Let's learn how to write chemical formulas for compounds that have polyatomic ions in
them.
So what's a polyatomic ion?
Well, it's what happens when a bunch of atoms come together and they connect together and
form a big clump.
That clump itself has a charge.
So here's a list of some of my polyatomic ions and it has stuff on it like Nitrate which
is NO3- and what that means is that one Nitrogen atom forms a clump with three Oxygen atoms
and this whole thing, these four atoms together, have a charge of one minus (-1).
Or we've got stuff like Ammonium here, which is NH4+, which means you have one Nitrogen
connected to four Hydrogens in a big group and that group of atoms together has a charge
of one plus (+1), okay?
So here is the name of a compound, Calcium Nitrate.
Let's go through the steps that we have to take to write the chemical formula for this
compound.
The two things you're going to need for this, the first is a periodic table.
I'm using this kind of weird version where I've left out a lot of the elements because
I think they're distracting but I've kept in the ones that are important for what we're
doing.
And then, you'll need a list of polyatomic ions, okay?
This is a short list of just my favorite polyatomic ions and I bet that your teacher or textbook
has a much longer list of polyatomic ions with a whole bunch of extras they want you
to learn, okay?
If that's the case, it's no big deal because if you learn how to write compounds with these
polyatomic ions, you'll also be able to do it with whatever extra ions they want you
to use, okay?
So anyway, ions!
Since we're talking about polyatomic ions here, the compounds were going to be writing
formulas for are all ionic compounds which means that we're going to have to find out
what the charges on both these things are in order to write the formula for it, okay?
So let's start out with Calcium.
What's the charge on Calcium?
Well, that's pretty easy.
Calcium is right here on the periodic table and it's in this column.
Everything in this column has a two plus charge (+2) so I'm going to write this right up here,
Ca2+.
Okay, now Nitrate.
Nitrate is a polyatomic ion and so it's on this chart here.
Nitrate here is NO3-.
This whole group of atoms have a one minus charge.
So now I ask myself, when I have one Calcium atom and one clump of this Nitrate, do the
positive and negative charges balance?
They don't because I have two plus (+2) in the Calcium but I only have one minus (-1)
from the Nitrate.
So in this stage, I can add more of either one of these or of both until I get the charges
to balance.
Since I have two plus (+2) here and each one of these Nitrates bring me one minus (-1),
I'm going to add another Nitrate (NO3-) so that now I have two minus total which will
balance out my two plus (+2) from Calcium, okay?
So now I have the charges balanced and now I want to write this in a chemical formula
that shows how many of each I have to have in order to balance out the charges.
So I need one Calcium so I'm going to write that as Ca without anything after it and that
means that I have one of them if you don't write anything after it and then I want to
say that I have two Nitrates, so NO3.
There's my Nitrate.
I want to say that I have two of this whole thing, right?
I want to multiply this whole thing, NO3, by two.
So what I'm going to do is use parentheses, I surround the polyatomic ion with parentheses
and then put down here how many at this whole thing I want.
It's going to be Ca(NO3)2.
It means one atom of Calcium and two whole clumps of Nitrate.
Let's do a couple more.
Ammonium Nitride, we have to bring our two tools back here, the periodic table and the
list of polyatomic ions.
Okay, so Ammonium, you may recognize from this list is a polyatomic ion and it's NH4+,
a whole clump of atoms has that one plus charge.
Now Nitride is right here on the periodic table, it's what we called Nitrogen when it
has a charge.
It has a charge of three minus (-3).
Now don't get confused!
Some people confuse Nitride with Nitrate and Nitrite which are polyatomic ions and often
times people are like how do you know which is the polyatomic ion?
How do you not get confused with things that are on the periodic table?
Sorry to say this but the only good way to do this is to memorize them.
People are always saying that it's a good idea to memorize polyatomic ions so when they
pop out like Ammonium, you know right away where to find that and you don't have to waste
time trying to find Ammonium on the periodic table and it's not going to be in the periodic
table because it's a polyatomic ion, okay?
Anyway, we got Ammonium (NH4+) and Nitride which is N3-.
Now we have to balance the charges if they're not already balanced out.
