okay good afternoon let's start remember
last time we have just finished chapter
eight and today we will start chapter
nine but before we simply start chapter
nine I would like to say a few things
about the formal charge and the
oxidation number of the atoms in a
molecule so formal charge and oxidation
number now we know how to excuse me I
guess I mean we know how to assign the
formal charge from the Lewis structure
if you simply know the Lewis structure
of the molecule we can assign the formal
charges to the atoms let's take for
example water the Lewis structure of
water molecule is like this and then the
formal charge is simply found by
counting the number of electrons belong
to each atom in a molecule I need water
so when we assign the formal charge as I
said we simply find out how many valence
electrons an anthem has in the molecule
now as we count the number of valence
electrons remember last time as we said
the shared electrons are assumed to be
equally shared between the atoms in a
bond so this way in fact if the number
of valence electrons belongs to the atom
is less than it is valence electrons at
neutral state then the oxidation number
becomes plus if it is more than the
oxidation number becomes simply minus so
for the oxygen for example in this case
we have two four and one electron from
here one from there six electrons as a
matter of fact and that simply means
that the oxidation a formal charge on
the oxygen here is zero right
normally the number of valence electrons
on a neutral oxygen atom is six so here
we also have six electrons in the
molecule which belongs to the oxygen
again assuming that the shared electrons
are equally shared and that becomes zero
and for the hydrogen case again one
electron from out of these two electrons
shared electron one belongs to the
hydrogen that means the hydrogen atoms
has one valence electrons
normally hydrogen atom has one valence
electrons at zero state right neutral
case so the oxidation the the formal
charge on the hydrogen is zero the
formal charge on this hydrogen is also
zero like this now if I simply
considered for example hydrogen peroxide
right hydrogen peroxide and write down
the lewis structure for hydrogen
peroxide we have the oxygen bonded to
the oxygen again we have
no the skeletal structure of the
molecule to put the higher the atoms in
the right place and two oxygens are
bonded together like this and then we
have another hydrogen here bonded to
this and another one over here like this
now
how many valence electrons again for the
hydrogen peroxide molecule we have two
from hydrogen's and twelve from the
oxygens this makes 14 electrons now then
I can simply fill out how many electrons
are used here two four six and eight
more electrons I can put up here to make
eight electrons around each atom
particularly in this case oxygen because
remember they when you have an hydrogen
you can only put two electrons around
the hydrogen so this is simply the lewis
structure for the hydrogen peroxide okay
let's try to find out the formal charges
now the formal charge again would be as
follow two four one from there and one
from here that is simply six and the
formal charge on this oxygen is zero and
here is also six electrons around the
oxygen valence electrons and that makes
also zero formal charge for this oxygen
and zero formal charge for the
hydrogen's again like this now let's
consider another example if I take
nitrate ion right as an ion and find out
the Lewis structure write down the Lewis
structure and first we simply write down
the skeletal structure skeletal
structure is nitrogen is in the center
and the oxygens are bonded to this
central nitrogen like this in this
fashion then we have to find out the
number of valence electrons that we will
we can use as we make the Lewis
structures and five valence electrons
from the nitrogen and six from each
oxygen six times three
eighteen plus one minus charge means one
electron so I have to add that there too
and as a result here I have simply 18
electron and then 23 24 electrons 24
lectrons so if I simply put the
electrons around the atoms such that
eight electron Iran each and we can
simply make it again as follow like this
remember we put electron pairs around
the terminal atoms not the central
terminal atoms to make eight electrons
around each and now if you count the
number of electrons this makes 24 24
lectrons
but the nitrogen has six electrons
instead of eight so what we do we make a
double bond here and simply two lone
pairs leave here on this oxygen two lone
pairs and this simply continues again
remember we have the resonance form this
bond may be here may be there and so on
now let's find out the formal charge the
formal charge on this oxygen is two four
six and one electron from there minus
one and formal charge for this oxygen is
also minus 1 here and the formal charge
for this oxygen zero like this and the
formal charge and the nitrogen again one
electron from there two from their 3-1
