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Writing Formulas with Polyatomic Ions

Feb 22, 2020
Let's learn to write chemical

formulas

for compounds containing

polyatomic

ions

. So what is a

polyatomic

ion? Well, it's what happens when a group of atoms come together and connect and form a big group. That group itself has a burden. So here's a list of some of my polyatomic

ions

and it has things like nitrate, which is NO3, and what that means is that one nitrogen atom forms a group with three oxygen atoms and all this, these four atoms together, They have a charge of one less (-1). Or we have things here like ammonium, which is NH4+, which means you have a nitrogen connected to four hydrogens in a big group and that group of atoms together has a charge of one plus (+1), okay?
writing formulas with polyatomic ions
Here is the name of a compound, calcium nitrate. Let's review the steps we must follow to write the chemical formula of this compound. The two things you will need for this, the first is a periodic table. I'm using this kind of weird version where I left out a lot of elements because I think they're distracting, but I kept the ones that are important to what we're doing. And then you'll need a list of polyatomic ions, okay? This is a short list of my favorite polyatomic ions and I bet your teacher or textbook has a much longer list of polyatomic ions with a bunch of extras they want you to learn about, okay?
writing formulas with polyatomic ions

More Interesting Facts About,

writing formulas with polyatomic ions...

If that's the case, it's not a big deal because if you learn to write compounds with these polyatomic ions, you'll also be able to do it with whatever additional ions they want you to use, okay? Anyway, ions! Since we're talking about polyatomic ions here, the compounds were going to write

formulas

for all ionic compounds, which means we're going to have to figure out what the charges are on both things so we can write the formula for okay? So let's start with calcium. What is the charge on calcium? Well, that's pretty easy. Calcium is here on the periodic table and in this column.
writing formulas with polyatomic ions
Everything in this column has a charge of two plus (+2), so I'll write this up here, Ca2+. Okay, now nitrate. Nitrate is a polyatomic ion and that is why it appears in this table. The nitrate here is NO3-. This entire group of atoms has a charge of one less. So now I'm wondering, when I have a calcium atom and a group of this nitrate, do the positive and negative charges balance out? They don't do it because I have two more (+2) in Calcium but I only have one less (-1) in Nitrate. So at this stage I may add more than one of these or both until the charges balance out.
writing formulas with polyatomic ions
Since I have two more (+2) here and each of these Nitrates gives me a minus (-1), I'm going to add another Nitrate (NO3-) so that I now have two minuses total, which will balance out my two plus (+ 2) Calcium, okay? Now I have the charges balanced and now I want to write this into a chemical formula that shows how many of each I have to have to balance the charges. So I need a calcium, so I'm going to write it as Ca without anything after it and that means I have one of them if you don't write anything after it and then I want to say I have two nitrates, so I NUMBER 3.
There's my nitrate. I mean I have two of all this, right? I want to multiply all this, NO3, by two. So what I'm going to do is use parentheses, surround the polyatomic ion with parentheses, and then write here how many I want in all of this. It will be Ca(NO3)2. It means one atom of calcium and two whole lumps of nitrate. Let's do a couple more. Ammonium nitride, we have to bring our two tools here, the periodic table and the list of polyatomic ions. Okay, so ammonium, you might recognize from this list is a polyatomic ion and it's NH4+, a whole group of atoms has that positive charge.
Now nitride is here on the periodic table, it's what we call 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 people often ask: how do you know which polyatomic ion is? How not to get confused with the things that are on the periodic table? I'm sorry to say this, but the only good way to do this is to memorize them. People always say that it's a good idea to memorize polyatomic ions so that when they come out like ammonium, you immediately know where to find them and you don't have to waste time trying to find ammonium on the periodic table and it doesn't work. be on the periodic table because it is a polyatomic ion, okay?
Anyway, we have ammonium (NH4+) and nitride, which is N3-. Now we have to balance the charges if they aren't already. Here we have one plus (+1) and three minus (-3). Since each of these has a charge of one plus (+1), I'm going to add two more for 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 surround them in these parentheses and then write the number. This is how I show that I want three of the Ammoniums and I only want one Nitride because it has a charge of three minus (-3) so I can write it like this without anything after it and that means I have one of them.
Well, sodium carbonate. Back to the periodic table, back to the list. Well, sodium here is in my column one plus (+1), so I'll write Na+ and carbonate is a polyatomic ion, so don't waste your time trying to find carbonate on the periodic table. Just memorize this list and you'll know right away that it's polyatomic. Carbonate is CO3 and this whole chunk of atoms together has a charge of two minus (-2). Now I want to balance the charges. One more (+1) versus two less (-2) so I add another of the atoms that has two more (+2) and now I am going to write the chemical formula as Na2, to show that I have two of them, CO3.
Sometimes people think that the polyatomic ion should be in parentheses. No no no! You only do that if you multiply it by a number like two, three or four, but if you only 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 you only have one of these pieces of carbonate, okay? We will do two more. Ammonium Phosphate, back with the periodic table and polyatomic ions. Both turn out to be polyatomic ions and again you will recognize it immediately after you have memorized it.
Ammonium is NH4+ and phosphate is PO4^3-. I want to balance the loads here. I'm going to add two more ammoniums for a total of three more charges (+3) to balance my three minus charges (-3). So I have ammonium (NH4), I want three of those, it's a polyatomic ion, so it's listed in parentheses, then there's a three after it to say I want three of them and then I have PO 4 here. . . Put it right after. Again, since I only want one of the phosphates, just one of the PO4s, there are no parentheses and I just write PO4 and that's all I need to say that I have one of them.
Well, here's the last one. Magnesium phosphate, periodic table, list of polyatomic ions, find the charges of 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 balance the loads and this is one of the most complicated because sometimes you have to think a little before you can figure out how it's going to work. I'm going to add another phosphate here to get a total of six less (-6) and now I can add two more magnesium to get a total of six more (+6). Then I add another Mg2+ and now I have four more (+4) and now one more, I have three of them for a total of six more (+6).
So I need three magnesium and two phosphates for these charges to balance. And now I'll write this as Mg3 and PO 4. I need two phosphates. I separated those parentheses, PO4 multiplied all that by two, so I use parentheses and put a two in there. Ok, some take home messages. I know it's annoying, but it's very useful to memorize the list of polyatomic ions so you don't waste time looking up something like phosphate in the periodic table or confusing phosphate with phosphide, which is P3- and very different from PO4-. So if you memorize polyatomic ions, you won't make those mistakes.
The other question people often ask is that they look at a list of polyatomic ions and wonder: how am I supposed to know this? Do you know that if someone said Nitrate or Nitrite and I knew it was NO2-, is there a way to know what the charge would be without having to memorize it? Or you know, discover the charge on Sulfur? Unfortunately, the answer is no. It is very, very difficult to determine what the charge of a group of atoms would be if you have not memorized and reviewed it. As much as I always like to tell you not to 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, yes, I hate doing. but you have to like it, smile, bear it and memorize these guys.
Seriously, when you write a lot of formulas, it will really pay off in the long run. Anyway, this is how we write chemical formulas for compounds containing polyatomic ions.

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