okay good afternoon let's start where we
left last time remember chapter 9 we
have started chapter 9 let's try to
refresh our memories and remember what
we did last time remember again this
chapter is about molecular geometry and
bonding theories so first we will talk
about the molecular geometry or
molecular shape these are exactly the
same things actually and they are
determined by the bond angles and also
the bond lengths particularly the size
and the shape of the molecules is
determined by these two factors bond
angles and bond length now if we have
simply two atoms molecule the structure
is simple because it's a linear molecule
there will be no other way to put the
two atoms other than putting them on the
same line so this structure will be
linear but when you have more than two
atoms in a molecule as we said last time
then the structure of the molecule may
not be predicted directly from the Lewis
structure again you can have the Lewis
structure of the molecule but the Lewis
structure does not directly tells you
the geometry or the shape of the
molecule
right because the Lewis structures is
sort of a two dimensional structures and
we put all the atoms on the same plane
and put the lone pair electrons but as
we said last time if we know the Lewis
structure of a molecule we can simply
predict the geometry of the molecule
geometry of the molecule and the
approach is very very simple as I show
you last time first we consider the
number of electron pairs around the
central atom in the Lewis structure of
the molecule so these electron pairs may
be bonding pairs or lone pairs the just
the number of electron pairs now
to get the minimum repulsions between
these electron pairs the calculation
coulombs calculation tells us that we
have to put the electron pairs in a such
a way to get minimum repulsions in
three-dimensional space in
three-dimensional space so if you have
for example only three electron pairs
around the central atom to get minimum
repulsions will be put in those on the
corner of an equilateral triangle those
electron pairs and if you have two
electron pairs around the central atom
you have to put those across on a line
it's simply passing through the central
atom so to get that minimum rapport
shion's and the structure of the
electron pairs will be linear if you
have four electron pairs around the
central atom again the calculation says
that to get the minimum repulsions
we have to put those electron pairs
around the central atom on a corner of a
tetrahedron tetrahedron and a
tetrahedron again is a three-dimensional
geometry and it has four faces and all
faces are equal equilateral triangles
they are all the same equilateral
triangles and there are angles actually
like over here if I have simply two
electron pairs I can put those I should
put those one over here one here on the
same line so simply electron pair
geometry becomes linear now if I have
three electron pairs around the central
atom then we have to put those on the
corner of an equilateral triangle then
we simply we get the minimum repulsions
and the shape becomes a triangular
planar and all angles are 120 degrees
actually and if I have four electron
pairs as we said last time you have to
put them on the corner of a tetrahedron
this is the tetrahedron and they are all
equivalent all of these electron pairs
are equivalent in terms of positions and
these are also
equivalent and these are also equivalent
in terms of positions so the angles here
all are equal and that's one of nine
point five degrees bump angles in
tetrahedral for the electron pair
geometry is actually all 109.5 degrees
now if I have five electron pairs around
the central atom again the electrostatic
consideration tells me that if I put
those electron pairs on the corner of
trigonal bi pyramid actually we get the
minimum repulsions so this is the
trigonal bipyramid
and this is simply the base which is the
kool aid through triangle right here so
this is 120 degrees at 120 degrees these
are actual positions that's 90 degrees
now the difference here between these
and that is as follows here all electron
pairs in all these three geometries are
equivalent they are exactly the same
position in terms of positions
environment and so on but now when we
get to this it simply trigonal
bi-pyramidal geometry we have two sets
of equivalent electron pairs and that is
these two are one equivalent set and
these three over here are the other
equivalent set and these electron pairs
here are called axial electron pairs on
the axes simply which is perpendicular
to this equilateral triangle and these
are these three here are called equally
equatorial electron pairs or equatorial
positions so we have two different
positions axial and equatorial and again
the angles these are 90 degrees and this
is 120 hundred and 20 and so on now if I
have six electron pairs around the
central atom as we said last time to get
the minimum repulsions I have to put
them on the corner of an octahedron
octahedron is this geometry here and it
has simply
eight faces and all faces are equal and
they're all equilateral triangles
equilateral triangles so the angles in
simply octahedral geometry are all 90
degrees and again all of these electron
pairs are equivalent they are all the
same it really doesn't matter which one
you take but they're all the same
equivalent in terms of their environment
in terms of their interactions with the
surroundings and so on now after this we
said that once you know the electron
pair geometry then you can also
determine or predict the plays of the
terminal atoms which are bonded to the
central atom by sharing these electron
pairs so to get the maximum sharing
actually for example in this case we
