Leah here from leah4sci.com and in this video we're going to look at some practice problems
drawing lewis structure, identifying molecular shape, hybridization and polarity for molecules.
As a reminder, for lewis structure we're looking to draw every atom, electron, and bond on
the molecule.
That will give us the molecular shape which is the 3-dimensional structure.
The hybridization tells us what orbitals refuse to create the bonding on that central atom,
for example Sp3, Sp2, or Sp.
And the polarity shows us if we have a difference in electronegativity so that the overall molecule
has partial negative or partial positive charge or if it's completely neutral.
If you're not comfortable with these concepts, go back to the tutorial videos on my website
where I walk you through all this.
You can find them linked below or go to leah4sci.com/introOrgo and leah4sci.com/OrgoBasics.
Ready to jump in?
Let's start with a warm-up.
CH3Cl.
The first thing we want to do for the lewis is identify the central atom, typically a
carbon.
We know that carbon can have up to four groups attached, and if we have H3 and Cl, these
are the 4 groups so I just place them around the carbon atom.
This gives me 3 hydrogen and 1 chlorine.
Once we have the atoms connect them all with a single bond and then let's see for missing
any electrons.
How many valence electrons do we expect to have on this molecule?
Well carbon should have a total of 4, hydrogen should have 1 each, gives me 1 x 3 is 3.
Chlorine should have 7 for a total of 14 electrons.
You can find these numbers on the periodic table but in organic chemistry you are going
to come across 10 atoms so often that it's worth memorizing.
And I show you how to memorize them specifically in the periodic table video linked below.
Let's see what we have for electrons, 1,2,3,4 around carbon.
1,2,3 connecting the hydrogen, that's 7 and 8, 1 carbon to chlorine, 9,10, 11, 12, 13,
14.
All of the atoms have a verified complete octet, 8 carbon, 8 for chlorine, and hydrogen
is an exception where it only needs 2.
We have our Lewis structure, what can we tell about the shape of this molecule?
Well, it kinda looks like a flat cross, but it isn't because the Lewis Structure is 2-dimensional
and this molecule is 3-dimensional.
Anytime you have a central atom with 4 groups attached to it, you should recognize that
the hybridization for this is sp3 having 4 groups which means that the molecule is a
tetrahedron or tetrahedral.
What about polarity?
Carbon and hydrogen have a nonpolar bond.
Carbon and Chlorine have enough of the difference of electronegativity where the electronegative
chlorine is pulling electrons away from carbon making carbon partially positive giving me
a dipole in the direction of chlorine.
Let's try another problem looking at HCN.
For the Lewis Structure let's start with the central atom which is a carbon, and place
the other groups around it.
Now we don't have 4 groups to put around carbon so we'll take what we have put hydrogen on
one side, nitrogen on the other and connect each one with a single bond.
How do we do we're done?
We don't until we account for all the electrons.
Hydrogen should have only 1 valence, carbon has 4, nitrogen has 5, for a total of 10.
Right now, I have 1,2,3.4, that's no good.
The question then, is what to do from here?
This is where students get stuck, how do I figure out the number of lone pairs or maybe
pi bonds?
The methods that I teach in my video is linked below.
Is the first get every atom a complete octet and then compare it to the number of electrons
that you should have.
If you have too many electrons, we're going to cut out duplicates on neighboring atoms
and put pi bonds between them.
I know this may sound confusing so if you haven't seen that video make sure you watch
that first, but if you're comfortable with it, let's take a look.
To complete carbon's octet, we want 8.
So far I have 4,5,6,7,8.
To complete Nitrogen's octet, we only have 2 so we have to add 3,4,5,6,7,8.
But here is the problem.
If we count the pair electrons we have 2,4,6,8,10,12,14.
We have 14, we should have 10, that means we have 4 extra, we have to get rid of it.
How do we get rid of 4 extra?
We take a pair, we take a pair, that's 4 gone, replace it with a pi bond, we replace 4 with
2, we have 2 extra so we do the same thing again over here.
Got rid of 4, replace it with 2, and we have no extras.
Let's verify just to make sure we did it correctly.
