0:00:00.880,0:00:06.000
welcome back uh in this video we're going to 
build what we learned on lewis structures in

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the previous video and see how we can take this 
shape that we've shown right the connectivity of

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the atoms through the lewis structure and then 
conclude about what the actual shape of that

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molecule will be because we'll see that there's 
going to be slightly more to this phenomenon

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this molecular geometry than just the number 
of bonds that a certain central atom is making

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that is to say just because we have two atoms that 
one atom is connected to doesn't mean that it's

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always going to have the same shape and we'll 
learn the factors that determine the different

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possible shapes uh in this following video because 
what we'll see is the shape that a molecule

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actually takes whether it's a straight line or 
whether it's bent or whether it's a triangle that

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is going to determine what we call the polarity 
of this molecule and that is to say whether or not

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the molecule can act almost as if it's a permanent 
little magnet and therefore have interactions with

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other molecules nearby because this polarity is 
going to determine many many properties of these

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molecules that we'll see in the upcoming video as 
well and so understanding very clearly the shape

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of these molecules we'll see has just a really 
tremendous effect on the molecules properties and

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it's just very important to understand well and 
is rather interesting when you think about the

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different molecules that you know already and that 
they have unique shapes that all instances of that

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molecule will take up all right so now we're 
going to be talking about molecular geometry

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we should be pretty comfortable with predicting 
the lewis structures for these covalent molecules

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and now what we're going to see is how we can 
connect those lewis structures which show how

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the different atoms are connected into the actual 
shape of molecules because we do know that they

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exist in a 3d world and their geometries are going 
to reflect that as well and the main theory that

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we're going to be using here is called the vesper 
theory valence shell electron pair repulsion

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and this says that when we have electrons 
around a central atom right so if there's

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an atom in the middle that's the one around which 
we're going to have geometry not the hydrogen at

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the end because it's kind of just capping off 
the end and doesn't really have any geometry

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but these central atoms are going to have shapes 
based on the idea that negative charges will repel

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and recall the electrons or where the negative 
charge exists on an atom and whether we have a

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covalent bond or a lone pair of electrons these 
are in both cases negative charges that are

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going to want to repel right whether we're 
bonding or nonbonding right meaning a bond

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or a lone pair we're going to repel one another in 
order to take up positions to maximize the space

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that those bonding or non-bonding electrons have 
around the central atom and so let's consider our

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sort of quintessential example this is methane 
ch4 and it has this specific shape shown below

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where the angle is going to be 109 and 
a half degrees around the central atom

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and how do we get that well this is the shape 
that the different bonding domains there are

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only bonding domains here right four bonds no 
lone pairs around the carbon this is the shape

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that the methane molecule will take up in order 
to accommodate these four bonding domains and no

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non-bonding domains right we can kind of see that 
two of the hydrogen shown in the top right picture

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will be within the plane of the page right 
these are just the lines right it's sitting

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kind of right in the page and then we have a 
dashed hydrogen that's kind of going backward

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and a sphere hydrogen the solid triangle looking 
like bond that's coming forward and so we see here

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is that when we are accommodating these 3d the 
3d space that this molecule wants to occupy that

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the bond angle is 109.5 degrees and this gives 
us what we'll know as the tetrahedral geometry

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and if we were to connect all these hydrogens 
it would sit kind of like a triangular

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prism right with equilateral triangles on each 
side so keep in mind right that each domain wants

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to occupy as much space as possible and so when 
we have four domains we're going to be based on

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this tetrahedral geometry which we just saw with 
our methane molecule if we only have two domains

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such as the following molecules shown here right 
this hcn or the carbon dioxide doesn't matter if

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there's single bonds or multiple bonds right 
one single bond and one triple bond means two

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bonding domains two double bonds means two bonding 
domains uh we'll have this linear geometry right

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if these two domains want to spread as much across 
from each other as possible they'll do so by being

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directly across right 180 degrees and if we have 
three domains we have a trigonal planar geometry

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now we're going to see that when we add in 
some lone pairs things get just a tad bit more

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intricate but these are the parent structures we 
want to keep in mind right whether we have bonding

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or nonbonding electrons right in this case we're 
only bonding electrons around the central atom

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we're going to take up these parent structures 
right and we're going to see coming up what

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happens if we are to replace some of these non 
some of these replace some of these bonding

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domains with lone pairs so let's consider right 
h2o versus co2 right and so what we have here

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is carbon dioxide which has two bonding domains 
versus water h2o which actually has two bonding

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domains and two non-bonding domains and you may 
see already that i've drawn these two slightly

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differently right these depictions show water 
differently than it does carbon dioxide right

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carbon dioxide has this linear geometry where 
water is showing something else what is different

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uh well the difference is in these lone pairs 
and therefore water is going to be based on a

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tetrahedral geometry because it actually has four 
electron domains two bonding and two non-bonding

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the difference though is that it's not 
actually going to be the tetrahedral