have to have if you have two terminal
atoms if both of these electron pairs
are shared so one of the terminal atoms
should be here on this line again and
this query should be here on that line
that means that the molecular geometry
which only accounts the positions of the
atoms not the electron pairs will be
linear again here right so that simply
means the positions of the electron
pairs around the central atom also
determines the positions of the atoms
which are bonded to the central atom
right central atom and here again the
same things here if I have three atoms
bonded all of these electron pairs are
shared then the molecular geometry will
be again trigonal planar and all of
these are simply shared if I have atoms
here all sharing these electron pairs
around the central atom and the geometry
will be tetrahedral and if all shared
here it will be trigonal bi-pyramidal
and if all shared here it will
octahedra so you can ask me this what is
the difference I mean when we find the
electron pair geometry fine here and
then we find the molecular geometry and
electron pair geometry and the molecular
geometry seems to be the same right yes
if all the electron pairs are shared by
the terminal atoms then the molecular
geometry and the electron pair geometry
are the same right but if there are some
lone pair electrons lone pair electrons
that means nonbonding electron pairs
around the central atom then the
geometry molecular geometry becomes
different than the electron pair
geometry right electron pair geometry
now let's see what we have here now in
this slide in the book instead of saying
electron pairs it prefers to call it
simply electron domains what is what
does electron domain means domain
actually means the plays the space that
is simply occupied by those electrons
right so if the electron spends their
time in a given space that space becomes
that electron domain right that electron
domain so it uses the electron domain as
I told you but electron pairs is the
same things we just call them electron
pair or you can call electron domain now
here it says we can refer to the
directions to which the electron points
as electron domains that means if I have
the electron pair here for example that
means the electron pair domain for these
shared electron will be this part here
this space right and here this space and
here this space over here right so this
is true whether there is one or more
electron pairs pointing in that
direction the central atom in this
molecule a has four electron domains
okay now here is
one point again as we said last time if
we simply make the elect loose structure
of this molecule this is the Lewis
structure and we look at here how many
electron pairs around the central atom
yes for it is not for actually right
there is five electron pairs but two of
those electron pairs four of them are
shared between the two atoms which
results we call a double bond right so
here you see again when we consider the
electron pair geometry around the
central atom if there are two electron
pairs shared by the central atom and the
terminal atom that two electron pair
will count as if it is one electron pair
right or if simply three electron pairs
sometimes to have a triple bond remember
three electron pairs are shared we will
count that as one electron pair again
for example here we say that there are
four electron pairs in terms of
determining the electron pair geometry
actually or electron domains we can call
it same things and this is again one two
three this will count one actually and
this is four so the electron pair
geometry in this molecule will be what
four electron pairs tetrahedral right
tetrahedral electron pair geometry will
be tetrahedral and the molecular
geometry we will see it again as we see
over here let's now let's see what now
again the book uses this inflated
balloons model or balloon analogy to
represent to represent actually electron
pairs around the central atom or
electron domains right for example here
this corresponds to one electron
- electrons domain actually it's an
electron pair we call it just electron
pair and this is another one and it
shows you again how to get the minimum
repulsion what should be the arrangement
of these balloons which corresponds to
the electron domain or electron pairs
this is again linear minimum repulsions
for two domains electron domains for
three electron domains this is going to
be minimum repulsion and this is again
remember equilateral triangle right it's
planar equilateral triangle and here is
the tetrahedral same things as we have
said before now these are again the
electron domains the same things I show
you when you have two electron domains
and this and that there or we call it
when we have two electron pairs so the
geometry electron pair geometry will
simply here it says electron domain
geometry it's the same things linear
trigonal planar tetrahedral trigonal
bipyramidal and octahedral we have all
actually shown those okay now let's say
simply work out these examples to show
you how we find the molecular geometry
from the Lewis structure starting from
the Lewis structure and then find the
electron pair geometry then put the
terminal atoms to find the molecular
geometry remember the main things the
aim is to determine the molecular
geometry not the electron pair geometry
but to determine the molecular geometry
we have to go through the electron pair
geometry first right as we have
explained it now let's simply consider
the examples and explain how we get the
molecular geometry and electron pair
geometry now we have done these last
time let's take the beryllium guy
fluorite right last time I write it
better than dichloride is the same
