Let's talk about how to write chemical formulas for ionic compounds.
So what this means is we're going to start the chemical name like Magnesium Chloride
and learn the steps that we have to go through to take this chemical name and use it to write
a chemical formula like MgCl2, okay?
Let's start with the first example, Lithium Oxide.
So when I'm doing these kind of problems, the first thing that I want to do is find
both these elements on the periodic table.
I'm using this kind of weird version of the periodic table that I just wrote out.
I left out a lot of the elements because they're not important for what we're doing here and
I thought that they're kind of distracting but don't be confused.
This really is no different from periodic table you probably have in your book, it's
just that it's missing a lot of the elements.
Anyway, Lithium . . . where do I find that?
It's right here, Li, and Oxide.
Oxide is just another word for Oxygen, it's what we call Oxygen when Oxygen has a charge
and I'm going to talk a little bit more about that later.
Anyway, Oxide is just another word for Oxygen and Oxygen is over here.
They're on opposite sides of the periodic table and check this out too.
There's this big thick staircase that separates Lithium from Oxygen.
What's a staircase doing?
If you remember, the staircase separates the metals on this side of the periodic table
from the nonmetals on this side of the periodic table.
So Lithium's a metal and Oxygen is a nonmetal.
This is important because we have a metal and a nonmetal connected together and that
means that we're dealing with an ionic compound.
Ionic compounds are always metals and nonmetals.
So we have metals and nonmetals making an ionic compound, that means that the atoms
in that compound have a charge, so I want to find out what the charge of those atoms
is and I can do that by looking at where they live, what column they live in on the periodic
table.
Here's what I mean.
So Lithium lives within this column.
Everything in this column has a plus one charge, so I'm going to write this right here . . . Li+1
. . . everything in this column has a two plus charge (+2), three plus charge (+3),
you may want to write this on your own periodic table that you have for your reference.
Everything here has a 3 minus charge (-3) and Oxygen which lives in this column has
a two minus charge (-2) so I'm going to write it right here.
So Li+1, O2-.
Okay, so now I ask myself, does the plus charge (+) and the minus charge (-) balance when
I have one atom of both of these?
The answer is no because I have one plus charge but I have two minus charges so the charges
don't balance.
But I want to figure out how to balance them.
I need to have them balanced in order to write the chemical formula, so what I can do is
I can add more Lithium atoms, I can add more Oxygen atoms, or I can add both of them until
I get the charges to balance out.
I have two minus (2-) here and only one plus (+1) here so what I'm going to do is I'm going
to add another Lithium atom, Li+, so that now I have two positive to balance out my
two negative and now they balance.
So in order to get the charges balanced, I have to have two Lithiums and one Oxide or
one Oxygen.
Now when I write the chemical formula, I'm literally going to say how many of each of
these atoms I need in order for the charges to balance.
We've said that it's two Lithium atoms so I'm going to write "Li" and then the two after
it that indicates that I have two lithium atoms and then one "O" to show that I have
one Oxygen and I'm not going to write anything after that.
If you have a letter without anything after it, it means that you just have one of them.
So Lithium Oxide, two Lithiums and one oxygen, is how we get the charges to balance for this
compound.
Here's the next one, Potassium Nitride.
Just as before, I'm going to take out the periodic table and I'm going to find both
of these guys on it.
Potassium is K, it's right over here and Nitride is what we call Nitrogen when it has a charge
on it and so Nitrogen is over here.
Check it out!
Metal, non-metal, separated by this big thick staircase and so we know we're dealing with
an ionic compound and whenever we think about ionic compound . . . metals and nonmetals
. . . we want to think charges, what charges does the atoms have?
Potassium lives in this column here, the same column as Lithium and so it has a one plus
charge (+1) so I'm going to write that up here, K+.
And Nitrogen lives in this column where everything has a minus three charge (-3) so here we have
N3-.
When I have one atom of K+ and one atom of N3- do the charges balance out here?
Doe the plus and minus balance out?
It doesn't, I got one plus and I've got three minus, that doesn't balance right.
But I could add more atoms of either type in order to get the charges to balance.
So since I don't have as much pluses as I have minuses I'm going to add a bunch of more
Potassium.
Here I add another, now I have 2+, and now I'm going to add one more and I have 3+ . . . 3+
Potassium on this side balances out the 3- Nitride on this side.
