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in this video we're going to go over
oxidation numbers and how to find it so
let's say if we're given the element in
zinc what is the oxidation number of
zinc now the first rule that you need to
know is that the oxidation state of any
pure element is always zero so the
oxidation state of oxygen gas as a pure
element is zero fluorine gas as the pure
element is zero even
phosphorus as a pure element is zero so
there's no charges and it's only one
pure element is not a compound the
oxidation state will always be zero so
that's the first rule you need to keep
in mind now the second thing is the
oxidation state of ions the oxidation
state of the zinc two plus ion is
basically the charge of what you see
there it's positive two the oxidation
state of the Fe plus three I am is
simply positive three now sometimes you
might have diatomic ions for example the
mercury two plus ion individually each
mercury ion has an oxidation state of
one because there's two of them so you
need to write an equation Q mercury
Adams has a net charge of positive two
so if you divide both sides by two you
can get the individual oxidation state
of each mercury particle which is plus
one so there's another example this is
the peroxide ion to find the oxidation
state of each oxygen atom in this ion
you can write an equation is two oxygen
atoms with the total charge of negative
two so individually each oxygen atom has
a charge of minus one so that's the
oxidation state of oxygen individually
in the peroxide honor
this is the superoxide iron so if you
want to find the oxidation state you
need to divide the total charge by two
so each oxygen atom has a net charge of
negative 1/2 so two of them combined
will have a net charge of negative one
so keep this in mind anytime you have a
pure element the oxidation state will
always be zero and if you have an ion
let's say if it's a mono atomic ion the
oxidation state is the same as that ion
now let's talk about compounds whenever
you have fluorine inside a compound when
it's not a pure element fluorine is
always going to have a negative 1
oxidation state fluorine is the most
electronegative element when oxygen is
in a compound it's going to have a
negative 2 oxidation state unless it's
bonded to fluorine or unless you hear
the name peroxide or superoxide whenever
you hear the name peroxide oxygen has a
negative 1 oxidation state if you hear
the word superoxide it has a negative
1/2 oxidation state if you hear the word
oxide then the oxidation state is
negative 2 which is 9 at the seventh
time now hydrogen will have an oxidation
state of plus run when bonded to a
nonmetal
when bonded to a metal hydrogen will
have an oxidation state of negative one
and really the key is electronegativity
hydrogen is more electronegative than
most metals that's why it bears a
negative charge but hydrogen is usually
less electronegative and then most
nonmetals and so that's why there's a
positive charge so typically the element
that's more electronegative is the one
that usually carries the negative charge
now let's work on some examples
what is the oxidation state of magnesium
and chlorine in this compound by the way
most halogens are usually negative one
chlorine technically has a negative one
charge like fluorine if we write an
equation mg plus 2cl
this whole compound is neutral so
therefore the total charge is zero
now if chlorine has a negative 1
oxidation state that means magnesium has
to have a positive 2 oxidation state you
can literally solve it and it makes
sense magnesium is an alkaline earth
metal which typically has a positive 2
charge go ahead and find the oxidation
states of aluminum and fluorine in this
example well we know that fluorine is
negative 1 in a compound
always an aluminum based on where it's
located in a periodic table it's
typically positive 3 within an ionic
compound and you could solve it to Al
plus 3 F should add up to 0
because then that charge is 0 so each
fluorine atom has an oxidation state of
negative 1 so now we got to add 3 to
both sides so aluminum has an oxidation
state of positive 3
is another example find the oxidation
state of vanadium and oxygen and is
kapha so this is called vanadium oxide
so whenever you hear the word oxide
oxygen has a negative two charge so we
got two vanadium atoms plus five oxygen
atoms with a net charge of zero so each
oxygen atom has an oxidation state of
negative 2 5 times negative 2 is
negative 10 and then add 10 to both
sides so 2 V is equal to 10 next divide
both sides by 2 so 10 divided by 2 is 5
and so the oxidation state of vanadium
is positive 5 now let's go over some
examples
containment polyatomic ions consider
sulfate what is the oxidation state of
sulfur and sulfate well no oxygen is
usually negative 2 so let's write an
equation sulfur plus 4 oxygen atoms has
a net charge of negative 2
so each option atom has an oxidation
state of minus 2 and 4 times negative 2
that's negative 8 next we need to add 8
to both sides negative 2 plus 8 is
positive 6 so this is the oxidation
state of sulfur and sulfate let's look
at another example phosphate go ahead