things if I make the lewis structure for
this it would be again as follow we have
simply electron pairs here electron
pairs there we have the fluorine
fluorine again eight electrons around
the fluorine right this barium
difluoride so again when we try to
determine the molecular geometry or
predict the molecular geometry first we
start from the Lewis structure right we
write down the Lewis structure why we
write down the Lewis structure to find
out how many electron pairs around the
central atom or how many electron
domains write the same things around the
central atom now from that I have two
electron pairs and now I can simply put
over here electron number of electron
pair around central atom okay how many
two right now here let's put the
molecular geometry no electron pair
geometry first electron pair geometry
electron pair geometry linear right
linear right again we have to know that
the angle here as we say 180 degrees
linear that's simply what it means now
let's put it over here the molecular
geometry
and the molecular geometry is also
linear right because I have the fluorine
here Florian there and that is on the
same line why they should be on this
this line to get the maximum sharing of
these electron pairs right to get that
maximum sharing electron pairs so it's a
linear like this so when I simply draw
the structure of the molecule in other
words geometry of the molecule I will
just draw it as follow fluorine
beryllium and fluorine and I put here
180 degrees like this now this is the
molecular geometry see again I don't
really have to show the electron pairs
shared on share and so on I don't really
care where the electron pairs are what I
care is where the atoms are in
three-dimensional space with respect to
each other what are the angles and so on
so that is a linear now the next is
let's consider the boron trifluoride
right we make the Lewis structure boron
trifluoride like this
that's right fluoride so we just count
the electrons show all the electrons I
am trying to make because this becomes
the Lewis structure when I say write
down the Lewis structure you have to put
these electrons around each atom so in
other words the Lewis structure have to
show all of the valence electrons in the
molecule write all of the valence
electrons so this is the Lewis structure
now how many electron pairs around the
central atom simply three now what is
the electron pair geometry trigonal
planar let's write down three guano
planar like this now what is the
molecular geometry you can easily guess
that since I have all
electron pairs are shared by the
terminal atoms
it will also be trigonal planar right so
this is again trigonal planar now how
about the structure structure is I put
the boron here and put the fluorine here
again I have to put these dashes because
they indicate the bumps right and I had
put the angles like this show the angles
this will be 120 degrees and all of
these are also 120 degrees equilateral
triangle now let's consider the third
and all to - over here now the lewis
structure for this is as follow ok this
is the lewis structure and three atoms
again now how many electron pairs around
the central atom how many electron pairs
three or four actually there are four
but when I consider the geometry of
these electron pairs I will extreme is
three right so the electron pair
geometry simply be let's see I write it
down three but we have to remember those
two electron pairs or two electron
domains is count in 1 in this case right
so the geometry electron pair geometry
trigonal planar again trigonal planar
like this now how about the molecular
geometry it will be v-shaped right
because I only consider actually the
positions of the atoms oxygen nitrogen
and oxygen there is also the residence
structure this double bond may be there
but it really doesn't matter
that's okay so now the we will call it V
shape or bent bent is also used to
express the molecular structure of this
molecule now when I write down the
geometry draw the geometry it would be
something like this okay again I don't
really show these simply shows the the
buns but I don't have to put their
double bond there to show the geometry
actually okay now how about the angle
the angle since the three electron pairs
around the central atom geometry is
trigonal planar equilateral triangle on
the corner of it equal to angle the
angle should be what 120 degrees right
hundred twenty DS but the angle here
will not be 120 degrees but it will be
less than hundred and twenty degrees why
because the lone pair electrons here
these lone pair electrons here will
occupy a large space and they will repel
the bonding pair closer to each other
what does that means that means the
angle will be less than 120 degrees how
much less I can't really tell you
exactly it may be 118 it may be 115 but
less right so it will not be definitely
90 degrees but will be less than 120
degrees so I can simply say less than
hundred twenty degrees reason is lone
pair electrons so again when we have
lone pair electrons on the central atom
we should consider that because they
have an effect on the bond angles right
probably not the name of the overall
structure geometry name but the bond
angles simply changes when you have lone
pair electrons compared to when you
don't have any lone pair around central
atom clear okay
so let's continue here the next is if I
have the CH 4 this is
methane molecule and the Lewis structure
is this right four hydrogen's four
electron pairs around the central atom
no lone pair electrons right so what is
the number of electron pairs for what is
the electron pair geometry tetrahedral
right tetrahedral tetrahedral what is
