Practice Problems for VSEPR Theory - Chemical Bonding, Inorganic Chemistry Video Lecture - IIT JAM

we're going to do a whole bunch of
practice problem so you'll get super
good at looking at a lewis structure
like this and figuring out what it's
3-dimensional Vesper shape with me we'll
start out right away with pf3
I've got on this paper these four
questions and these are like the thought
process that you want to go through
every time you figure out the Vesper
structure for one of these shapes okay
so the first question here is how many
things are there around the central atom
the first thing you always want to ask
yourself how many things are there
around the central atom here's the
central atom it's this p here and it's
surrounded by one two three bonds and
one lone electron pair so the things
around the central atom they can be
either lone electron pairs or bonds so
the point is three bonds one lone
electron pair for total things okay
so now that we know how many things
surround the central atom for in this
case we ask what structure is this
molecule based on and what I mean by
that is what if all four of these things
surrounding the central atom what if
they were all other atoms okay if they
were all other atoms we get this shape a
tetrahedral shape where we have four
atoms surrounding a central atom we call
this tetrahedral and because this has
four things around the central atom we
say that it's based on this tetrahedral
structure now here's what's important in
pf3
only three of the things surround the
central atom are other atoms one of the
atoms from this tetrahedral structure
has been replaced by a lone electron
pair here's what I mean this is what pf3
looks like see that it's very similar to
the tetrahedral shape there are four
things that surround this but one of
these atoms from the tetrahedral shape
has been replaced by a lone electron
pair so the four things in tetrahedral
are four atoms the four things in PF
three are three atoms
and one lone electron pair so the shape
looks kind of like a pyramid from the
side and we call this trigonal pyramidal
it's what we get when we have four
things around a central atom three of
them are other atoms and one is a lone
electron pair let's talk about bond
angles really quickly so in tetrahedral
every the angle between every pair of
bonds is a hundred and nine point five
degrees that's when we have all atoms
around the central atom but when we got
unshared electron pairs these unshared
electron pairs they push a little harder
against these atoms unshared electron
pairs they need a little bit more space
than regular atoms do so what that means
is that all of these atoms are going to
get pushed kind of in this direction and
so the angle between them this bond is
going to be smaller because they're
moving in this direction
so for the the trigonal pyramidal shape
here instead of 109.5 that we'd have in
the tetrahedral we get an angle that's a
little smaller than that and it's more
like a hundred and seven degrees just
keep in mind that it's similar to one
109.5 it's just been compressed a little
bit because of these unshared electrons
that are pushing on the bonds a little
bit more no.2 and the whole thing is an
ion it has a one minus charge charges
don't have any impact on the Vesper
shape though so don't worry at all if
you see something that has a charge we
just want to go through this thought
process number one how many things are
there around the central atom so single
bonds and double bonds they count for
the same doesn't matter what type of
bond it is all that matters is that it
is a bond okay so we have one bond two
bonds and then one unshared electron
pair so for this molecule we have three
total things around the central atom two
bonds one unshared electron pair 3 total
things
so now that we know we have three total
things we ask what structure is this
molecule based on in other words what
structure would have three things around
a central atom and all of them would be
other atoms that would be this shape
which is the trigonal planar shape it's
three atoms around a central atom and
all of them are lined up in this plane
which is why we call it trigonal planar
okay now on to number three which atoms
from this structure are replaced by lone
electron pairs so no.2 has two bonds to
oxygen these two oxygens but then it has
this lone electron pair so that means
that one of these three atoms is
replaced by a lone electron pair in no.2
so the structure is going to look like
this all right look at how similar it is
to the trigonal planar shape but this
atom here has been replaced by the
unshared electrons that are up there so
that is going to give us this bent shape
now when we talk about angles in the
trigonal planar molecule there is a
hundred and twenty degree angle between
all of these bonds this unshared
electron pair in the bent molecule
though pushes these atoms a little bit
harder than this atom does because
unshared electron pairs like to have a
little extra space so that means that in
the bent molecule these atoms are going
to be a little bit closer to each other
that angle is going to be less than 120
degrees because of this pushing it's
going to be more like about a hundred
and sixteen degrees okay so that's a
bent molecule you can get when you have
two bonds and one lone electron pair
around a central atom
no.3 one - remember don't worry at all
about the charge on this molecule it
doesn't matter for vesperal the first
question that you should ask yourself is
number one how many things are there
around the central atom
there's a double bond here doesn't
matter that it's a double bond it's just
a bond so there are 1 2 3 bonds to other
atoms are
the central atom so that means that for
number two what structure is this
molecule based on it's going to be based
on the trigonal planar shape which looks
like this okay
trigonal planar now for number three
which atoms are replaced by lone
electron pairs for no.3 one - none of
them are replaced by unshared electron
pairs three atoms surround the central
atom so that means that this molecule
has this shape it is trigonal planar
with a hundred and twenty degrees
between each of the atoms Oh F - number
one how many things are there around the
