In this video
I'll explain how to do these 9 Lewis structures. The way to use the video, do one of the Lewis structures and
then you have the time underneath it go to that place in the video and check your work when you're done go back and do
some other ones.
They'll get harder as you go on and that's to help you learn more complex Lewis structures.
Just a quick note, whenever you have a pair of electrons like this
that's called a single bond and we can write a single bond
just by putting a line like this so in our NH3 Lewis structure here
we can replace the bonds. These are the pairs of electrons between two atoms with single lines, and that'll mean the same thing.
It's the same Lewis structure just a different representation.
We'll start with the Lewis structure for OF2. First we find the valence electrons.
So oxygen is in group 16 sometimes called 6a. It has six valence electrons, plus
Fluorine which has seven, but we have the two fluorines, so
we'll multiply that by two and that gives us 6 plus 14 a total of 20 valence electrons
Next we'll put the least electronegative atom in the center of the Lewis structure
That's going to be our oxygen and then on either side. Let's put the fluorine atoms
We'll place a pair of electrons between the foreign and the oxygen here to form a chemical bond and then over here.
One we've done that we'll complete the octet around the fluorine atom so we'll won't have eight valence electrons around each fluorine atom.
So now the Floyd's have eight valence electrons they have an octet we've used 16 that means we have four more valence electrons.
We'll move back now to that central atom we'll put two valence electrons here,
and then to here and we've used all 20 valence electrons and each of the atoms in this Lewis structure has an octet
So we're done with the lewis structure for Of2.
For the CH4 Lewis structure, let's find the number of valence electrons
Carbon is in Group 14 sometimes, called 4A, it has four valence electrons.
Hydrogen has one valence electron we have these four hydrogen's
So let's multiply this by 4 if we had that up four
Plus four equals eight so we have eight valence electrons for the CH4 Lewis structure
We'll put the carbon in the center hydrogen atoms always go on the outside so we put the carbon in the center
And then we'll put the four hydrogen atoms around it
Next we'll put a pair of electrons between each of the atoms to form a chemical bond so put two here
At this point the carbon has an octet
We've used 2 4 6 8 . We've used all our valence electrons, and the hydrogen's they each have two
This is an important rule
Hydrogen's only need two valence electrons to have a full outer shell so the carbon has an octet the hydrogen's each have two
We're done with this Lewis structure.
H2 is probably about the simplest lewis structure we can draw so if we count the valence electrons up
Hydrogen is in Group one it has one valence electron, but we have the two hydrogen's we multiply those together one times two is two
So we have a total of two valence electrons
So we'll put our two hydrogen atoms right here next to each other and then we're going to move two valence electrons
Between them to form a chemical bond and when we do that. We've used both of the valence electrons
Hydrogen has two we said that hydrogen is an exception it only needs two and that's the lewis structure for H2 hydrogen gas
CO2, Carbon dioxide, is a little bit more of a complicated Lewis structure, but it's not too bad
Let's count the valence electrons we have four four carbon
oxygen has six
We have these two oxygens and that gives us a total of 4 plus 12
16 valence electron
Will put the least electronegative atom in the center of the Lewis structure? That's the carbon and then we'll put an oxygen on either side
Next we'll move electrons to the center to form the chemical bonds
Once we form those chemical bonds. We'll complete the octet around the oxygen atoms we have a total of 16 valence electrons
And now we've completed the octets on the oxygen atoms, but we've used 16 valence electrons
And we don't have any more the carbon in the center though it only has four valence electrons around it. It doesn't have an octet
to solve this problem
We can move valence electrons from the oxygen atom to the center between the carbon of the oxygens so that they share
Valence electrons so when we do that
The oxygen atom it still has eight valence electrons
But now the carbon atom has six. We're almost there. Let's take two from this oxygen move them to the center now
This oxygen here still has eight
This oxygen has eight and the carbon in the center has eight valence electrons
And we're using all 16 of the valence electrons here no more
So this is the lewis structure for co2 we call this a double bond
For the BH3 lewis structure boron has three valence electrons, it's in Group 13 sometimes called 3A.
Hydrogen has one and we have three hydrogen's so we have three plus three
total of six valence electrons for the BH 3 of the Lewis structure since hydrogen always goes on the outside
We'll put the boron in the center, and then we'll put the 3 hydrogen's
Around it we'll start by putting pairs of electrons between atoms to form chemical bonds
We've created the chemical bonds, but we've used 2 4 6 valence electrons
And we only have six valence electrons
So how do we complete the octet for the BH3 lewis structure? It turns out that with Lewis structure there are quite a few
exceptions. The octet rule is a very general rule. It works a lot of the time, but not all the time so for Boron,
it's one of the exceptions. Boron only needs six valence electrons. We'll see more of these exceptions later on.
