Welcome back, guys!
In this new video, we’re going to look at some basic rules that we can follow in order
to draw a Lewis dot structure.
Just realize that I know it might seem complicated right now like your professor is just throwing
elements on the board, writing bonds and throwing on some dots.
But if we follow these rules that I’ve built up and made from looking at various examples,
you'll be able to be better prepared when it comes to drawing a Lewis dot structure.
Here if we take a look at the first rule, it says rule number one: Least electronegative
element goes in the center.
We're going to say remember, electronegativity, when it comes to our periodic table, it increases
going from left to right.
and it increases going from the bottom up to the top of any group.
Remember with chemistry, there's one thing you can always depend on in chemistry.
There's always an exception to something you come up with.
We say that the least electronegative element goes in the center but that's not always the
case.
When it comes to hydrogen and fluorine, we're going to say they never go in the center,
never ever go in the center.
They only make one bond.
That’s a huge rule to remember.
Sometimes, hydrogen may be the least electronegative element but we can never put it in the center.
Hydrogen only wants to make one bond because every time you make a bond, you gain one electron.
Hydrogen on our periodic table has an atomic number of 1.
If it gains one more electron, it’s going to have an atomic number of 2 and become just
like helium.
It doesn't need to gain more than one more electron, so that's why it only makes one
bond.
Fluorine, fluorine doesn't want to make more than one bond.
It only needs one electron to become just like Neon, so that's why it only makes one
bond.
Just remember this big rule here.
Next we’re going to say rule two: The number of valence electrons that an element has is
equal to its group number.
You're not going to see bonds between transition metals so don't worry about that.
Next, we’re going to say carbon must make four bonds.
If we take a look at the periodic table, and this one’s kind of incomplete but neon is
right here.
Just realize, how far is carbon away from being a noble gas?
It’s four spaces away – 1, 2, 3, 4.
Every time we make a bond in Lewis dot structure, we pick up an electron.
Carbon would need to make four bonds in order to gain four more electrons to become just
like neon.
That's why we say carbon must make four bonds.
But there are rare occasions where it doesn't make four bonds.
In those rare occasions, it's going to make three bonds.
We’re going to say if carbon were positive or negative, then it would make three bonds.
Next, we're going to say nitrogen likes to make how many bonds?
Nitrogen is three spaces away from being a noble gas.
Therefore, nitrogen wants to make three bonds.
Every bond it makes, it picks up an electron.
Nitrogen likes to make three.
Based on that trend, we can say that oxygen likes to make two bonds.
It’s two spaces away from being a noble gas so it wants to make two bonds so that
it picks up two electrons, so it has the same number of electrons just like neon.
Then we’re going to say halogens.
They’re group 7A elements.
When they are not in the center of our Lewis dot structure, they only make one bond.
Remember, fluorine can never go in the center.
But the other halogens – bromine, chlorine, and iodine – we could put them in the center.
The last and final rule.
This is called the expanded valence shell theory.
We said earlier that nonmetals when they form covalent compounds want to follow the octet
rule.
The octet rule says that that central element wants eight electrons around it, eight valence
electrons around it.
But here's the thing.
That is not always the case.
We get into expanded valence shell theory territory.
In this case, we’re going to say that non-metal starting from period 3 down two period 7 can
have more than eight valence electrons around them.
They actually use their D shells, their D orbitals to compensate the extra electrons
they’re picking up.
Remember, hydrogen is here.
This is period 1, 2, 3, 4, 5, 6.
There’s also francium down here which will be 7.
We’re saying period 3 nonmetals, so phosphorus, sulfur, chlorine and lower all the way down
to 7, so selenium, bromine, iodine and of course, the noble gases as well.
We're going to say here when it comes to the noble gases making bonds, we’re usually
only going to see Krypton and lower making bonds.
Because they're larger, they’re able to form bonds with other elements.
Neon, helium, and argon are higher up.
They’re too stable to want to form any type of bonds with anyone.
We usually don't see any bonds forming between helium, neon, and argon.
But the bottom three, you can see bonds where they're forming connections to other elements.
Again, remember these rules.
They could save you on the exam.
This is just a composite of different rules and patterns that I've noticed from reading
various books.
I just condensed it as far as I could to seven basic rules.
Learn these basic rules.
It will help you a lot when it comes to drawing different types of Lewis dot structure.