific rules.
The two general rules.
We're going to say for an atom in its elemental form, what does elemental form mean?
Elemental form means two things.
It means that the metal is by itself such as sodium.
Sodium metal is by itself.
Or the element is connected to itself.
Oxygen here is connected to another oxygen.
Sulfur here is connected to a bunch of sulfurs just like itself.
If you're by yourself or you’re connected to copies of yourself and you have no charge,
that is your elemental form.
Let's write that down.
Elemental form means you are by yourself or connected to copies of yourself
and you have no charge.
If you fit these types of categories, then your oxidation number is equal to 0.
The second general rule is for an ion, whatever the ion’s charge is, that's its oxidation
number.
Sodium here is by itself but it has a plus one charge, so its oxidation numbers is plus
1.
Calcium is by itself but it has a plus 2 charge, so its oxidation number is plus two.
NO3 minus, that’s our nitrate ion, our polyatomic ion.
The oxidation number for the entire compound is negative 1.
If we want to find the oxidation state of each of the individual elements, that's when
we move on to our specific rules.
Our specific rules, we're going to say group 1A, plus 1.
Group 2A, plus 2 based on the charge distributions that we learned earlier.
What we’re going to say if a hydrogen, hydrogen is in group 1A but a hydrogen is not always
plus 2.
We're going to say it's plus 1 when it's connected to non-metals, so if it's connected to Cl
or perchlorate or in water.
In these three examples, water is connected to only non-metal so it’s plus 1.
But if a hydrogen is connected to a metal or boron, it’s minus 1.
If we had NaH or BH3, then in those cases hydrogen would be negative 1.
We’re going to say fluorine is always minus 1 no matter what.
Oxygen on the other hand can be a pain because oxygen has various oxidation states.
It's all based on what kind of compound it’s in.
We're going to say for oxygen, when it's a peroxide, it's going to be negative 1.
What exactly is a peroxide?
A peroxide has the formula X2O2.
X equals a group 1A element, so hydrogen, lithium, potassium – all of those are group
1A metals.
A good example here, you could have hydrogen peroxide or sodium peroxide.
Both are peroxides because both have the formula X2O2.
In this case, oxygen would be minus 1.
Oxygen will be negative a half when it’s a superoxide.
A superoxide has the formula XO2.
X again is a group 1A element.
Example, you could have sodium superoxide or lithium superoxide.
We're going to basically say if oxygen is not a peroxide or superoxide, we can say that
it's -2.
This is the normal state of oxygen because we seldom see peroxide or superoxide.
But if we do not see one of those two, remember oxygen would be minus 2.
Group 7A elements or halogens.
They're going to be minus 1 except when they're connected to oxygen.
When they're connected to oxygen, we're going to have to use some mathematical knowledge
in order to do it.
Simple algebra.
We’ll get to see how this works when we do practice questions dealing with this concept.
Remember, we have our two generals and then we have our more specific rules.