the molecular geometry also tetrahedral
right it is also a tetrahedral
tetrahedral like this if I draw the
geometry of the methane molecule again I
have to make a tetrahedron here put the
hydrogen's like this and this one then
combine those indicator in the face like
this now how about the angles angles
will be exactly 109 degrees 109.5
degrees for ideal tetrahedral geometry
right ideally the geometry all of these
hydrogens are equivalent that means
there's no difference in terms of their
environment and so on okay now let's
continue the next example I picked up is
ammonia let's make the Lewis structure
Lewis structure again is as follow this
is the Lewis structure what is the
electron number of electron pairs around
the central atom for right there number
of electrons around the central atom is
four let's put that there for now what
is the electron pair geometry ok
trajeron right four electron pairs
tetrahedral let's write it tetrahedral
now what is the molecular geometry
molecular geometry will also be no
molecular geometry
will not be tetrahedral right because we
have lone pair electrons the molecular
geometry becomes trigonal pyramidal
right trigonal pyramidal we know which
can pyramid in Turkish trigonal
pyramidal and the free goal actually the
the it is equilateral triangle the base
will be equilateral triangle actually
so let's write the name three guano
trigonal pyramidal is the name of the
geometry what is the shape if I draw
what is the geometry of the molecule can
I draw that yes I can simply show it
like this now this is the equilateral
triangle here the base now I have the
lone pair electrons but remember when I
just show the geometry of the molecule
I don't indicate the electron pairs
right it's not important but when it
comes predicting the angles bond angles
then we have to consider whether I have
a lone pair electrons or not and simply
this angle here all the angles will be
simply equal in this case but this angle
is one all seven degrees which is less
than one oh nine point five perfect
tetrahedral why because again this lone
pair electron occupies a larger space
and push the bonding electron pair
closer to each other making the angle
less than one oh nine point five right
109.5 okay now if I simply continue here
let's consider the water molecule and
the water molecule the Lewis structure
is like this I can put that there
okay now what is the number of electron
pairs around the central atom
four right let's write at four what is
the electron pair geometry or electron
domain geometry would be the same thing
if it considered those balloons actually
and it will also be this is tetrahedral
like this now how about the molecular
geometry molecular geometry I have to
only consider simply these two
hydrogen's and the oxygen now if you
consider the tetrahedron there right and
take out two of the hydrogen's take out
two vertices from the tetrahedron you
get a v-shape right you get a V shape
and the V shape or bent remember and if
I draw the geometry it would be again as
follow I will just put it like this this
is the structure of water now how about
the angle experimentally the angle is
fine all point 105 degrees 105 degrees
this is 105 degrees somewhere now you
see again the angle normally should be
109.5 perfect tetrahedral but it is less
than that actually it is less than the
nitrogen this is 107 this is 105 why
because uh knock sujin we have two
electron pairs that electron pairs
simply pushes the abundant pair
electrons in the water more than they do
in ammonia right ammonia molecule so
this is again the reason so when we have
simply the lone pair electrons and we
asked to predict the angle bond angle we
have to consider those repulsions
actually between the nonbonding and
bonding pairs okay
now the next I picked up the phosphorus
pentachloride if I make the Lewis
structure it is again as follow
one two three four five and I have
chlorine chlorine chlorine chlorine like
this and electron pairs around the lone
pair electrons around the chlorine eight
electrons around chlorine now 10
electrons around the phosphorous
remember again more than eight electrons
this is possible only for the third
period and down the elements there right
not for the second period so what is the
number of electron pairs number of
electron pairs five what is the geometry
geometry as we have seen here trigonal
bipyramidal
electron pair geometry is trigonal
bipyramidal now how about the molecular
geometry it is also trigonal
bi-pyramidal right it is to be gone over
by pyramidal let's write it trigonal
bi-pyramidal by randall how about the
shape geometry can I draw it yes simply
and over here chlorine chlorine chlorine
now this these three chlorines actually
are on the corner of equilateral
triangle how about the angles angle here
is 120 degrees now how about the angle
here between those this is 90 degrees 90
degrees now remember again in this
geometry trigonal bi-pyramidal geometry
these two chlorines are equivalent but
they are different than these three
chlorines which are
equivalent among themselves right so
these two are called axial chlorine
atoms and these three chlorine atoms are
called equatorial chlorine atoms right
so again we have two different positions
axial and equatorial positions there
okay now let's continue the next example
I have picked up is sulfur tetrafluoride
now the Lewis structure is again as
follow we have the sulfur and and
fluorine here more in here okay now
again how many electron pairs around the
central atom five right now what is the
electron pair geometry trigonal
bi-pyramidal trigonal bipyramidal
trigonal bi-pyramidal now what is the
electron molecular geometry now the
molecular geometry is called seesaw or
