- [Instructor] So we have the formula for an ionic compound
here, and the goal of this video is
what do we call this thing?
It clearly involves some cobalt and some sulfur,
but how do we name it?
Well, the convention is, is the first element to be listed
is going to be our cation, and if we look at cobalt
over here, we see that it is a D-block element
and D-block elements are tricky because you don't know
exactly how it will ionize.
So we know that this is going to be our cation,
it's going to be our positive ion, but we don't know
what the charge on each of those cobalt
is actually going to be.
So now let's look at the anion, let's look at the sulfur,
or as an anion, the sulfide.
So let me underline that.
And on the periodic table, we see sulfur is out here
that in its group, it would want to gain two electrons
in order to have a complete outer shell.
It's just like oxygen, it wants to gain two electrons.
So the sulfide anion will look like this.
So it will have sulfur when it ionizes
will have a two minus charge, just like oxygen,
just like everything else in this group.
It would want to gain one, two electrons so that
its outer shell looks like that of a noble gas,
looks like that of argon.
We can use this as a clue to figure out what must be
the charge on the cobalts because we have
three of the sulfides.
Each of the sulfides has a two minus charge,
and we have three of them, so that's going to give us
a six minus charge all in.
And then the cobalt,
we have two of them.
And so these two cobalt have to offset
this six minus charge.
They have to have a six plus charge.
Well that means that each of them need
to have a three plus charge.
If each of these have a three plus charge and you have
two of them, then you're gonna have six plus on the positive
side and you're gonna have six minus from the sulfides.
And the reason why this is useful for us
is now we can name this.
We would call this ionic compound
Cobalt III,
cobalt and you would write it
with Roman numerals here, Cobalt III Sulfide,
Cobalt III Sulfide.
Now I know what you might be thinking.
Hey, when we looked at other ionic compounds,
I didn't have to write the charge of the cation there
and the reason why the convention is to do it here, and
I don't have to write in upper case there,
so let me rewrite it as
cobalt (III) sufide.
The reason why I wrote that three in this case
is because the cobalt can take on multiple charges.
It's known as polyvalent, polyvalent.
If something is in group one, you know it tends to have
a positive one charge.
Group two tends to have a positive two charge.
If it's a halide, it tends to have a negative one charge.
But these metals here in this D-block here,
they could be ionized in multiple ways.
And so that's why we have to figure out what the charge
of the cobalt is, and we write it here in the name.
We would call this cobalt (III) sulfide.
You could have other ionizations of cobalt in other ionic
compounds, but this one is cobalt (III) sulfide.
And you could go either way.
You could say, okay if the cobalt has a charge of three plus
you could figure out its formula right over here,
or as we just did in this video, you could go
from the formula to the actual name.