from therefore normally has to be 5
therefore the formal charge on the
nitrogen is +1 now this is again one of
the resonance form remember there are
two more resonance forms for the nitrate
ion the Lewis structure and the formal
charges
remember when we add up all formal
charges on each atom in a molecule
net charge must be equal to the overall
charge on the molecule or on the iron
again the net charge on the ion here is
minus one nitrate ion and if you add the
formal charges you get also minus one
now how about the oxidation number or
oxidation state can we assign oxidation
state our oxidation number to each atom
in a molecule if we know the Lewis
structure yes the answer is yes how
similar to the formal charge
but the shared electron between two
atoms counts totally for the more
electronegative atom and it counts zero
to the less electronegative atom right
now for example let's consider water
when we find out the formal charge for
example of the hydrogen here we said
that the these two electrons equally
shared so one electron belongs to this
hydrogen here from this bond one here
but as we find the oxidation number we
say okay if I have shared electron
between two atoms I look at the
electronegativity of those two atoms and
for example in this case the
electronegativity of oxygen is greater
than hydrogen so I count these electrons
all two of them
two more electro negative atom right so
this way what happens hydrogen here has
no electrons right if those two
electrons simply counts to the oxygen
and then the number of valence electrons
on the hydrogen actually there's only
one normally no electrons so what does
that means that means that the oxidation
number of hydrogen atom in water is +1
okay plus 1 so we write out here simply
plot a 1 plus or let's put it that way
one plus for the oxidation
or oxidation state now the same thing is
also a tree here one plus B now how
about the oxygen the oxygen cases let's
count the number of electrons we have
two four remember these two electrons
will count for the oxygen because the
oxygen is more electronegative right so
and that makes six and then this one
also accounts for the oxygen because
oxygen is more electronegative so that
makes eight electron eight valence
electrons on the oxygen atom now what
does that means that means that the
oxidation number is minus two two more
electrons then neutral oxygen so this is
simply becomes 1 minus no 2 minus charge
here as the oxidation number is this
clear so when you have a formal charge
and the oxidation number they will not
be the same things and they but if you
have the Lewis structure of the molecule
you can easily calculate or I should say
assign the formal charge or oxidation
number and there are different
differences in fact as we simply proceed
to assign the formal charge and the
oxidation number particularly account on
the shared electrons between two atoms
if they have different
electronegativities so we can for
example do the same things over here and
for the oxidation number for example
this hydrogen will be 1 plus again just
like the same as there because those two
electrons belongs to the oxygen and this
is also 1 plus again because that counts
to the oxygen now how about the oxygens
here in this case we have electron pair
between two atoms which are the same
atoms right what does that means that
means those two atoms have the same
electronegativity now there is no way in
this case that the shared electron will
be equi have to assume that it's equally
shared
so the number of valence electrons on
the oxygen here will be to these two
belongs to that four six one from there
that makes seven and what does that
means one more electron then mutual
oxygen atom in the valence shell and
this becomes the oxidation state makes
the oxidation number or the oxidation
state for this oxygen minus one and this
one is also minus one so we can simply
continue clean like that and assign the
oxidation number to the atoms in the
molecule as a matter of fact we should
also remember that some of the oxidation
numbers in a molecule or in an ion must
be equal to the overall net charge on
the molecule or on the ion so that's and
other things we should remember so the
next question here is usually the
students ask what is this formal charge
is that really the net charge on the
atoms in a molecule the answer is no so
what is the net charge I don't know
exactly but will probably be less than
what the formal charge assigned right
for example the formal charge on the
hydrogen in this case it's plus probably
it's not close no it's a zero but it is
probably not zero right what is it it
may be a little bit plus Delta plus
again now on the other side extreme
cases how about the oxidation number are
the oxidation number indicates the net
charge on each atom in a molecule and
the answer again is no right
so both the oxidation number of an atom
in a molecule and also the formal charge
of an atom in a molecule are not equal
to the net charges on the atom so the