central atom okay there are two bonds
and then there are two lone electron
pairs so that means that there are four
things
total around this atom so what structure
is this molecule based on if you have
four things around a central atom your
structure is going to be based on the
tetrahedral shape now for number three
here it asks which atoms are replaced by
lone electron pairs there are four atoms
here in red and tetrahedral but in oh F
- there are only two atoms surrounding
the central atom the other two atoms
have been replaced by these unshared
electron pairs so that means that Oh F 2
is going to look like this here are the
two lone electron pairs this one and
this one these two lone electron pairs
that have been that have replaced two of
these atoms in the tetrahedral structure
so if we look at it from the side
it looks a lot like the tetrahedral
shape but it's missing you could say
that it still has these two atoms but
it's missing this atom in this atom
they've been replaced by these unshared
electron pairs this is going to give the
molecule a bent shape and four angles
these unshared electron pairs are going
to push these atoms together harder than
other atoms would okay because those
unshared electron pairs need a little
bit of extra space so in a regular
Tetris
no molecule the angles between the bonds
are 109.5 but in the bent where these
are pushing harder it's going to be more
like a hundred and five degrees because
we have two electron pairs that are
pushing so they really push these two
atoms closer together so we get a bent
molecule with 105 degrees when we have a
central atom surrounded by two bonds and
two lone electron pairs HCN how many
things around the central atom got one
bond here one triple bond here doesn't
matter that it's a triple bond it's just
two bonds a bond here a bond here so if
we have two things around a central atom
the structure is based on this this
linear thing that we've got here here's
a central atom add them on the side add
them on the side which atoms are
replaced by lone electron pairs none
because we got an atom here and an atom
here so that means that the final shape
is going to be this linear shape and
since we're calling it linear these
three atoms are in a line that means
that we're going to have 180 degrees
between these two angles so if you have
two atoms now two bonds surrounding a
central atom
you get a linear shape 180 degrees
between the ABS nh4 1 plus how many
things around the central atom 1 2 3 4
so that means it is based on this
tetrahedral shape for things around a
central atom and for number 3 none of
these things around the central atom are
unshared electron pairs so we're going
to have 4 atoms surrounding the central
atom we're going to have a tetrahedral
shape as our final shape and the angles
between each bond are going to be 109
point five degrees okay you might be
getting the hang of this that's great
I'm going to do you will more practice
problems because it is so useful to just
keep going through these steps that have
become second nature every time you look
at one of these lewis structures to
determine the Vesper shape you want to
go through these three or four steps to
figure out what it's actually going to
look like it should just become
automatic so that's why I'm doing
these practice problems like over and
over again so that can just become very
natural and you won't ever doubt
yourself when you're doing these sort of
problems okay
SB h3 number one how many things are
there around the central atom one two
three bonds one lone electron pair so
there are four things four things total
around the central atom and it's going
to be based on a tetrahedral shape now
which atoms are replaced by lone
electron pairs in s BH three one of
these four things is a lone electron
pair so that means that its shape is
going to be like this because this lone
electron pair replaces this atom this is
the trigonal pyramidal shape and it's
going to be an angle of 107 between
these bonds a little bit smaller than
100 9.5 of tetrahedral because this lone
electron pair pushes these atoms a
little bit harder than an atom would oh
three which is ozone how many things are
on the central atom one two bonds and
one lone electron pair three things
total which means that this is going to
be based on a trigonal planar shape here
it is how many things are replaced by
lone electron pairs well instead of
having three atoms is in the trigonal
planar shape we're going to have two
atoms and one lone electron pair so this
is the shape that I'm going to get here
is a lone electron pair that replaces
this atom and that means that I'm going
to have a where is it oh there okay okay
it is a bent molecule a bent molecule
with less than 120 degrees between these
angles because this is this is pushing a
little harder angle about 116 degrees
one more and then we'll be done se cl2
how many things are there around the
central atom there are four things two
bonds two lone electron pairs so this is
this has four things total so it's based
on this tetrahedral shape which atoms
are replaced by lone electron pairs in
this molecule two of these atoms and
because of that it's going to have a
shape that
looks like this two atoms from here two
of them have been replaced by lone
electron pairs this one in this one lone
electron pair lone electron pair and
this is also going to give a bent shape
and the angles between the atoms in this
Bend molecule are going to be about 105
degrees because these lone electron
pairs are pushing pretty hard against
these two apps so that's what we're
going to get anyway these steps these
steps that I keep talking about are the
ones you always want to go through every
time you're looking at a little
structure and figuring out what the
Vesper shape is once you get the hang of
it it's really not that hard you think
about the structure that the molecule is
based on and then you just figure out
how many of the atoms are not there how
many of the atoms have been replaced by
lone electron pairs and that will tell
you what the final structure is so keep
practicing this keep these steps in mind
until it's just second nature and you
won't have any trouble at all with these
problems good luck