—
Mr. Andersen shows you how to draw Lewis Dot Diagrams for atoms and simple molecules.
—
—
Transcript Provided by YouTube:
00:04
Hi. It’s Mr. Andersen and today I’m going to show you how to draw Lewis Dot
00:09
Diagram. Lewis Dot Diagrams are going to look like this. If we start with cesium (Cs). Cesium
00:14
is a metal. It’s got an atomic number of of 55 but it’s in the first column so that makes
00:19
it really easy on the periodic table. That means it has one valence electron. So if I
00:23
were to draw its Lewis Dot Diagram it’d look like this. You’re going to have Cs for the
00:27
atomic symbol of cesium and then you’re going to have one dot. And that represents one valence
00:31
electron. Now that one valence electron makes it really really reactive. And so if you mix
00:36
Cs with water, let’s take a look at this video, “Let’s try cesium, our fifth alkali metal”,
00:47
you get a huge explosion and the reason you get that huge explosion is due to that one
00:52
valence electron. So by the end of this video you should be able to draw Lewis Dot Diagrams
00:57
and you also should be able to draw Lewis Dot Diagrams for molecules. In other words
01:01
when you mix Cs with water you’re going to make cesium hydroxide. You’re going to make
01:05
hydrogen gas. And so we should be able to draw those Lewis Dot structures for molecules
01:10
as well. So let’s get started.
01:12
If we look on the periodic table, the first thing is you’ll keep jumping back to the periodic
01:18
table over and over and over again, so just get used to that. So if we look at it the
01:23
first thing that jumps out is that there is this verticality. In other words, all of these
01:28
alkali metals right here have the same properties and that’s because they all have one valence
01:33
electron. And so learn this right away. That they’re one valence electron in the first
01:37
column, 2 valence in the next one. So this would be 3 and 4 and 5 and 6 and 7 and 8.
01:46
So all of these, you probably know this, they are called the noble gases, are very stable
01:51
and that’s because they have 8 valence electrons. Where as these alkali metals and these halogens
01:56
right here are incredibly unstable. And then we have kind of a combination is the middle.
02:01
What you just notice is I ignored all of these, the transition metals and their electron configurations
02:05
are a little more complex. But please learn this right away. 1, 2, 3, 4, 5, 6, 7, 8 valence
02:12
electrons. You can’t do Lewis Dot Diagrams unless you know how many valence electrons
02:15
they have.
02:16
So let’s start with the first one. This is my method. I can’t remember where I picked
02:20
this up. But first thing I do is in my brain I draw cross hairs across the symbol. So we’re
02:26
going to do neon, let’s jump back again to the periodic table. Where’s neon? Neon is
02:31
going to be way over here. Here’s neon. And so it’s going to have how many valence electrons?
02:37
8. So let’s do the biggest one right away. 8 valence electrons. So again I’ve drawn this
02:42
in my head. I have this cross hairs. So I’m just going to start putting in electrons like
02:46
this. So here’s 1 valence electron, 7 to go. Here’s 2, 3, 4. Now it doesn’t matter if I
02:55
start on the top, or if I start on the right or the bottom. It doesn’t matter, but once
02:58
I choose a place to start I have to either go clockwise or counter-clockwise and keep
03:02
going from there. So let’s go again, 1, 2, 3, 4. Now I’ve got to put 5 right here. This
03:08
is 6. This is 7 and then this is going to be 8. So the Lewis Dot Diagram for Neon is
03:14
going to put, just do these all in right here, it’s going to look just like that. So neon’s
03:21
really, really happy. It’s a stable gas.
03:23
Let’s go to the next on. Hydrogen. Hydrogen, you should know, has one valence electron.
03:27
So where’s that going to go? Right here. Now I put hydrogen and helium at the top because
03:31
their rules are a little bit different. Because remember, the periodic table looks the way
03:35
it does because these first two elements, hydrogen and helium, can only have 2 valence
03:43
electrons in that out energy level. So let’s go to hydrogen. Hydrogen’s going to have one
03:47
of the electrons in it’s outer energy level, but helium, instead of going around, it clockwise,
03:53
it can only have two in that outer level and so this would be the correct Lewis dot structure
03:57
for that.
03:58
Let’s go do some more. So silicon is going to have 4 valence electrons. If you don’t
04:02
know, you just check on the periodic table. So it’s going to have 1, 2, 3, and 4. Or lithium
04:10
only has one. So it’s going to have one valence electron or one Lewis dot. Let’s go to carbon.
04:17
Carbon is the same group as silicon so it’s going to have 4 as well. If we’re doing oxygen,
04:24
how many does oxygen have? Oxygen has 6 valence electrons and so if I were to draw that again,
04:29
imagining those cross hairs in my brain, 1, 2, 3, 4 and here’s the 5th and then here’s
04:37
the 6th. So how many electrons would it like to pick up? Two more. Or if we go to nitrogen.
04:42
Nitrogen has 5 valence electrons so 1, 2, 3, 4, 5. Okay. So what are the two rules?