net charge is definitely different than
the formal charge and also different
than
the oxidation number so we have to keep
that in mind
okay now let's this is again what I
wanted to say from the last chapter now
we can go to chapter 9 now chapter 9 as
you see about the molecular geometry and
bonding theories what is it now what are
we going to learn in this chapter now if
I I have a loose structure of a molecule
I actually put all the atoms on the same
plane doesn't matter how many atoms I
have in the molecule so that simply
means the relative positions of the
atoms in a molecule in three-dimensional
space are not expressed by the Lewis
structure of the molecule right the
Lewis structure only tells me which atom
is bonded to which and what are the
number of electrons that shared between
the atoms and what are the number of
electrons that are not shared between
the atoms again which we call them lone
pair so the question is how do we know
the structure or molecular geometry we
call it or the shape of a molecule if
the molecule is made of more than two
atoms right for example if a molecule is
only made up from two atoms we know the
structure the geometry of that because
there is no other way for example if I
have the hydrogen chloride molecule this
hydrogen chloride molecule the geometry
or the shape is linear right so the
hydrogen is here one atom there chlorine
is over here so the shape is linear that
means both atoms are on the same line so
we don't need anything else actually to
predict the job
- you're the shape of the molecule but
if I have more than two atoms in a
molecule or in anion I can't really tell
the shape of the molecule or the iron or
geometry of the molecule or the iron by
simply looking at the Lewis structure
right but what I can do by using the
Lewis structure if I have the Lewis
structure of the molecule or the iron I
can predict what will be the shape or
the geometry of the ion s right or the
molecule is now in this case here these
are simply some shapes this is carbon
dioxide for example here linear and
sulfur dioxide is bent shape and so3 is
trigonal planar and nitrogen trifluoride
is again trigonal pyramidal shape or the
geometry and chlorine trifluoride is T
shape again the question is can I
predict the geometry for these shapes of
these molecules by using the Lewis
structure answer is yes very very simple
model actually let's look at if this
thing's works this one also doesn't work
okay right here now let's look at what
determines the shape of a molecule this
is very important or geometry remember
again shape and geometry is the same
things it's simply the relative
positions of the atoms with respect to
each other in three-dimensional space
three-dimensional space and the main
things that determines the shape of the
molecule is what we call the bond angles
that means the angles made by the lines
which combines the nuclei of the atoms
in the molecule off the atoms in the
molecule now very simple approach is
again as follow it says simply says put
the electron pairs whether they be
bonding or not
repel each other here I guess just
living that I can explain you more
simply on the board here first we start
with how can we predict the relative
positions of the electron pairs around
the central atom in a molecule
now electron pairs around the central
atom again we have a central atom those
electron pairs that we see on the Lewis
structures right and the approaches
purely electrostatic since the electrons
are negatively charged so electron pairs
have to be as far apart as possible from
each other to get the minimum repulsions
of the molecule and the molecule will
prefer the lowest energy which
corresponds that minimum repulsion is
between the electron pairs around the
central atom right so let's consider for
example this is our central atom and
suppose that we have two electron pairs
around the central atom where we know
that from the Lewis structure the
central atom is known number of electron
pairs Arana is known also now if I have
simply two electron pairs by using
simple Coulomb's law and mathematical
approach simple mathematic I can find
out where I should put those electron
pairs around the central atom to get the
minimum charge minimum repulsions and
the calculations or electrostatic
considerations actually tells me that I
have to put those electron pairs around
the central atom in such that they will
be on a line across the atom like this
so that means if I have those lone pair
electrons
here two of them here the other two
should be here and on the same line what
happens if I have them not on the same
line but over here repulsion will be
greater so this arrangement of the
electron pairs around the central atom
for two electron pairs right gives me
the minimum repulsions and the electrons
will prefer to be in that positions in
the act within the molecule right now so
the geometry here let's call electron
pair geometry electron pair geometry or
