- [Instructor] Let's get some practice
naming ionic compounds.
So I have a formula for an ionic compound right over here,
but how would I say this?
And if you get inspired, pause the video
and try to work it out on your own.
Well we could see that it has some magnesium,
and it has some phosphorus, and the convention is,
is the positive ion is written first.
So if this follows the convention,
the magnesium is going to be our cation.
And when you look at magnesium in the periodic table
right over here it makes sense
that it is going to be the cation
because it is a group two element, an alkaline earth metal,
and when those get ionized they tend to lose two electrons.
They have two electrons in their outermost shell and so,
when they ionize they tend to lose those.
So magnesium when it ionizes tends to be, let me,
when magnesium ionizes, it tends to magnesium two plus
and so this is probably each of these three magnesium ions
are probably magnesium two pluses,
so we could if we want, just to see what is going on
in this compound, we could write it like this,
which isn't what you would typically see
but this is just to see what the constituent ions
of this compound actually are.
Now as I mentioned, the cation tends
to be written first so that's the magnesium,
and the anion tends to be written second,
and that is the phosphorus here if we follow the convention.
So that's the phosphorus right over here,
and does it make sense that phosphorus would be an anion?
Well phosphorus is out on the right-hand side
of the periodic table right over here.
In order for it to complete its outermost shell,
it wants to gain one, two, three electrons,
so when it ionizes it makes sense
that it gains those three electrons.
So that phosphorus becomes phosphide, three minus,
and so these two phosphoruses in this formula,
we could write like this.
So we have two phosphide anions,
so we could just write it like this.
Now the whole reason why I wrote out
the constituent ions like this is to make sure
that it'll all end up being neutral.
The magnesiums, the three magnesium ions,
will have a collective six plus charge,
and the two phosphides are going to have
a collective six minus and so,
when you add the six pluses and the six minus,
they are all going to cancel out.
So let me write this, you have six plus,
and then you have six minus, which once again,
makes us feel good that we're understanding
this ionic compound well because an ionic compound,
they didn't write any net charge here,
so this is going to be neutral.
The cations and the anions are going to cancel out.
But once again, what do we call this thing?
Well the convention is, is we just write the elemental name
or we say the elemental name for the cation,
so this is going to be magnesium, magnesium.
And then we say, we say the -ide version of the anion.
So this is phosphorus, we say phosphide,
so this is going to be magnesium phosphide.
And actually I don't have to capitalize it,
so let me write it all lowercase, magnesium . . .
Magnesium . . .
Phosphide, and you might be saying hey,
this doesn't have a lot of information in it,
how do you, if someone just told me magnesium phosphide,
how do I construct what I did,
what we originally started with?
Well the answer is, you would have to say okay,
magnesium is our cation, it's right over here,
it's a group two, it's going to have a positive two charge
when it ionizes, so you would look at that,
and you'd say okay, phosphide,
well that tends to have a three minus charge
when it ionizes which we saw over here, and then you'd say,
okay well what ratio would I have to have
between these two things in order for them to cancel out?
Well for every three of these, of the magnesiums,
I have two phosphides, well then they're going to cancel out
and so that's how you could go
from the name to the actual formula,
which we will actually do in the next video.