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shape that we looked at before because these 
electrons don't really have any mass compared to

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the hydrogens or the oxygens you know 
compared to one proton an electron is just

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so much smaller that we count it as basically 
massless and dimensionless effectively as well

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so they will definitely affect the shape 
right the existence of these lone pairs

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means that the water is going to want to be in a 
fundamentally tetrahedral geometry but those lone

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pairs aren't really going to be part of the shape 
it's really just going to be those two bonding

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pairs connecting the oxygen and the hydrogen 
that are forced away from that linear geometry

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and so again we'll call that parent geometry 
the electron geometry and for co2 that electron

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geometry is based on two zones and we're therefore 
going to have a linear molecular geometry as well

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because there are no lone pairs right linear 
electron geometry based on that linear structure

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and we're going to also have of course the linear 
what we'll call molecular geometry in other words

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co2 is a line water on the other hand is based on 
that tetrahedral electron geometry because it has

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four zones however two of those zones are lone 
pairs and don't actually contribute to the shape

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itself they are only distorted by those electrons 
the lone pair electrons and therefore what we'll

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have here is a bent molecular geometry where water 
is not linear like co2 even though both have two

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bonding domains the existence of those lone pairs 
forces the linear structure into a bent structure

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so again the existence of these lone pairs are 
very important we're going to see that this

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shape affects physical properties of water 
we know that the physical properties right

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are actually very different co2 is a gas at room 
temperature and water is a liquid this shape is a

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lot to do with that right for now though keep in 
mind that the geometry is going to be determined

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both by the lone pairs and the bonded pairs but 
that the lone pairs don't actually take up space

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they just help determine the space and we want 
to determine the parent structure based on all

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the electron domains bonded and non-bonded 
so here's a nice little chart that shows us

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you know the different electron domains right 
again we want to consider both the bonded and

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the non-bonded and what we have here is with two 
regions we have a linear angle with three regions

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we have a trigonal planar with 120 degrees of 
separation and we should be getting pretty used

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to this tetrahedral geometry by now again we want 
to determine these parent structures based on

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both bonding and non-bonding electrons and we're 
going to call this the electron geometry and we're

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going to determine the molecular geometry based 
only on the bonded pairs but keeping in mind that

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they may be affected by the lone pairs if there 
so happens to be no lone pairs like in the example

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of co2 on the previous slide the electron and 
molecular geometries are going to be equivalent

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so here is another kind of good chart that shows 
4 up to four bonding and nonbonding domains uh

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what the different shapes are for these different 
potential molecules right uh if we look at the top

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row right if there's two electron regions the only 
option is to have no lone pairs and we have linear

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in the second row if we have three regions we 
could potentially have all bonded or one of those

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could be non-bonded right one lone pair and two 
bonded pairs in which case right similar to the

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water example that lone pair will force the bonded 
pairs away from that linear geometry and make it a

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bent or angular structure in tetrahedral right 
that's our 109 degree spacing when we replace one

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of those bonds with a lone pair we end up having a 
trigonal pyramid right so just one of the the top

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bond removed and if we have two removed we have 
that bent structure for water one thing to keep

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in mind here is that when we have a lone pair it's 
actually a little bit greedier for space compared

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to the bonded pairs and so when we have the bent 
or angular with three regions or the trigonal

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pyramid or the bender angular like water the angle 
is actually going to shrink up just a little bit

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because of the existence of those lone pairs so 
let's go ahead and take a look at this chart or

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perhaps you're just kind of thinking of things and 
what is the shape of a molecule with three bonds

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and one lone pair go ahead and pause the video 
and see if you can come up with the right one

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all right hopefully you're able to come up with 
an answer and it is indeed c the trigonal pyramid

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three bonds one lone pair that is four total 
electron regions one of which is a lone pair

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and that gives us our trigonal pyramidal 
shape keep in mind that if we do have

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more than one central atom such as in the 
following lewis structure we're going to

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have geometry around each of the central 
atoms and in this case there are three

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right there's this carbon which has four bonding 
regions and that's our tetrahedral geometry

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there's this carbon here which has three bonding 
regions no lone pairs and therefore is trigonal

0:11:07.680,0:11:12.720
planar remember that whether it's a double bond 
or a single bond doesn't matter it's still one

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bonded domain and finally we have this oxygen here 
two bonds two lone pairs similar to water and it's

0:11:18.800,0:11:25.040
going to have a bent structure with four domains 
around it but two are non-bonding only two are

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bonding that gives us a bent structure all right 
give this one a try now as well we have two lewis

0:11:31.680,0:11:36.160
structures that you should first draw and then 
determine what the molecular geometry of those

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structures are remembering that we want to look 
at our central atom and it's going to be based

0:11:41.760,0:11:47.440
on the bonded and non-bonded electrons so go ahead 
and pause the video try this one and we'll discuss

0:11:51.920,0:11:57.760
all right hopefully you're able to come up 
with an answer right so we have ch2cl2 and h2co