We have one plus (+1) here and we have three minus (-3).
Since each of these has a one plus charge (+1) I'm going to add two more so that I have
a total of three plus (+3) to balance out my three minus (-3) on the Nitride, okay?
When I need more than one polyatomic ion, I surrounded them with these parentheses and
then I write the number afterwards.
That's how I show that I want three of the Ammoniums and that I only want one Nitride
because it has a charge of three minus (-3) so I can just write it like that without anything
after it and that means that I have one of them.
Okay, Sodium Carbonate.
Back to the periodic table, back to the list.
Okay, Sodium here is in my one plus (+1) column so I'm going to write Na+ and Carbonate is
a polyatomic ion so don't waste time trying to find Carbonate on the periodic table.
Just memorize this list and you'll know right away that it's a polyatomic.
Carbonate is CO3 and this whole chunk of atoms together has a charge of two minus (-2).
Now I want to balance out the charges.
One plus (+1) versus two minus (-2) so I add another one of the atoms that has a two plus
(+2) and now I'm going to write the chemical formula as Na2, to show that I have two of
them, CO3.
Now sometimes people think that you should surround the polyatomic ion with parentheses.
No, no, no!
You only do that if you're multiplying it by a number like two or three or four but
if you just want one, all I want is one Carbonate, you don't use anything at all.
No parentheses, no number one after this, just CO3 and that means that you just have
one of these chunks of Carbonate, okay?
We'll do two more.
Ammonium Phosphate, back with the periodic table and with the polyatomic ions.
Both these turn out to be polyatomic ions and again you'll recognize it right away after
you've memorized this.
Ammonium is NH4+ and phosphate is PO4^3- . I want to get the charges to balance here some
I'm going to add two more Ammoniums for a total of three plus charge (+3) to balance
out my three minus charge (-3).
So, I've got Ammonium (NH4), I want three of those, it's a polyatomic ion, so it gets
parentheses, then there's a three after it to say I want three of them and then I've
got PO 4 here . . . put it right after.
Again, since I only want one of the Phosphates, only one of the PO4's, there are no parenthesis
and I just write PO4 and that's all I need to say that I have one of them.
Okay, here's the last one.
Magnesium Phosphate, periodic table, polyatomic ion list, I find the charges for Magnesium
and Phosphate.
Magnesium is here and it's Mg2+ and Phosphate, we just did this, it's right here, PO4^3-
. Now I want to get the charges to balance and this is one of the tricky ones because
sometimes you have to do a little thinking before you can figure out how this is going
to work.
I'm going to add another Phosphate here to get a total of six minus (-6) and now I can
add two more Magnesiums to get a total of six plus (+6).
So I add another Mg2+ and now I have four plus (+4) and now one more, I have three of
them with a total of six plus (+6).
So I need three Magnesiums and two Phosphates for these charges to balance.
And now I'll write this as Mg3 and PO 4 I need two Phosphates, I got a break out those
parentheses, PO4 multiply that whole thing by two, so I use parentheses and put a two
there.
Okay, a few take home messages.
I know it's a pain but it's really helpful to memorize the list of polyatomic ions so
you don't waste time looking for something like Phosphate on the periodic table or confusing
Phosphate with Phosphide which is P3- and very different than PO4-.
So if you memorize the polyatomic ions, you won't make those mistakes.
The other question that people often ask is they look at a list of the polyatomic ions
and they're like, how am I supposed to know this?
You know if someone just said Nitrate or Nitrite and I knew it was NO2- , is there a way that
I could figure out what the charge would be without having to memorize this?
Or you know, figure out the charge on Sulfur?
Unfortunately, the answer is no.
It's really, really hard to figure out what the charge on a clump of atoms would be if
you haven't just gone through and memorized this.
So as much as I always like to say don't memorize things and I usually like to teach the rules
behind how you can figure it out yourself instead of relying on memory, polyatomic ions
are one of the few things that, yeah I hate doing it but you just kind of have to like
you know grin and bear it and memorize these guys.
Seriously when you're writing lots and lots these formulas it's really going to pay off
in the long run.
So anyway, that's how we write chemical formulas for compounds that have polyatomic ions in
them.