you can say shape it is the same things
in this case is linear right so this is
simply linear like this okay next is if
I have three electron pairs around the
central atom again by the same approach
purely electrostatic model I can simply
show that if I have three electron pairs
around the central atom I have to put
them on the corner of an equilateral
triangle around the central atom Luka
later triangle so what does that means
this simply means that the angles here
will be 120 degrees right and this one
is 120 degrees that 120 degrees this is
simply act the electron pairs are at the
corner of equilateral triangle and that
gives me the minimum repulsions what
happens if I put the electrons let's say
on somewhere else not the corner of an
equilateral triangle but other place the
repulsion will not be minimum will not
be minimum so now when I have this
electron pairs in this fashion and the
electron pair geometry is called tree
guano planar trigonal planar that is the
geometry electron pair geometry around
the central atom now the name
is what happens if I have four electron
pairs by the same approach if I have
four electron pairs the calculations
which considers the repulsion between
those electron pairs tells me that I
have to put those electron pairs on the
corner of a tetrahedron right now what
is a tetrahedron now tetrahedron is a
three dimensional geometry where it has
four faces and all faces are equilateral
triangle and all those faces are equal
and they are all equal later triangle so
that simply means that if I try to show
it here I will have simply the electron
pairs on the corner of tetrahedron so
the shape would be something like this
and I have four faces all phases are
equilateral triangles and they are
equally away from the central atom and
this geometry is called tetrahedral
right tetrahedral geometry so the
electron pair geometry is tetrahedral
tetrahedral now the next is just by
using what happens if I have five
electron pairs around the central atom
now remember we can put more than eight
electrons around atoms and which means
that in that case I can have five
electron pairs right now again five
electron pairs let's say I have to put
those again considering the repulsion
between them in such a way that they
have to be on the corner of trigonal
bipyramidal geometry trigonal
bipyramidal geometry means that i have
okay
the axial positions over here and I have
the electron pairs here here let's put
that over here
and also
here like this so this is the
arrangement of the electron pairs around
the central atom if I have five electron
pairs to give the minimum repulsions to
give the minimum repulsions now if I
combine this shape here and like this I
can show you those better there actually
and the geometry again electron pair
geometry electron pair geometry becomes
Trego know by parameters remember pi
remit we know the pyramid trigonal
bi-pyramidal means i have two trigonal
pyramid i put them base to base to get
this trigonal bi-pyramidal geometry
trigonal again by pyramidal okay now the
next is again what happens if I have six
electrons around the central atom how
should I simply arrange them around the
central atom where I should put them to
get the minimum repulsion and in that
cases it would be again as follow six
electron pairs the electron pairs has to
be on the corner of octahedron around
the central atom octahedron and the
octahedron is remember a geometry which
have eight equilateral triangular face
right equilateral triangular face so I
have simply this over here like this and
I can combine that again to give the
octahedron so here the geometry becomes
octahedral octahedral geometry okhla
heater geometry now
what is next the next is these are
electron pair geometry how about the
molecular geometry right regular
geometry now if I know actually the
electron pair geometry is I can also
find the molecular geometries again as
follow let's consider the examples okay
now suppose that i have beryllium
difluoride molecule what is the geometry
right off the barium titanite molecule
first i will write down the lewis
structure right and if I write down the
lewis structure for beryllium dichloride
it is again as follow here I have the
chlorine and here chlorine like this
maybe four right all right like that all
right
now this lewis structure tells me that i
have two electron pairs around the
central atom right now where should i
put the electron pairs
it's a I have to put the electron pairs
in a linear geometry which would be like
two electrons here and then three
electrons here this is little bond
distances are exaggerated like this on
the same line
now the chlorine atoms have to approach
in this direction and that direction
should be somewhere on this line to get
the maximum overlap or maximum sharing
between the atoms clear so the chlorine
items here will be here like this and it
would be here like this again I have the
electron pair like this now how do I
know that again the aim here is to get
