I'm going to describe here how we determine or
assign the oxidation statesor the oxidation numbers
for atoms in molecules
This is essentially a bookkeeping method
That is useful when we are balancing redox equations
What we is apply a set of rules
And we have to apply the rules in the order
So what I'll show you is the rules in order
and give you an example of how to apply each of these rules
So let's llok at the first rule
And this says that if we have an element in the oxidation state
the atoms in the element are zero
So for example if we have oxygen molecule
Then the oxdiation state of each atom in
the molecule is zero
The second rule
This says for any molecule or ion
the sum of the oxidation states must equal the
the net charge on that molecule or ion
Again let's look at an example
Look at a chromium compound this particular ion
with for oxygens bound to a chromium
A net charge of minus two
We'll see later how to assign these but
for now I'll tell you oxygen
each oxygen has an oxydation state of minus 2
Four oxygens, that's minus eight
And the chromium, in order for the charge to add up
to minus two, must be plus six
The next rule says that if we have an alkalai metal
in a compound, then its oxydation state is plus one
So for example
sodium choride salt
Sodium is plus one, chlorine must be
minus one to satisfy rule two
Next rule says we assign an oxidation state
to flourine of minus one
Keep in mind this is in a compound
So we look at calcium flouride
Fluorine will be minus one, this tells us calcium
since there are two fluorines must be plus two, so
so the sum of oxidation states is zero
Next rule says that the alkaline earths, namely
the elements in the second column on the periodic table
have an oxydation state of plus two
So for example magnesium oxide
Easy plus two oxygen would be minus two
The next rule then is for hydrogen
Says that the oxydation state of hydrogen in a compound
is plus one
for example, hydrogen chloride
Oxydation state of hydrogen, plus one
Chlorine minus one
Methane, oxidation state of hydrogen
plus one, four hydrogens
So carbon must be minus four so
that the total charge, which is zero, equals
the sum of the oxidation states
Our next rule is for oxygen
So we assign oxygen minus two in a compound
And remember all these rules are in a compound
And so, for example look at HClO4
A rule said that we assign first the hydrogen, plus one
Then we assign the oxygen minus two
So thats four oxygens, minus eight
Plus one for hydrogen means plus seven for chlorine
So the total oxidation state is zero
Look at a different compound
Hydrogen peroxide. We have to assign the rules
in order, so plus one for hydrogen
Which means minus one for oxygen in this case
So rule six takes precedence over rule seven and so
in these peroxides, oxygen has a different oxidation state
So we apply these rules in order
and then calculate the oxidation states of
the remaining elements based on the rule that
the sum of the oxidation states must equal the charge
So let's look at a just a contrast since it's
important to understand that there's a big difference
the oxidation state and the formal charge
So the formal charge again we have a rule
for calculating the formal charge
When we use the formal charge
we are trying to decide which lewis structure
might be the better structure
based on minimizing the formal charge
So here's our rule for calculating formal charge
We take to total number of valence electrons
in the free atom
then we subtract the total of lone pair electrons
for that atom in the compound
and then subtract half of the number of shared electrons
the number of electrons that that atom is
sharing with other atoms
So let's look at an example
So we look at this ion, first let's look at the oxidation numbers
So we can contrast them to the formal charge
By a rule we assign oxygen, minus two
We have three oxygens, so a total of minus six
we have one chlorine and the total charge
must add up to minus one, the total of oxidation states
and so that means the chlorine is plus five
Now let's look at the lewis structure for this compound
So here's the lewis structure, now I want
to calculate the formal charge
Let's look at the oxygen atoms first
Oxygen has six valence electrons
And the we are gonna subtract
the number of lone pair electrons, which is six
and then we subtract half of the number of shared electrons
Which in this case is two
So this says each oxygen atom has a formal charge of minus one
For the chlorine atom, seven valence electrons
minus lone pair
two electron, minus one half the shared electrons
that's six. So the formal charge for chlorine is plus two
And then we check that's the sum of the formal charges
minus three plus two is minus one
and that indeed the charge on this ion
So things are consistent
And notice the formal charges are very different
from the oxidation numbers, and we use them for different purposes