irregular tetrahedral some books calls
it irregular tetrahedral let's see what
kind of shape it is let's write down
here see so
see so is I guess I so test era right
see so is the device that you put the
woods on it and you cut it so this looks
like that shape wise it's called seesaw
and let me draw it here is the sulfur
and here is the fluorines or i should
probably okay for those fluorines here
like this four fluorines and now there
is a lone pair electron here right and
that is the lone pair electron now this
is the C so now this is axial and this
is sort of like perpendicular to that
right now the angle has to be normally
will be 120 degrees this hang go here
and that will be 90 degrees but since I
have unshared electron pairs on the
central atom those angles will be
distorted than the ideal trigonal
bi-pyramidal geometry right what we
expect from that now one point here is
this what is this position over here now
if you consider the trigonal bipyramidal
geometry this position here this
position that position this position
which is empty in this case our
equatorial positions right equatorial
positions now these positions are axial
positions you see again I left the lone
pair electrons in equatorial positions
so when you have trigonal bi-pyramidal
electron pair geometry if one of those
electron pairs are not shared there as
lone pair they have to be in the
equatorial position why to get the
minimum repulsions again right
the electrostatic interaction dictate
that and since the lone pair electron
will have more will need more space
there so you put them in the equatorial
knot over here actually you put it over
here right so here I have the lone pair
electron so this is called the C's so
geometry C sub geometry it is actually
you can imagine it as follow here's the
axial positions like this and here you
have the two fluorines like that right
that is the seesaw or sometimes called
irregular
tetrahedral irregular tetrahedral now
let's take the next example the next
example is again CL chlorine trifluoride
if I make the Lewis structure chlorine
in the center and again around the
chlorine I have five electron pairs like
this 1 2 3 4 5 now only three of those
are shared by the terminal fluorine
atoms like this you should always count
the number of electrons now how many
electron pairs around the central atom
chlorine five right five now let's put
that five what is the electron pair
geometry trigonal bi-pyramidal just the
same as here tree gone old trigonal
bi-pyramidal now how about the molecular
geometry now we have this trigonal
bi-pyramidal geometry and two of those
equatorial positions that means that
actually this and that position let's
say are occupied by the lone pair
electrons and there's only one fluorine
there and it would be simply like this a
game
the lone pair electrons are always
occupies the equatorial positions to get
the minimum repulsions
and i have the structure of the molecule
like this sulfur no chlorine sorry this
is chlorine trifluoride chlorine
difluoride now how about the angles
normally this angle should be 90 degrees
that should also be 90 degrees but it
will be less than 90 degrees why because
I have too long pair electrons here push
that so that angle will be less than 90
degrees and how much less that depends
on the molecule what atoms I have and so
on but it will be less than maybe 89
maybe 80 88 and so on degrees and the
what is the name the name is called T
shape because this looks like a teeth
right so the shape of the molecule or
geometry is called T shape for the
Quarian trifluoride again we have two
lone pair electrons in equatorial
positions okay now let's continue if I
have xenon difluoride remember the xenon
is a noble gas and it is the only noble
gas that can form compounds and if I
simply write down the loose structure
remember this as eight electrons right
and each fluorine have seven electrons
and if I make that it would be again as
follow five electron pairs around the
central one two three four five
electrons around the xenon two of those
are simply shared by the terminal atoms
fluorine let's put that over here like
this actually in as I write down the
Lewis structure I don't really care
where the fluorines are right I can put
it there like this now how many
Brown pears I have simply five electron
pairs one two three four five electron
pairs let's write it down five what is
the geometry electron pair geometry tree
guano trigonal bi-pyramidal now what is
the molecular geometry now I have three
lone pair electrons in a trigonal
bi-pyramidal electron pair geometry and
those two three lone pairs will be have
to be in the core toriel positions and
the shared electron pairs will be in X
she'll positions what does that means
this structure will be linear right it
will be just a linear structure put it
here linear structure and let's simply
show that the xenon over here and the
fluorine like this like that now how
about this angle exactly 180 degrees
right now you see again I have three
lone pair electrons but they're on the
equatorial equatorial positions on the
corner of an equilateral triangle and
they will have exactly the same effect
in pushing the electron bonding electron
pairs and in all directions actually and
they will stay linear just like in
original trigonal bi-pyramidal geometry
so the geometry is linear clear any
questions so again you see how we
determine the molecular geometry from
the Lewis structure and also depending
on our off the number of electron pairs
we can guess what will be the INGOs bond
angles right the bond angles okay I
guess we have to stop here let's have
ten minutes break exactly ten minutes
and come back again