So what's a polyatomic ion? A polyatomic ion is a group of atoms that has a charge. Here's what I mean. You probably already know that individual atoms can have a charge, they can form ions. So we can get things like Na+, Mg2+, and O2-. Polyatomic ions or what happens when more than one atoms comes together and they form a big group of atoms and that group of atoms itself has a charge. So polyatomic ions are things like NO3-, so we have one Nitrogen and three Oxygens all connected together form a group and that whole group has a charge of one minus (-1). "Poly" means "many," so here, polyatomic ions, we're talking about ions that are made up of many atoms, more than one atom. Let me show you a few examples of how this actually works. So here are two of my favorite polyatomic ions, Ammonium and Carbonate. Ammonium here is made up of one Nitrogen connected to four Hydrogens and Carbonate is one Carbon connected to three Oxygen atoms. Now the atoms that are in each of these polyatomic ions, they have protons and electrons in them. So what we can do for both Ammonium and Carbonate is we can count up the number of protons that are in each of these atoms. If I do this for Ammonium I get eleven protons and thereby count up the total number of electrons that are in this whole group, I get ten. So these two numbers don't balance out. I have more protons than electrons which means I have more positive charge, I have I net charge of one plus (+1) because I have one more proton. So my net charge here is one plus (+1) and I can write the formula for this polyatomic ion, it's NH4 because I have a Nitrogen and four Hydrogens with a plus one (+1). I usually will also take the structure of this Ammonium and put the whole thing in brackets and put a plus outside to indicate that this whole group of atoms together has a positive charge. I can also call this thing a cation, a polyatomic cation because it is what we can ions that have a positive charge. Now Carbonate, we can do the same thing with it. We can count out the total number of protons that are in the atoms that make it up and we can also count up the total number of electrons that are in this whole group. We get 30 protons total and 32 electrons total. So we have more electrons and protons which means we have more negative charge, it is going to have a net negative charge, and since there are two more electrons it's going to have a net charge of minus two (-2). I can write the formula for this polyatomic ion as CO32-, one Carbon and 3 Oxygens totaling the charge of two minus (2- ) and then I write my structure in brackets and put a two minus (2-) here to show that this whole thing has a net charge of two minus (2-). We call this guy a cation because it has a positive charge so we can call Carbonate a polyatomic anion for negative ion. As I said before, there are a whole bunch of polyatomic ions that have a small number of them on this list right here. A lot of times people ask me, they say, okay if I knew Nitrate was NO3 but I didn't know the charge, would it be possible for me to figure out the charge just on my own? Unfortunately no, it's very difficult to figure out how many electrons a polyatomic ion has and so you need that to figure out the charge. Polyatomic ions are really important for when you're writing chemical formulas of compounds and since you need to use them a lot, they're one of the things in chemistry that you really just have to memorize. So I made a video on that! It's called learning polyatomic ions and if you need to learn these, which you probably do, you might want to check that video out. I talk about a number of strategies that you can use to better learn these. Also, if you're interested in the actual bonding that's taking place in the atoms that make up a polyatomic ion, it's a little bit more advanced but you might want to check out the video called "How are polyatomic ions made?" where I talk about the covalent and ionic bonding and sharing of electrons that takes place as these polyatomic ions form. So anyway, that's what polyatomic ions are! They are groups of atoms that come together and have either more electrons than protons or more protons than electrons and because of that, this whole group of atoms together has a charge.