At the end of this video, you'll be a pro
at drawing the basic Lewis structures.
There really are only five main steps that
you need to know.
Let's get started.
Step 1: find the total number of valence electrons
for the molecule you're drawing the Lewis
structure for.
Let's try one.
We'll use the periodic table to figure out
how many valence electrons each element has.
Hydrogen is in Group 1 on the periodic table,
so it has 1 valence electron, but the subscript
tells us we have two Hydrogens, so we'll need
to multiply that by 2.
Oxygen is in group 6 or 16; it has 6 valence
electrons.
When we add that up, 2 + 6, we get a total
of 8 valence electrons for H2O.
Remember: valence electrons are the outer
shell electrons.
They're the ones that form chemical bonds
and interact with the world around them.
Let's try one more: cyanide.
For cyanide, we can look on the periodic table
again.
Carbon has 4 valence electrons and Nitrogen
has 5.
The trick for this one, is, though, it's a
negative ion.
This negative sign right here tells you there's
going to be one extra valence electron.
So 4 + 5 + 1: 10 valence electrons for the
cyanide ion.
Step 2: put the least electronegative atom
in the center.
So how do you know it's the least electronegative
atom?
Just remember that Fluorine is the most electronegative,
followed by Oxygen.
So as you move away from Fluorine and Oxygen,
atoms become less electronegative.
That means those atoms will go at the center.
Let's see how it works: When I draw the Lewis
structure for something like NO2, I can see
that Nitrogen is further away from Fluorine
than Oxygen.
That means when I draw my Lewis structure
I'll put Nitrogen at the center, and Oxygens
on the outside.
For something like PCl3, I can see that Phosphorus
is further away from Fluorine than the Chlorine
atom.
So I'll put Phosphorus at the center and the
Chlorine atoms on the outside, like this.
Some Lewis structures, like HCl, only have
two atoms.
In that case, you don't have to worry about
it, because there's no center.
An important note: Hydrogen always goes on
the outside of Lewis structures.
Always.
Seriously.
Let's look at HCl again.
We know it has 8 valence electrons, and we'll
put the first 2 in our structure between the
H and Cl to form a chemical bond.
That's important: by putting those two electrons
there, we're showing that the Hydrogen and
the Chlorine are chemically bonded.
Step 4: Complete the octets on the outside
atoms.
Octet means 8.
For HCl, we have a total of 8 valence electrons--one
for the Hydrogen, 7 for the Chlorine.
So we've used 2 already to form the chemical
bond.
For the Chlorine, we'll put 4, 6, and then
8.
We've used all the valence electrons and we've
completed the octet for Chlorine--it has 8
valence electrons.
For basic Lewis structures, the big exception
is going to be Hydrogen.
It only needs 2 valence electrons for a full
outer shell.
So Hydrogen has a full outer shell with two
valence electrons.
And Chlorine, its octet is full with 8, and
we've used only the 8 valence electrons that
we have for the HCl molecule.
We're done with this structure.
There's one more rule that we need to look
at.
It doesn't apply to HCl.
Actually, it doesn't apply to a lot of molecules.
But what happens when you've used all your
valence electrons and you still haven't completed
the octets for each atom in the molecule?
That's step 5.
So sometimes you run out of valence electrons
and still haven't filled the octets.
Hey, it happens, right?
So, in Step 5 what we're going to do is, if
we run out of valence electrons, we'll move
some of the valence electrons from the outside
to the inside and share them, and form double
and triple bonds.
Here's how it works: So for O2, Oxygen, if
I follow all the rules, I count the valence
electrons.
I have 12.
I put a bond between the atoms, and then fill
the outer shell, I end up with a situation
where I have an octet for this Oxygen--it
has 8--but this Oxygen only has 6 valence
electrons.
It doesn't have an octet, and I don't have
any more valence electrons.
To solve the problem, I can take these valence
electrons right here and move them to the
center to share them and form a double bond
between the two Oxygen atoms.
I'm still only using 12 valence electrons,
but now this Oxygen has 2, 4, 6, 8 valence
electrons.
And this Oxygen has 2, 4, 6, 8 valence electrons.
So by sharing those valence electrons, I can
get the octet on each of the Oxygens, and
still use the 12 valence electrons that I
had for the O2 molecule.
If I wanted, I could replace these right here
with two lines to show the double bond, where
each line represents a pair of electrons.
For Lewis structures like N2, I would even
use a triple bond to achieve octets.
So how do you get good at drawing Lewis structures?
The first thing is, know the steps.
They're your guide.
The second thing is practice.
You need lots of practice to get good at this,
for it to be second nature.
Use the links in this video, or go to my website
and try more videos.
First, try it yourself and then watch the
video.
Pause when you're stuck, see what's going
on there, and try to correct the problem.
This is Dr. B., and thanks for watching.