the maximum overlap if I if I put the
chlorine atoms not here on this line but
somewhere here like this then the
overlap or sharing of this electron
pairs will not be maximum between the
three atoms so again you see the
electron pairs that shared between the
center around the central atom
determines the position actually of
those electron pairs determines where
will be the terminal atoms that will
share those electron pairs clear so now
what is the geometry of the molecular
geometry in that case molecular geometry
I put it like this and that angle is 180
degrees the angles are important because
the shape of the molecule or the
geometry of the molecule mainly
determined by the bond angle and this
bond angle is 180 what does that means
they means that the molecule is linear
you see now the point here is as we
start to determine the molecular
geometry first we try to find out how
many electron pair geometry around the
central atom that's the first step where
we find it from the Lewis structure then
depending on the number of electron
pairs are on the central atom I
determine the electron pair geometry
around the central atom then I put the
terminal atoms which will share those
electron pairs in a position to get
maximum overlap maximum share in
electrons and then then I look at the
molecular geometry right so the electron
pair geometry when I consider the
geometry of this molecule I never think
about electron pairs what I think about
what I simply
consider the positions of the atoms with
respect to each other in a
three-dimensional space and again bond
angles simply determines that so I say
that the beryllium dichloride is a
linear molecule clear very simple very
very simple again now let's say consider
boron trifluoride right boron
trifluoride first we write down the
Lewis structure again the Lewis
structure is as follow we have the
fluorines fluorines like this and we
have the electron pairs but those are
actually not important important ones
are the number of electron pairs around
the central atom right and goes like
this electrons I complete the Lewis
structure now how many electron pairs 1
2 3 where I should put those electron
pairs around the central atom to get
minimum repulsions on the corner of a
trigonal equilateral triangle right
equilateral triangle then I say alright
let me take that here
boron and equilateral triangle two
electrons here two electrons there two
electron here angle here is 120 degrees
all of the angles are 120 degrees like
this right now next is to put the
fluorine where should I put the fluorine
logically I can say that if I put the
fluorines
on the line of the central atom and this
lone pair or electron pair that will be
shared right over here I will get the
maximum overlap and maximum Sharon so
the fluorine will be over here and the
other fluorine will be here on this line
and the other fluorine will be here on
that line so then they share this
electron here so how about the final
shape of the molecule the final shape of
the molecule will be again as follow
I have fluorine chlorine and fluorine
and the angles are 120
degrees 120 degrees like this and the
shape of the molecule is trigonal planar
right we call a trigonal planar what
does that means that means all the atoms
are on the same plane and the terminal
atoms are on the corner of an
equilateral triangle and the central
atom is again in the center of this
equilateral triangle right that's what
it is okay
now the electron pair geometry here is
linear and the molecular geometry is
also linear electron pair geometry here
is trigonal trigonal planar molecular
geometry is also trigonal planar now the
question is are the electron pair
geometry and the molecular geometry are
always the same
no if I have lone pair electrons around
the central atom that means the
electrons that are not shared between
two atoms both belong only one atom in
that case the electron pair geometry and
the molecular geometry are different are
different right now let's look at those
examples
now uh ting dichlorite okay I guess take
another example let's consider one more
I guess that would be better now let's
let's take the ch4 okay what is the
molecular geometry if I write down the
Lewis structure this is like this number
of electron pairs around the central
atom for electron pair geometry let's
say electron pair geometry is
tetrahedral right tetrahedral
tetrahedral now how about the molecular
geometry now by the same approach I can
take the carbon over here their electron
pairs are on the corner of a tetrahedron
that's what the tetrahedral geometry
tells me right on it on the vertices on
the vertices of the simply tetrahedron
like this now the hydrogen's should
approach in this line here this carbon
and lone pair to get the maximum Sharon
and this should also be in this line
here to get the maximum Sharon and this
is also be here in this line to get the
maximum Sharon another one here to get
the maximum Sharon as a result the
molecular geometry becomes also
