Hi. It’s Mr. Andersen and today
I’m going to show you how to draw Lewis Dot Diagram. Lewis Dot Diagrams are going to look
like this. If we start with cesium (Cs). Cesium is a metal. It’s got an atomic number of of
55 but it’s in the first column so that makes it really easy on the periodic table. That
means it has one valence electron. So if I were to draw its Lewis Dot Diagram it’d look
like this. You’re going to have Cs for the atomic symbol of cesium and then you’re going
to have one dot. And that represents one valence electron. Now that one valence electron makes
it really really reactive. And so if you mix Cs with water, let’s take a look at this video,
“Let’s try cesium, our fifth alkali metal”, you get a huge explosion and the reason you
get that huge explosion is due to that one valence electron. So by the end of this video
you should be able to draw Lewis Dot Diagrams and you also should be able to draw Lewis
Dot Diagrams for molecules. In other words when you mix Cs with water you’re going to
make cesium hydroxide. You’re going to make hydrogen gas. And so we should be able to
draw those Lewis Dot structures for molecules as well. So let’s get started. If we look on the periodic table, the first
thing is you’ll keep jumping back to the periodic table over and over and over again, so just
get used to that. So if we look at it the first thing that jumps out is that there is
this verticality. In other words, all of these alkali metals right here have the same properties
and that’s because they all have one valence electron. And so learn this right away. That
they’re one valence electron in the first column, 2 valence in the next one. So this
would be 3 and 4 and 5 and 6 and 7 and 8. So all of these, you probably know this, they
are called the noble gases, are very stable and that’s because they have 8 valence electrons.
Where as these alkali metals and these halogens right here are incredibly unstable. And then
we have kind of a combination is the middle. What you just notice is I ignored all of these,
the transition metals and their electron configurations are a little more complex. But please learn
this right away. 1, 2, 3, 4, 5, 6, 7, 8 valence electrons. You can’t do Lewis Dot Diagrams
unless you know how many valence electrons they have. So let’s start with the first one. This is
my method. I can’t remember where I picked this up. But first thing I do is in my brain
I draw cross hairs across the symbol. So we’re going to do neon, let’s jump back again to
the periodic table. Where’s neon? Neon is going to be way over here. Here’s neon. And
so it’s going to have how many valence electrons? 8. So let’s do the biggest one right away.
8 valence electrons. So again I’ve drawn this in my head. I have this cross hairs. So I’m
just going to start putting in electrons like this. So here’s 1 valence electron, 7 to go.
Here’s 2, 3, 4. Now it doesn’t matter if I start on the top, or if I start on the right
or the bottom. It doesn’t matter, but once I choose a place to start I have to either
go clockwise or counter-clockwise and keep going from there. So let’s go again, 1, 2,
3, 4. Now I’ve got to put 5 right here. This is 6. This is 7 and then this is going to
be 8. So the Lewis Dot Diagram for Neon is going to put, just do these all in right here,
it’s going to look just like that. So neon’s really, really happy. It’s a stable gas. Let’s go to the next on. Hydrogen. Hydrogen,
you should know, has one valence electron. So where’s that going to go? Right here. Now
I put hydrogen and helium at the top because their rules are a little bit different. Because
remember, the periodic table looks the way it does because these first two elements,
hydrogen and helium, can only have 2 valence electrons in that out energy level. So let’s
go to hydrogen. Hydrogen’s going to have one of the electrons in it’s outer energy level,
but helium, instead of going around, it clockwise, it can only have two in that outer level and
so this would be the correct Lewis dot structure for that. Let’s go do some more. So silicon is going
to have 4 valence electrons. If you don’t know, you just check on the periodic table.
