Writing Formulas with Polyatomic Ions
Let's learn how to write chemical
formulas
for compounds that havepolyatomic
ions
in them. So what's apolyatomic
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 mypolyatomic
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 atomstogether,
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 favoritepolyatomic
ions
and I bet that your teacher or textbook has a much longer list ofpolyatomic
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 youlearn how to write compounds with these
polyatomic
ions
, you'll also be able to do it with whatever extraions
they want you to use, okay? So anyway,ions
! Since we're talking aboutpolyatomic
ions
here, the compounds were going to bewriting
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? Theydon'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 ofpolyatomic
ions
. Okay, so Ammonium, you may recognize from this list is apolyatomic
ion and it's NH4+, a whole clump of atoms has that one plus charge. NowNitride 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 thepolyatomic
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 thatit'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 apolyatomic
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 eachof 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 thatI 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 apolyatomic
. 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, justCO3 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 bepolyatomic
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 minuscharge (-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 Iadd 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 thepolyatomic
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 outwhat 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