04:50
Again the two rules are number 1, choose a side. You start on the top or do you start
04:54
on, it doesn’t matter, but once you start, you’re either going to go clockwise or counter-clockwise.
05:00
And you’re always going to fill in one valence electron at a time until you come back again.
05:04
And you’re going to fill those other ones in. So that’s how you do atoms. That’s pretty
05:07
easy.
05:08
Let’s start doing molecules though. So let’s say we take something like hydrogen gas, H2.
05:13
The way I do it is I first of all draw my Lewis Dot structures for each of the atoms.
05:18
That’d be one hydrogen and here’s the other hydrogen. And now I kind of imagine this thing
05:24
in my brain. How am I going to get this electron and this electron to match up? Well if you
05:29
thing about it, if I were to rotate this and I were to rotate this, let’s do that over
05:34
here, so if I rotate that one electron over here so it’s facing the hydrogen it would
05:38
look like that. If I rotate the other hydrogen this way, then it’s going to share its electron
05:44
here. Sometimes if it gets to confusing which dot comes from where you could instead of
05:49
using a dot for this one we could use an x to represent this electron. So now I could
05:53
rotate this one and then I’d have an x right here.
05:56
Now I’m not done yet because we never draw
05:59
just 2 electrons like that in a Lewis structure. We want to show that there’s an actual bond
06:03
where they’re sharing those electrons. So I draw a hydrogen with a horizontal line to
06:07
another hydrogen. So this would be the Lewis structure for H2 or hydrogen gas. This line
06:13
represents a pair of electrons that the two of them are sharing in a covalent bond.
06:18
Okay. Let’s do the next one, H2O. It’s going
06:21
to get a little harder. Let’s draw them out. So we’ve got 1 hydrogen. It’s got one valence
06:26
electron. We’ve got another hydrogen with another valence electron. And then we’ve got
06:31
an oxygen, let me fill that in. It’s going to look like that. Okay, so now in my brain
06:37
I have to figure out, how are these going to match up. Well I can kind of do that. So
06:41
I’m going to put 1 here, my oxygen, start with that. I’m going to represent these as
06:46
xes, my two hydrogen so you don’t get confused. So I’ve got that. Then we’ve got that, so
06:52
now let’s put our hydrogen in there. So we’ve got a hydrogen here. There’s one, its electron
06:56
it’s sharing. Here’s the other hydrogen and here’s its electron it’s sharing. So how do
07:00
we draw the Lewis structure? Let’s kind of make this a line then. So we’re going to have
07:04
an oxygen to a hydrogen to a hydrogen and then we’re going to have these electron pairs
07:11
on this side and then on that side as well. So this would be the correct way to draw the
07:15
Lewis structure for water.
07:16
And let’s do a last one then. So the last one is going to be carbon dioxide. Up to this
07:21
point we’ve only done single bonds, but let’s try and do a double bond. So what do we got
07:25
here? Two oxygens, one carbon, so let’s do the oxygen first, draw the Lewis structure
07:30
for that. And we’ve got a carbon in the middle. Carbon’s going to have 4 valence electrons,
07:38
so it looks like this. Make that a little nicer. And we have another oxygen. Oxygen
07:43
is going to look like that. Okay, now I’ve got to figure this out. How am I going to
07:49
rotate this? Well if you think about this, this carbon we could kind of rotate it maybe
07:53
45 degrees like that. So I’m going to put the carbon in the middle and then it’s going
07:58
to share electrons with the oxygen on this side. So these two electrons are going to
08:08
share with these two electrons over here and then we’re going to form what’s called a double
08:12
bond. Now if I rotate that again we’re going to have our pairs on the side. We’ve got another
08:17
couple pair that we’re sharing to the oxygen and then we’re going to have our electrons
08:22
around the outside like that. So again, we’re not quite done because we want to make sure
08:26
those actually are drawn as double bonds. So we’d have a double bond to the oxygen.
08:31
Double bond to the other oxygen. Let’s make that actually an O. And we have our pairs
08:36
of electrons here on the side. Pairs of electrons here on the side.
08:40
So Lewis Dot structures tell us a lot about
08:42
the valence electrons in an atom. But more importantly they’re going to hint at its chemical
08:47
structure. Sometimes it’s hard to do this in your brain and you have to get the atom
08:51
building blocks out. And we can actually do that in class. So I hope that’s helpful.\
08:55
Thanks to Mr. Lewis, Gilbert Lewis we’ve got
08:59
these Lewis Dot Diagrams. The sad story about Mr. Lewis, right here that end on a sad note,
09:06
is that he did so much work in chemistry, did so much amazing work. Really figured out,
09:12
especially electrons and how electrons work. Sad note is that he was nominated for the
09:17
Nobel Prize 35 times and he never won. Some scientists think that Gilbert did himself
09:23
in with a little bit of cyanide in his office after meeting with one of his co-workers who
09:29
had actually just received a Nobel Prize. So kind of sad, but his legacy, Lewis Dot
09:34
Diagrams, lives on in chemistry students from here until the end of time. So I hope that’s
09:39
helpful. Thanks.
—
This post was previously published on YouTube.
—
Photo credit: Screenshot from video.