tetrahedral right molecular geometry
becomes also tetrahedral so let's write
it down over here
and this is the molecular geometry
tetrahedral like this and that's that's
it now in case of tetrahedral how about
the angles here the angles are 109.5
degrees right 109 109 point five degrees
and this one 109.5 degrees all angles
are equal and 109.5 degrees in this case
so in other examples where we have the
electron pair geometry and the molecular
geometry are the same why because
there's no lone pair electrons lone pair
electrons now let's consider ammonia
molecule right alright first we will
write down the lewis structure for
ammonia and the lewis structure is again
as follow we know that simply i have a
lone pair there now how many electron
pairs around the central atom 4 electron
pairs right so the electron pair
geometry electron pair geometry electron
pair geometry will be what tetrahedral
right tetrahedral tetrahedral how about
the molecular geometry ok let's look at
where will be them how will be the
molecular geometry and that's is going
to be hydrogen's hydrogen and that's the
lone pair electrons there right so you
see again the hydrogen have to approach
or should be in a position on the line
which simply combines the central atom
and the electron pairs that electron
pairs and this should be also the same
that should be also the same but that
electron pairs is unshared there's no
hydrogen's there right now what about
the angles the angle here normally it
has to be what 109.5 degrees this is
ideally this is also 109 this is all 109
and the geometry the electron pair
geometry is tetrahedral how about the
molecular geometry molecular geometry
will be considered by only thinking
about the positions of nitrogen and
these three hydrogen's and that is
trigonal pyramid right trigonal pyramid
so the Electra the molecular geometry
molecular geometry the molecular
geometry is trigonal trigonal pyramidal
now here is the again question how about
the bond angles that means what is the
bond angle between this hydrogen
nitrogen and this hydrogen over here and
that one and so on now normally it
should be what 109.5 but in reality this
angle in fact is less than 109.5 like in
the case of methane case right less than
this and the angle actually becomes it's
a little bit sort of the hydrogen is
shifted down and the angle this angle is
107 degrees and that angle is also 107
degrees and the other one is also 107
degrees so it's like this right and this
is a trigonal pyramidal and the angles
are little bit
less than what we expect from the
tetrahedron electron pair geometry why
because this electron pair lone pair
electrons occupies a larger space than
the shared electron pairs and because of
that lone pair electron
simply repels the bonding pair electrons
down and closes the angle and makes it
107 instead of 109
right so that is the difference in fact
between methane and this ammonia
molecule so you see again the electron
pair geometry and the molecular geometry
are different why I have lone pair
electrons clear lone pair electrons
okay now let's can continue and consider
another molecule
and that is water right so if I take the
water and I have simply write down the
Lewis structure the Lewis structure will
be as follow
I have hydrogen here like this and two
lone pairs and another hydrogen right so
what is the electron pair geometry
around the oxygen
yes tetrahedral why four if I have four
electron pairs it is always tetrahedral
to get what minimum repulsions mineral
repulsions so the electron pair geometry
electron pair geometry is tetrahedral
just the same as ammonia and just the
same as methane molecule right four
electron pairs tetrahedral electron pair
geometry saturator
but when I consider the molecular
geometry I only consider the atoms no
lone pair electrons or abundant pair
electrons so now the molecular geometry
here is V shape or it is called bent
it's simply like this right so I have
the oxygen here hydrogen there hydrogen
there question is what will be this
angle do I have an idea about that
actually it should be 109.5 like in
methane case because if I have a
tetrahedral arrangement of the electron
pairs around the central atom the angles
in fact is 109.5 but in this case the
angle is 105 some way about fo not 105
degrees degrees and what does that means
that means it's even smaller than the
ammonia case the bond angle Y you can
easily predict it in the case of ammonia
I have one lone
electrons on the nitrogen central atom
but in the case of water I have two long
pair lectrons like this and those lone
pair electrons push the bonding pairs
closer to each other and the angle
becomes 105 which is less than even for
the ammonia cases so you see again B let
me write it down here the molecular
geometry molecular geometry is V shape
we call it V shape or bent both are used
in fact for this type of molecule V
shape or bent okay I guess we will stop
here any questions so we will continue
on Monday
from this point on