and find the oxidation state of
phosphorus in phosphate so once again
oxygen is still negative 2 so we got a
phosphorus atom plus 4 oxygen atoms and
then that charge is negative 3 based on
what we see here so it's going to be P
plus 4 times negative 2 and 4 times
negative 2 is negative 8 and then add 8
to both sides so negative 3 plus 8
that's going to be positive 5 and that's
the oxidation state of phosphorus
let's look at another example let's try
Nightrain and also glory as perchlorate
go ahead and find the oxidation state of
nitrogen and chlorine in these two
pilots Emig ions so we have a nitrogen
three oxygen atoms and that's going to
equal in that charge of negative one so
always negative 2 3 times negative 2 is
going to be negative 6 and negative run
plus 6 if we add 6 to both sides that's
going to be positive 5
so that's the oxidation state of
nitrogen and for chlorine in per
chlorine it's going to be CL plus 4
oxygens equals in that charge of
negative 1 so this is going to be 4
times negative 2 which is a negative 8
and then negative 1 plus 8 that's going
to give us an oxidation state of
positive stuff so now I know how to find
the oxidation states of elements within
compounds and polyatomic ions now I want
you to understand the concept of
electronegativity and how it relates to
oxidation numbers electronegativity
increases towards fluorine on a periodic
table so as you go up into the right the
electronegativity increases so let me
give you some values of common elements
so let's say hydrogen is somewhere in
the corner over there and then we have
boron carbon nitrogen oxygen fluorine
chlorine bromine iodine phosphorus and
sulfur hydrogen has an electronegativity
value of 2.1 for boron is 2.0 carbon is
2.5 and then 3.0 3.5 floorings the
highest is 4.0 phosphorus it's two point
one is the same as hydrogen sulfur is
2.5 chlorine the street for no
and this is 2.8 iodine is 2.5 so keep
these values in mind so here's a
question for you
what is the oxidation state of oxygen
and flowing in oxygen difluoride now
oxygen has an electronegativity value of
3.5 fluorine is 4.0 so which one is more
electronegative electronegativity is the
ability of an atom to attract electrons
to itself so flowing is going to pull on
electrons in this molecule it's going to
have a stronger Pole than oxygen so
flowing is going to acquire a partial
negative charge whereas oxygen is
therefore going to acquire a partial
positive charge because fluorine pulls
on the electrons stronger than oxygen
can so in this example oxygen will not
have its typical charge of negative 2
the only time oxygen will have its
oxidation state of negative 2 is if it's
the most electronegative element in that
compound if it's not then it's going to
have a positive oxidation state keep in
mind any time flame is in the compound
it has an oxidation state of negative 1
and the reason for that is because
fluorine is the most electronegative
element on a periodic table so now we
can solve for oxygen so o plus 2 F
should have a net charge of zero because
there's no number here so fluorine is
negative 1/2 times negative 1 is
negative 2 so if we add 2 to both sides
oxygen is going to equal positive 2
which makes sense because it's partially
positive in this particular example
now let's look at two other examples
hydrochloric acid and sodium hydride
chlorine has an electronegativity value
of 3.0 hydrogen is 2.1 and sodium it's
like 1 point something I'm not sure what
the exact number is it could be like 1.5
1.7 but I know it's less than 2 so in
this example hydrogen there's a partial
positive charge chlorine there's a
negative charge because chlorine is more
electronegative than hydrogen so
therefore chlorine is going to have its
oxidation state of negative 1 which is
typical of most collisions hydrogen is
going to have an oxidation state of plus
1 as you mentioned before whenever
hydrogen is bonded to a nonmetal the
oxidation state is usually positive 1
now what about in sodium hydride well we
know that sodium is an alkali metal
which always have a positive run charge
so therefore sodium is going to have an
oxidation state of +1 but hydrogen has
an oxidation state of negative 1
typically when hydrogen is bonded to a
metal it usually has a negative Run
oxidation state and it makes sense
because hydrogen is more electronegative
than most metals so it usually bares the
partial negative charge that's why it
has a negative oxidation state sodium
has the positive charge still it has a
positive oxidation state and so you can
use electronegativity to help you
determine what the oxidation state will
be so let me give you another example BH
Stream
what is the oxidation state of boron and
hydrogen feel free to try that one now
hydrogen has an electronegativity value
of 2.1 and boron is 2.0 now as boron a
metal or nonmetal in this example
hydrogen is more electronegative so
hydrogen bares the partial negative
charge or boron bears the partial
positive charge so therefore hydrogen
it's going to have its oxidation state
of negative one because it's more
electronegative than boron so then this
is going to be B plus 3 H which is equal