O3 is also called ozone
We have six valence electrons for oxygen
And we have the three oxygens for a total of 18 valence electrons for this Lewis structure
We'll write the three oxygen atoms, and we'll just put them next to each other so one two
Three next we'll put electrons between the oxygen atoms to form the chemical bonds
We've used to we have 16 more. Let's complete the octets on the oxygens on the outside
So we've used 16 valence electrons we have 2 more and we'll put those on the central oxygen
And we have octets on the outside on the oxygens on the outside
But that central oxygen it only has 6 valence electrons so much like we did with carbon dioxide
We can move a pair of electrons to the centre
When we do that these two oxygens are sharing those electrons
So this oxygen has 8 the central oxygen has 8 and then this oxygen over here on the left. Also has 8
When we have Lewis structures like this there's another way we could draw
You can see the only difference here is we have the double bond here and on this one
We have the double bond on this side. These are called resonance structures often when we draw Lewis structures
You're asked to draw the resonance structures. Those are the different ways you can draw the Lewis structure here
We have a double bond on the right and here we have it on the left.
We have something a bit odd with this Lewis structure.
And that's that plus sign up there whenever you have a plus sign it means the ion compound
It's positive and to be positive it lost an electron so when we write up the number of valence electrons, nitrogen
It's 5 plus the four oxygens. We need to subtract one valence electron
Because the plus means we've lost one if this were a negative sign it wouldn't mean we gained one so five plus four is nine
Minus one we have a total of eight valence electrons for this NH4+ ion
we'll put the N in the center hydrogen always on the outside, and then we'll put the four hydrogen's around it
We have a total of eight valence electrons let's put a pair of electrons between each of the atoms to form a chemical bond
There so we've used all eight valence electrons than nitrogen it has a full outer
Shell as an octet and the hydrogen's they only need two they all have full outer shells
That's the lewis structure for nh4 plus. It's called the ammonium, ion
For SO4 2- , the sulfate ion, sulfur has six valence electrons
oxygen also has six we have four
Oxygens and this 2- up here that means we have an extra two valence electrons
So we'll add those right there. We have six plus
24 that's 30 plus two gives us 32 valence electrons for this lewis structure
We'll start by putting the least electronegative atom in the center. That's the sulfur and then we'll put the four oxygens around it
We have 32 valence electrons. We'll put two between each of the atoms to form the chemical bonds
Then we'll go around and complete the octets on the oxygen atoms
Whoo that was a lot of electrons if we add all these electrons up you'll see that we've used 32 valence electrons
The software has an octet has do all of the oxygen atoms
It looks like we're done with this Lewis structure everything has octet. We've used all the valence electrons. It's all good, right?
However, if you're in a more advanced chemistry class you will learn that sulfur and other atoms in the periodic table that are in Period Three
or below they can have what's called an Expanded Octet.
An expanded octet means that the central atom there would have more than eight valence electrons the
Way we tell is we check the formal charges
I'm not going to do that right here, but I'll show you what the correct structure looks like after we find the best formal charges
This is actually the best Lewis structure for the sulfate ion all of the formal charges are 0.
Each of the oxygen atoms has 8 valence electrons the central sulfur has 2 4 6 8 10 12
But that's okay it can do that because it's in Period 3 on the periodic table or below can have that expanded octet
So this is the best Lewis structure for SO4 2- ,the sulfate ion.
@e should also put brackets around this with that - - to show that it's an ion and
There you have it the Lewis structure for SO4 2- , the sulfate ion.
For phosphate we have 32 valence electron it's likely when you do the Lewis structure
You'll come up with something initially that looks like this
And that makes a lot of sense. We've used all 32 valence electrons each of the atoms has an octet
Looks like a pretty good lewis structure
Again when we have atoms that are in period three or below in the periodic table especially things like sulfur and phosphorus
We should check the formal charges to make sure that this is the best or the most favorable lewis structure. I won't do that here
There's a link to a fuller video you can watch, but when we do it. Here's what we end up with
The only difference is that we have a double bond here between
The phosphorus and the oxygen everything else stays a single bond
We still use the 32 valence electrons so checking the formal charges is a very good idea if you're an advanced chemistry class
One last thing we should put brackets around this since it is an ion
And then we'll write the three minus right up here, and that's the lewis structure for PO4 3-, the phosphate ion.