hello and welcome to the chemistry
solution this tutorial is on assigning
oxidation numbers oxidation numbers are
a bookkeeping system used to keep track
of electrons in a compound all bonded
electrons are assigned to the more
electronegative atom
remember that oxidation numbers are not
real charges on the atoms let's go over
some rules for assigning oxidation
numbers first the oxidation number for
any lone element is zero some examples
would be potassium or aluminum but the
oxidation number is also zero even for
molecules composed of all atoms of the
same element like o 2 or s 8 oxidation
numbers are usually assigned to the most
electronegative elements first which
means that the oxidation number assigned
to fluorine is always negative 1 the
oxidation number assigned to oxygen is
always negative 2 except when in the
compound of fluorine oxidation numbers
are assigned based on the ion they would
form using the group number on the
periodic table so elements in group 1a
and 2a would have oxidation numbers of
plus 1 and plus 2 respectively and
aluminum is assigned an oxidation number
of plus 3 and generally the oxidation
number for hydrogen is plus 1 if
combined with nonmetals and minus 1 if
combined with only metals and lastly the
sum of the oxidation numbers of all the
atoms must equal the overall charge on
the compound let's look at some examples
assign oxidation numbers for all of the
elements in the following compounds
looking at a first compound Oh F 2 we
know that we need to assign the
oxidation number for the most
electronegative element first and that
would be fluorine fluorine has an
oxidation number of minus 1 but there
are two fluorine atoms and so when
assigning oxidation numbers I like to
keep track by writing the oxidation
state right below the atom in the
compound and then multiplying it by the
subscript on that element for a total of
negative two now remember the
some of the oxidation numbers must equal
the overall charge on the compound
so for of2 the sum of the oxidation
numbers must be zero this means that
oxygen must have an oxidation number of
plus two because plus 2 and minus 2
equals zero so in this compound fluorine
has an oxidation state of minus 1 and
oxygen has an oxidation state of
positive 2 moving to the next example
again we assign the oxidation number for
the most electronegative element first
which means that oxygen has an oxidation
number of minus 2 you could also
determine this by looking at where
oxygen is on the periodic table and
remembering that oxygen normally forms
ions with a charge of negative 2 again I
write the oxidation number right below
the atom in the compound and then I
multiply it by the subscript on that
element here we have three oxygen atoms
each with an oxidation number of
negative 2 giving us a total of negative
6 remember that the sum of the oxidation
numbers for all of the atoms in the
compound must equal the overall charge
on that compound and in this case the
overall charge on the compound is 0
which means that the sum of the
oxidation numbers for iron must be equal
to positive 6 now looking at this
compound we have two iron atoms this
means that the oxidation state on each
iron atom must be +3 so plus 3 times 2
which is the subscript on iron gives us
positive 6 and positive 6 plus the
negative 6 which is the sum of the
oxidation numbers on the oxygen equals 0
so in this case oxygen has an oxidation
state of negative 2 and iron has an
oxidation state of plus 3 looking at our
third example remember that hydrogen
normally has an oxidation number of plus
1 when combined with nonmetals when we
multiply this oxidation number by the
total number of atoms of hydrogen in the
molecule this gives us a total of
positive 4 remembering that the sum of
the oxidation numbers must be equal to 0
for a neutral compound we know that the
oxidation state for carbon must be
negative 4 so each hydrogen atom has an
oxidation state of plus 1 and the carbon
atom has an oxidation state of minus 4
looking at our last example first we
assign the oxidation number to our most
electronegative element first this means
oxygen has an oxidation state of
negative 2 this makes the sum of the
oxidation numbers on the three oxygen
atoms minus 6 remember that the elements
in Group 1 have an oxidation state of
plus 1 this is equal to the charge of
the ions that these elements typically
form and remember that hydrogen has an
oxidation state a positive 1 when bonded
with nonmetals this means that the
oxidation state for carbon must be
positive 4 in order for the sum of all
of the oxidation numbers in this neutral
compound to be equal to 0 so oxygen has
a max addition state of minus 2 sodium
has an oxidation state of plus 1
hydrogen is also plus 1 and carbon is
plus 4 thanks for watching the chemistry
solution we hope you enjoyed this
tutorial
you