to zero so 3 times negative 1 is
negative 3 so boron is going to have an
oxidation state of positive 3 in this
example now let's consider these two
examples so Furyk acid or rather
hydrofluoric acid and also sulphur
dioxide now hydrogen has an en value of
2.1 sulphur is 2.5 an oxygen is 3.5 so
in sulphur dioxide oxygen has the
partial negative charge sulphur has the
partial positive charge now in h2s
hydrogen has the partial positive charge
sulphur has the function negative charge
9 a periodic table we have elements like
nitrogen oxygen fluorine typically
nitrogen has a negative 3 charge oxygen
- 2 fluorine negative 1 so for 2 sulfur
usually has a negative 2 charge if if
sulfur is the more electronegative
element so looking at h2s hydrogen is
brought in to a nonmetal that is more
electronegative than itself so hydrogen
is going to have the positive 1
oxidation state and there's two of them
so sulfur in this example has its normal
oxidation state of negative 2
so you can base your answer on a
periodic table if sulfur is the more
electronegative element now in so2 you
can't do that because sulfur doesn't
have the partial negative charge so you
can't based a charge on a periodic table
you can do so however for oxygen because
oxygen is the electronegative element in
that compound so you can use the
negative two charge for oxygen so oxygen
is going to have an oxidation state of
minus two and to find it for sulfur it's
going to be s plus two oxygen atoms
equals zero so that's 2 times negative 2
which is negative 4 so sulfur is going
to have a positive oxidation state of 4
due to the positive partial charge so
elements that are less electronegative
typically those are the ones you got to
solve for the ones that are more
electronegative you can find a charge
based on a periodic table if they carry
a negative charge now let's look at some
other examples nh3 and no2 go ahead and
find the oxidation state of each element
now in ammonia hydrogen has an en value
of 2.1 but nitrogen is more
electronegative
it's 3.0 so therefore nitrogen should
have its normal charge of negative 3 if
we write an equation n plus 3 H is equal
to 0 hydrogen is going to have a
positive run charge it's partially
positive whereas nitrogen is partially
negative so typically when hydrogen is
bonded to a nonmetal it's usually +1
which means n has to be negative 3 so as
you can see nitrogen is the
electronegative element in this example
and it has its periodic charge of
negative 3 which you can find out in
periodic table now in this case o is
more electronegative so nitrogen is
going to have a different oxidation
state it's not going to be its natural
oxidation state of negative 3
so in this example it's going to be
positive for typically when you have
elements like nitrogen sulfur phosphorus
if they carry and the element that's
more electronegative than itself those
are the elements together solve for the
one that usually has a partial positive
charge now try these two examples
methane and the carbon dioxide and
methane hydrogen has a positive one
charge hydrogen is less electronegative
than carbon so it's going to be
partially positive carbon is going to be
partially negative so solve them for
carbon we have C plus 4-h is equal to
zero so that's 4 times 1 so C is
negative 4 so when carbon is bonded to
hydrogen carbon has a negative oxidation
state when carbon is bonded to oxygen
it's going to have a positive oxidation
state when it's bonded to hydrogen it
has a negative oxidation state so oxygen
is negative 2 and there's two of them so
carbon is going to have to be positive 4
in this example now sometimes you might
have elements that have an average
oxidation state that's not a whole
number let's try these two c3h8 and
fe3o4
in this example hydrogen is less
electronegative than carbon so it's
going to be positive 1 so if we write
the formula 3 C + 8 H it's equal to zero
so that's going to be 8 times 1 and if
we subtract 8 from both sides 3 C is
equal to negative 8 so carbon on average
has an oxidation state of negative 8
over 3 now let's do the same thing for
fv304 so we got 3 iron atoms and 4
oxygen atoms within that charge of zero
so oxygen has an oxidation state of
negative 2 so 4 times negative 2 that's
negative 8 and if we add 8 to both sides
when you get this so Fe has an oxidation
state of 8 over 3
so 8 over 3 is about 2.67 now keep in
mind an individual iron atom cannot have
a charge of 2.67 it's usually a whole
number like positive 2 or positive 3
because electrons and protons there they
basically have numerical charges an
electron has a charge of negative 1 a
proton has a charge of positive 1 so a
typical ion will have a decimal charge
so what does it mean that the average
oxidation state is 2 point 6 7 so what
is meant by that in this compound there
are three are in ions and four oxygen
ions each oxygen has a charge of
negative two so the total negative
charge is negative 8 in order for the
compound to be electrically neutral the
total positive charge has to be positive
8 iron metal has two common oxidation
states positive 2 and positive 3 now
they all can't be positive 3 because 3
