Writing Formulas with Polyatomic Ions


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.

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