0:11:57.760,0:12:05.040
uh so for the top which is our dichloromethane 
uh we have a tetrahedral geometry right four

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bonded zones no lone pairs and therefore it is 
going to be tetrahedral right for our second

0:12:11.600,0:12:16.560
example this is formaldehyde we see that we're 
going to need a double bond between the oxygen

0:12:16.560,0:12:20.640
and the carbon in order to fulfill our octet 
rules and therefore what we're going to have is

0:12:20.640,0:12:26.720
three bonded zones no lone pairs and it will have 
a trigonal planar shape so this connection between

0:12:26.720,0:12:32.080
the lewis structure and the geometry is again very 
very important because what we're going to see is

0:12:32.080,0:12:37.200
that this geometry is going to determine something 
called the polarity which is going to determine

0:12:37.760,0:12:43.280
many physical properties and explains why 
water and carbon dioxide are so different

0:12:43.280,0:12:48.160
right water liquid at room temperature 
carbon dioxide gases at room temperature

0:12:48.160,0:12:51.840
and this really determines a lot of the 
chemistry that they engage in as well so

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polar molecules right recall that we have this 
value called electronegativity that says how much

0:12:58.800,0:13:05.680
does each atom want to have the electrons within 
a covalent bond and so when we have hcl chlorine

0:13:05.680,0:13:10.560
is more electronegative than hydrogen and 
therefore we have a polar bond and we have a

0:13:10.560,0:13:16.160
bond dipole that shows that the electrons want 
to drift towards chlorine compared to hydrogen

0:13:16.800,0:13:20.080
now this is a pretty straightforward 
example because there is only one bond

0:13:20.080,0:13:25.040
but in the more complicated examples we've seen 
what we're going to need to do is add up the bond

0:13:25.040,0:13:31.840
dipoles to get what we call a net dipole and in 
this case right the bond dipole is the net dipole

0:13:31.840,0:13:38.640
and so we say well it has a non-zero net dipole a 
total dipole of zero which we'll see in a second

0:13:38.640,0:13:45.840
means nonpolar and in this case we do have a net 
dipole and so we have a polar molecule which is

0:13:45.840,0:13:54.080
hcl here hydrochloric acid what about the case of 
h2o versus co2 right well we see that the carbon

0:13:54.080,0:14:00.080
oxygen double bond and the oh bond are both polar 
but does this mean that these molecules are both

0:14:00.080,0:14:05.920
polar well not exactly right because we see 
here that co2 has a linear geometry and the

0:14:05.920,0:14:10.960
bond dipoles are going to cancel however water 
has a bench geometry and the bond dipoles will

0:14:10.960,0:14:15.920
not cancel and we can see this in the image 
below right we know that carbon and oxygen here

0:14:15.920,0:14:21.360
are going to you know carbon's always carbon 
oxygen's always oxygen so the carbon oxygen

0:14:21.360,0:14:25.680
bond dipole on the right is going to be 
equivalent to the bond dipole on the left

0:14:25.680,0:14:31.200
and we know that it's a linear geometry so those 
bond dipoles will be pulling in equal and opposite

0:14:31.200,0:14:37.120
directions therefore the overall dipole on 
carbon dioxide is going to be equal to zero

0:14:38.000,0:14:42.400
in the case of water we do have the same case 
with the oxygen hydrogen dipoles they're both

0:14:42.400,0:14:48.160
equivalent the bond dipoles however because of 
the bent structure of water we're going to have

0:14:48.160,0:14:53.920
those two arrows add up they're going to cancel 
out in their kind of inward directions but the

0:14:53.920,0:15:00.000
upward direction will add and we're going to have 
an overall dipole for water and as a result this

0:15:00.000,0:15:06.640
shows that carbon dioxide is nonpolar while water 
is polar and again you know the strength of this

0:15:06.640,0:15:11.120
separation will depend on the electronegativity 
difference between these different atoms it's

0:15:11.120,0:15:15.600
really just vector addition adding up these arrows 
in three-dimensional space but this is really

0:15:15.600,0:15:21.680
i think a very clear-cut case that shows that 
molecular geometry is going to affect the overall

0:15:21.680,0:15:26.080
dipole and keep in mind right that if we're going 
to have a non-polar molecule everything's going to

0:15:26.080,0:15:31.360
cancel and this really does happen quite a bit 
right if you go back to our ch4 example that

0:15:31.360,0:15:37.600
one everything will cancel as well so only in 
cases where we do have some basically asymmetry

0:15:37.600,0:15:43.440
will we end up having a polar molecule in the 
next video we're going to talk more about this

0:15:43.440,0:15:49.200
polarity and how it leads again to what we call 
intermolecular forces which determines whether

0:15:49.200,0:15:53.760
something is you know solid liquid or gas at room 
temperature and again is really all about those

0:15:53.760,0:15:58.720
physical properties that we discussed so thank 
you for watching and i'll see you in the next one
