So what's a polyatomic ion? Well, 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 are what happens when more than one atom comes together and they form a big group of atoms, and then that group of atoms itself has a charge. So polyatomic ions are things like NO3, 1-, where we have one nitrogen and three oxygens all connected together.
to form a group and that whole group has a charge of one minus. 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 two. 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 11 protons. And then if I count up the total number of electrons that are in this whole group, I get 10. So these two numbers don't balance out. I have more electron...
I have more protons and electrons which means that I have more positive charge. I have a net charge of 1 plus because I have one more proton. So my net charge here is 1 plus and I can write the formula for this polyatomic ion as NH4 because I have a nitrogen and four hydrogens with a plus 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 a cation is what we call ions that have a positive charge. Now, carbonate, we can do the same thing with it.
We can count up 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 than protons, which means we have more negative charge.
This 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 2. I can write the formula for this polyatomic ion as CO3, 2 minus, 1 carbon, 3 oxygens, total net charge of 2 minus. And then I write my structure in brackets and put a 2-here to show that this whole thing has a net charge of 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.
I have a small number of them on this list right here. A lot of times people ask me, they say, okay, if I knew that say 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? And 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 use them a lot. They're one of the things in chemistry that you really just have to memorize.
So I've made a video on that. It's called Learning Polyatomic Ions. 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? We're talking 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.