What's a polyatomic ion?

So

what

is a

polyatomic

ion? A

polyatomic

ion is a group of atoms that has a charge. This is

what

I mean. You probably already know that individual atoms can have charges and can form ions. So we can get things like Na+, Mg2+ and O2-. Polyatomic ions or what happens when more than one atom comes together and forms a large 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 they form a group and that entire group has a charge of one minus (-1). "Poly" means "many," so here, polyatomic ions, we are talking about ions that are made up of many atoms, more than one atom.

Let me show you some examples of how this actually works. Here are two of my favorite polyatomic ions, ammonium and carbonate. The ammonium here is composed of a nitrogen connected to four hydrogens and the carbonate is a carbon connected to three oxygen atoms. Now, the atoms that are in each of these polyatomic ions have protons and electrons. So what we can do for both ammonium and carbonate is count the number of protons in each of these atoms. If I do this with ammonium, I get eleven protons, and so I count the total number of electrons in this whole group, I get ten.

So these two numbers do not balance. I have more protons than electrons, which means I have more positive charge, I have a net charge of one more (+1) because I have one more proton. So my net charge here is plus one (+1) and I can write the formula for this polyatomic ion, it's NH4 because I have nitrogen and four hydrogens with a plus one (+1). I usually also take the structure of this ammonium and put it all in parentheses and put a plus sign outside to indicate that this whole group of atoms together has a positive charge. I can also call this a cation, a polyatomic cation because it is what we can call ions that have a positive charge.

Now Carbonate, we can do the same thing with it. We can count the total number of protons in the atoms that make it up and we can also count the total number of electrons in this entire group. We get 30 protons in total and 32 electrons in total. So we have more electrons and protons, which means we have more negative charge, it will have a net negative charge, and since there are two more electrons, it will 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 parentheses and put two minus (2-) here to show that this Everything has a net charge of two minus (2-).

We call this type a cation because it has a positive charge, so we can call the carbonate a polyatomic anion for the negative ion. As I said before, there are a lot of polyatomic ions that have a small amount on this list here. A lot of times people ask me and they say, "Okay, if I knew that nitrate was NO3 but I didn't know the charge, would it be possible for me to calculate the charge on my own?" Unfortunately no, it is very difficult to calculate how many electrons a polyatomic ion has, so it is necessary to calculate the charge.

Polyatomic ions are really important when writing chemical formulas of compounds, and since you need to use them a lot, they are one of the things in chemistry that you really need to memorize. So I made a video about it! It's called learning polyatomic ions and if you need to learn them, which you probably do, you might want to watch that video. I'm talking about a series of strategies that you can use to learn them better. Also, if you are interested in the actual bonding that occurs in the atoms that form a polyatomic ion, it is a little more advanced, but you may want to watch the video called "How are polyatomic ions formed?" where I talk about covalent and ionic bonding and the exchange of electrons that takes place as these polyatomic ions form.

Anyway, that's what polyatomic ions are! They are groups of atoms that come together and have more electrons than protons or more protons than electrons and therefore, this entire group of atoms together has a charge.