Now when I write my chemical formula, I want to literally say how many atoms of each type
I need for the charges to balance.
I'm going to need three Potassiums so I'm going to write "K3" and I only need one nitrogen
or one Nitride so I'm going to write "N" with nothing after it and if I don't write anything
after, it means one.
So three Potassiums, one Nitrogen, the correct formula for this is K3N.
Okay, here's another one.
Sodium Chloride which is the name of common table salt.
Once again, I'm going to find them both on the periodic table.
Sodium right here is in the one plus (+1) column and Chlorine or Chloride is in this
column right here.
We've got metal and nonmetal so we're dealing with ionic compounds which means the atoms
have charges.
Now I want to figure out what the charges are.
As I said before, Na is in the one plus (+1) so an atom of Sodium has a one plus (+1) charge
here and then Chloride is what we call Chlorine when it has a charge and Chlorine lives in
this column so it has a one minus charge (-1) so Cl-.
Okay, so I have one atom of Sodium plus one (Na+) and I have one atom of Chloride minus
one (Cl-).
When I have an atom of both of these, does the positive and negative balance out?
It does, I don't need to add more of either one of these atoms, I'm totally set.
So to write the chemical formula for Sodium Chloride I want to say again how many of each
I need for the charges to balance out.
I'm just going to say NaCl.
I don't put any numbers after them because if I don't put a number it means I just have
one.
So I have one Sodium and one Chloride, okay?
You might be getting the hang of this.
I want to try to convince you to watch this one last example because it's a tricky one
that confuses a lot of people.
Aluminum Oxide.
Once again, periodic table.
Aluminum here, Oxygen here, metal, non-metal, it's ionic which means charges are important.
So Aluminum is in the three plus (+3) column, one atom or ion of Aluminum, an ion is what
we call an atom that has a charge.
An ion of Aluminum has a three plus charge (+3) and an atom of Oxygen or oxide here has
a two minus charge (-2), Al2+ O2-.Now I need to figure out how many of each of these atoms
I need for the charges to balance out and this example is a little bit trickier than
the ones before.
How are we going to be able to add atoms to get the same number of positive and negative
charge?
Let me give you a hint.
We're going to start by adding another Aluminum atom in here or Aluminum ion to be precise.
So now I have three plus here and three-plus here, each Aluminum atom has a charge of three
plus (+3) so I have a total of six plus (+6).
Now I can balance out the six plus by adding more of the Oxygen.
I add one more Oxygen and I have a total of four minus and I add one more I have a total
of six minus.
So this is how we balance out the charges in Aluminum Oxide with two Aluminum ions with
three plus (+3) each, total of six plus (+6), and three Oxygen ions with a total of two
minus (-2) each which gives us minus six (-6).
Now I'm going to write the chemical formula to show how many each I need to have.
That's going to be Al2 and O3.
I have two Aluminums and three Oxygens.
Okay, so now let me tell you what I'm going to do.
If you're still a little bit shaky on this, I'm going to do a few more practice problems
in a minute.
If you feel really good about this, you might want to just turn the video off and go and
watch videos I have on writing formulas for compounds with transition metals and for compounds
with polyatomic ions.
But before I do a few more practice problems I want to talk about two commonly made mistakes
so many people make when they're writing these formulas so you might just want to stick around
to watch that.
Here are the two mistakes people make.
Let's assume that you're asked to write the chemical formula for Lithium Oxide, you do
this you find out you want to have two Lithiums with one plus each to balance out the charge
on Oxygen, okay?
Here is one mistake that people make.
They say, okay, I have two Lithiums and one Oxygen so I'll write LiO1.
They say yeah, yeah, yeah there's only one Oxygen so it can't hurt to just add the O1
here, can it?
It can't hurt.
It does hurt!
It's just not right to do, you never want to put a one after an element.
So Li2O1 is wrong.
Instead, you want to write Li2O without any numbers after this and this is the right way
to do it.
Here's another mistake that people often do.
They don't remember to get rid of the charges when they write the chemical formula.
So they end up writing something like Li+2O2- where the charges that were on the atoms here
stay when they write the chemical formula.
This also unfortunately is wrong.
When you write the chemical formula you want to get rid of all of the charges so you just
write again Li2O.
Make sure there aren't any charges in the chemical formula when you finally right it,
okay?