Hydrogen has a single bound to carbon, we give carbon a triple bond to Nitrogen and
Nitrogen has 1 lone pair on the right side.
Let's count our electrons, 1,2,3,4,5,6,7,8,9,10.
The number of electrons are correct.
Does every have a full octet?
Nitrogen has 8, carbon has 8, Hydrogen has an exception to the rule, we have a structure.
To find the shape of the molecule, we look at that central carbon and it only has 3 groups
coming out of it, one to the right, one to the left making it a straight line or a linear
telling us that the hybridization of the central atom is an sp hybrid.
If you're curious, Nitrogen is also an SP hybrid because this got one and 2 groups coming
out of it.
We won't find polarity between carbon and hydrogen but Nitrogen is slightly more electronegative
than carbon.
And that means it's pulling on the electronegativity creating a dipole in the direction from carbon
to nitrogen.
For the next molecule let's look at SO2 which seems very innocent but can get very confusing.
Because we don't have a carbon, do we choose sulfur or oxygen of the central atom?
I like to start with the less electronegative atom and put the more electronegative ones
around it.
So if we have sulfur, we can put oxygen on either side and connect them with a single
bond.
What else do we need for this molecule?
Well, how many valence electrons do we have?
On the periodic table, sulfur is under oxygen so they have the same number of valence electrons
and that means if each should 6 times 3, 1 sulfur to Oxygen we expect a total of 18 electrons
and so far all I have is 4.
Let's start by completing all their octets and comparing what we have.
A quick count tells us that we have 2,4,6,8,10,12,14,16,18,20.
We have 20 electrons, we only want 18.
That means we have 2 extra that we're going to get rid off by creating a pi bond.
We get rid of a lone pair each on sulfur and oxygen and put a pi bond between them.
To redraw it clearly, we see an oxygen with 3 lone pairs bound to a sulfur with 1 lone
pair double bound to an oxygen with 2 lone pairs.
To find the shape of this molecule, we can't simply say it's linear with sulfur having
one group to the right and one group to the left because that lone pair also counts as
a group impacting the shape of the molecule.
So to draw it more correctly, we will have the lone pair on sulfur, taking up a lot of
space forcing the oxygen to shift upward a little bit, forcing that other oxygen to also
shift up a little bit, giving me a molecular shape of Bent.
Now if you were thinking, wait a minute, if I have 3 groups, it should be a trigonal planar.
That would be correct if there was an atom bound here.
Having 3 bound groups gives you trigonal planar.
Having only 2 bounds groups where the third one is a lone pair gives you a bent molecule.
And the hybridization for the central atom just like something that's trigonal planar
is going to be sp2 hybridized.
If you've already study resonance and formal charge, tutorials for both are linked below,
then you'll recognize that's a formal charge of minus 1 on the oxygen and a formal charge
of +1 on sulfur which should make you realize that we have an unconventional resonance structure
where that central sulfur atom can have 2 oxygens both double bound, and still have
a lone pair.
If you're thinking wait a minute Leah, doesn't this violate the octet rule?
Sulfur now has 1,2,3,4,5,6,7,8.9,10, you would be right to think so, except, that sulfur
is an exception to the octet rule and it can have more than a total of 8 electrons and
in its outermost shell.
What about polarity?
Given that sulfur and oxygen are somewhat close in electronegativity, we wouldn't expect
much polarity.
Alright, sulfur is still slightly more positive, oxygen is slightly more negative, and in this
situation, they appear to cancel out which is why it's important to recognize that we
have a bent structure where even though the oxygens are each pulling electronegativity
away from sulfur, both of them are pulling in a slightly upward direction which means
that the next dipole on this molecule will be cancelled out from the right and left but
will go slightly upward away from sulfur towards the oxygen.
Are you ready to try even more practice?
First, make sure to give this video a thumbs up and subscribe to my channel so you don't
miss out on any new videos and then if you're looking for even more practice, check out
my general chemistry basics for organic chemistry practice quiz which you can find on my website
along with the introductory videos that teach you these concepts by visiting my website
leah4sci.com/IntroOrgo.
The link again, leah4sci.com/IntroOrgo