So it’s going to have 1, 2, 3, and 4. Or lithium only has one. So it’s going to have one valence
electron or one Lewis dot. Let’s go to carbon. Carbon is the same group as silicon so it’s
going to have 4 as well. If we’re doing oxygen, how many does oxygen have? Oxygen has 6 valence
electrons and so if I were to draw that again, imagining those cross hairs in my brain, 1,
2, 3, 4 and here’s the 5th and then here’s the 6th. So how many electrons would it like
to pick up? Two more. Or if we go to nitrogen. Nitrogen has 5 valence electrons so 1, 2,
3, 4, 5. Okay. So what are the two rules? Again the two rules are number 1, choose a
side. You start on the top or do you start on, it doesn’t matter, but once you start,
you’re either going to go clockwise or counter-clockwise. And you’re always going to fill in one valence
electron at a time until you come back again. And you’re going to fill those other ones
in. So that’s how you do atoms. That’s pretty easy. Let’s start doing molecules though. So let’s
say we take something like hydrogen gas, H2. The way I do it is I first of all draw my
Lewis Dot structures for each of the atoms. That’d be one hydrogen and here’s the other
hydrogen. And now I kind of imagine this thing in my brain. How am I going to get this electron
and this electron to match up? Well if you thing about it, if I were to rotate this and
I were to rotate this, let’s do that over here, so if I rotate that one electron over
here so it’s facing the hydrogen it would look like that. If I rotate the other hydrogen
this way, then it’s going to share its electron here. Sometimes if it gets to confusing which
dot comes from where you could instead of using a dot for this one we could use an x
to represent this electron. So now I could rotate this one and then I’d have an x right
here. Now I’m not done yet because we never draw just 2 electrons like that in a Lewis structure.
We want to show that there’s an actual bond where they’re sharing those electrons. So
I draw a hydrogen with a horizontal line to another hydrogen. So this would be the Lewis
structure for H2 or hydrogen gas. This line represents a pair of electrons that the two
of them are sharing in a covalent bond. Okay. Let’s do the next one, H2O. It’s going to get a little harder. Let’s draw them out.
So we’ve got 1 hydrogen. It’s got one valence electron. We’ve got another hydrogen with
another valence electron. And then we’ve got an oxygen, let me fill that in. It’s going
to look like that. Okay, so now in my brain I have to figure out, how are these going
to match up. Well I can kind of do that. So I’m going to put 1 here, my oxygen, start
with that. I’m going to represent these as xes, my two hydrogen so you don’t get confused.
So I’ve got that. Then we’ve got that, so now let’s put our hydrogen in there. So we’ve
got a hydrogen here. There’s one, its electron it’s sharing. Here’s the other hydrogen and
here’s its electron it’s sharing. So how do we draw the Lewis structure? Let’s kind of
make this a line then. So we’re going to have an oxygen to a hydrogen to a hydrogen and
then we’re going to have these electron pairs on this side and then on that side as well.
So this would be the correct way to draw the Lewis structure for water. And let’s do a last one then. So the last
one is going to be carbon dioxide. Up to this point we’ve only done single bonds, but let’s
try and do a double bond. So what do we got here? Two oxygens, one carbon, so let’s do
the oxygen first, draw the Lewis structure for that. And we’ve got a carbon in the middle.
Carbon’s going to have 4 valence electrons, so it looks like this. Make that a little
nicer. And we have another oxygen. Oxygen is going to look like that. Okay, now I’ve
got to figure this out. How am I going to rotate this? Well if you think about this,
this carbon we could kind of rotate it maybe 45 degrees like that. So I’m going to put
the carbon in the middle and then it’s going to share electrons with the oxygen on this
side. So these two electrons are going to share with these two electrons over here and
then we’re going to form what’s called a double bond. Now if I rotate that again we’re going
to have our pairs on the side. We’ve got another couple pair that we’re sharing to the oxygen
and then we’re going to have our electrons around the outside like that. So again, we’re
not quite done because we want to make sure those actually are drawn as double bonds.
So we’d have a double bond to the oxygen. Double bond to the other oxygen. Let’s make
that actually an O. And we have our pairs of electrons here on the side. Pairs of electrons
here on the side. So Lewis Dot structures tell us a lot about the valence electrons in an atom. But more
importantly they’re going to hint at its chemical structure. Sometimes it’s hard to do this
in your brain and you have to get the atom building blocks out. And we can actually do
that in class. So I hope that’s helpful.\ Thanks to Mr. Lewis, Gilbert Lewis we’ve got these Lewis Dot Diagrams. The sad story about
Mr. Lewis, right here that end on a sad note, is that he did so much work in chemistry,
did so much amazing work. Really figured out, especially electrons and how electrons work.
Sad note is that he was nominated for the Nobel Prize 35 times and he never won. Some
scientists think that Gilbert did himself in with a little bit of cyanide in his office
after meeting with one of his co-workers who had actually just received a Nobel Prize.
So kind of sad, but his legacy, Lewis Dot Diagrams, lives on in chemistry students from
here until the end of time. So I hope that’s helpful. Thanks.