plus 3 plus 3 is 9 and they can't all be
positive 2 because 2 plus 2 plus 2 is 6
so some of them is positive 2 and some
are positive 3 so the question is how
many iron ions to have a +2 charge and
how many have a +3 charge in order to
get up to eight two of them has to have
a positive 3 charge and one of them has
to have a positive 2 charge if you
average the numbers 2 3 and 3 and
divided by 3 that's going to be 8 over 3
which averages out to 2 point 6 7 so
whenever you get a decimal value what it
really means is that that's the average
oxidation state individually some more
positive 3 and some are positive 2 so
the individual ions should have a
numerical oxidation state so when you
have multiple of them the average could
be a
no value because these they don't all
have to be the same they can be
different so hopefully this makes sense
in terms of why some oxidation states
have a decimal value now let's try the
polyatomic ions that have three
different elements in it go ahead and
find the oxidation state of every
element in that polyatomic ion so oxygen
has an oxidation state of negative two
hydrogen is positive one when it's
bonded to nonmetals so usually so all
we're going to do is find sulphur so H
plus s plus three oxygen atoms as in
that charge of negative one so hydrogen
is one oxygen is negative 2 and so 3
times negative 2 that's negative 6 and
then 1 plus negative 6 is negative 5 so
now let's add 5 to both sides
negative one plus five is positive four
so in this example sulfur has an
oxidation state of positive four go
ahead and try this one k2 cro4 find the
oxidation state of every element in that
example so we have two potassium atoms a
chromium atom and four oxygen atoms now
we know oxygen is going to have an
oxidation number of negative two
potassium is an alkali metal which all
of them have a positive one charge
chromium is the transition metal and it
has a variable charge so that's what I'm
going to solve for so this is going to
be 2 times 1 plus CR plus 4 times
negative 2 and all of that is equal to 0
so 4 times negative 2 that's negative 8
and 2 plus negative 8 is negative 6 so
therefore in this example chromium has
an oxidation state of positive 6
try this one potassium bicarbonate find
the oxidation state of carbon in this
example so we know oxygen is going to be
negative to potassium and alkali metal
is plus 1 and hydrogen hydrogen is
actually bonded to the oxygen in
bicarbonate if you were to draw the
Lewis structure
so therefore hydrogen is bonded to a
nonmetal through a covalent bond and so
it's going to be plus 1 so we have K
plus h plus c plus 3 o and that's equal
to 0 so K is positive 1 hydrogen is 1 an
oxygen is going to be 3 oxygen is
negative 2 but we're going to multiply
that by 3 so 1 plus 1 is 2 3 times
negative 2 is negative 6 and then 2 plus
negative 6 that's negative 4 so in this
example carbon is positive for potassium
bicarbonate you could break it up into
two ions k+ and hco3 minus so just by
looking at k+ that tells you that cain
has an oxidation state of positive 1 now
bicarbonate is basically the sum of the
hydrogen ion and the carbonate ion so
therefore you can see that hydrogen in
this example also has a positive one
charge and then from this you can find
the oxidation state of carbon you can
say c plus 3 own has in that charge of
negative 2
then you have to add six to both sides
so negative two plus six is positive
four so if you understand the ions and
all the polyatomic ions you can break it
down and efficiently to see that
hydrogen has a positive one charge in
this example and the same is true for K
so that's why it's good to know the
polyatomic ion Chi now have two more
examples fully BR CL 3 and IBR 5 find
the oxidation state of every element in
this example
so most halogens like fluorine chlorine
bromine iodine they typically have a
negative 1 charge but both bromine and
chlorine can't be negative so which one
is negative and which one is positive
keep in mind bromine has an
electronegativity value of 2.8 chlorine
is 3.0 iodine is 2.5 so in this example
chlorine bears the partial negative
charge bromine is partially positive so
therefore chlorine is going to have its
natural oxidation state of negative 1
bromine we need to calculate it so it's
going to be BR plus 3 CL and that's
equal to 0 so this is going to be 3
times negative 1 and so we can see that
bromine has an oxidation state of
positive 3 now in the second example
bromine is going to carry the partial
negative charge
iodine carries the partial positive
charge if it's written correctly
usually the electropositive element is
written first
the electronegative element is there in
second so the one that you see on the
right side is usually the one that
carries the natural charge that can be
found on the clear example so in this
case bromine is going to have its
natural oxidation state of negative 1
so iodine it's going to have an
oxidation state of positive five in this
example
so hopefully you understand the
relationship between electronegativity
and oxidation numbers so that's it for
this video thanks for watching and have
a good day
