Let's talk about how to write chemical formulas for ionic compounds. So what this means is we're going to start with a chemical name like magnesium chloride and learn the steps that we have to go through to take this chemical name and use it to write a chemical formula like MgCl2. Let's start with our first example, lithium oxide.
So when I'm doing these kind of problems, the first thing that I want to do is find both these elements on the periodic table. I'm using this kind of weird version of the periodic table. that I just wrote out, I left out a lot of the elements because they're not important for what we're doing here and I thought they were kind of distracting. But don't be confused.
This really is no different from the periodic table that you probably have in your book. It's just that it's missing a lot. a lot of the elements. Anyway lithium, you want to find that? It's right here, Li.
And oxide, oxide is just another word for oxygen. It's what we call oxygen when oxygen has a charge. I'm going to talk a little bit more about that later.
Anyway oxide is just another word for oxygen and oxygen is over here. They're on opposite sides of the periodic table. Check this out too. There's this big thick staircase that separates lithium from oxygen. from oxygen.
What's a staircase doing? Well, if you remember, the staircase separates the metals on this side of the periodic table from the non-metals on this side of the periodic table. So lithium is a metal and oxygen is a non-metal.
This is important because we have a metal and a non-metal connected together and that means that we're dealing with an ionic compound. Ionic compounds are always metals and non-metals. So when we have metals and non-metals making an ionic compound, that means that the atoms in that compound have a charge. So I want to find out what the charge of those atoms is and I can do that by looking at where they live, what column they live in on the periodic table. Here's what I mean.
So lithium lives in this column. Everything in this column has a plus one charge. So I'm going to write this right here, Li1+. Everything in this column has a 2 plus charge, 3 plus charge. You might want to write this on your own periodic table that you have for your reference.
Everything here has a 3 minus charge and oxygen which lives in this column has a 2 minus charge. So I'm going to write it right here. So Li1 plus O2 minus. Okay, so now I ask myself, does the plus charge and the minus charge balance when I have one atom of both of these?
The answer is no because I have one plus charge but I have two minus charges. So the charges don't balance. don't balance but I want to figure out how to balance them. I need to have them balanced in order to write the chemical formula.
So what I can do is I can add more lithium atoms, I can add more oxygen atoms or I can add both of them until I get the charges to balance out. I have 2 minus here and only 1 plus here. So what I'm going to do is I'm going to add another lithium atom.
Li plus, so that now I have 2 positive to balance out my 2 negative and now they balance. So in order to get the charges balanced, I have to have 2 lithiums and 1 oxide or 1 oxygen. Now when I write the chemical formula, I'm literally going to say how many of each of these atoms I need in order for the charges to balance. You said that's 2 lithium atoms, so I'm going to write Li and then the 2 atoms.
after it that indicates that I have two lithium atoms and then one O to show that I have one oxygen and I'm not going to write anything after that. If you have a letter without anything after it, it means that you just have one. So lithium oxide, two lithiums and one oxygen is how we get the charges to balance for this compound. Here's the next one, potassium nitride.
Just as before, I'm going to take out the periodic table and I'm going to find both of these guys on it. Potassium is K. It's right over here. And nitride is what we call nitrogen when it has a charge on it. And so nitrogen is over here.
Check it out. Metal, nonmetal, separated by this big thick staircase. So we know we're dealing with an ionic compound.
And whatever we think ionic compound, metals and nonmetals, we want to think charges. What charges did the atoms? atoms have.
Potassium lives in this column here, the same column as lithium and so it has a 1 plus charge so I'm going to write that up here, K plus. And nitrogen lives in this column where everything has a minus 3 charge so here we have N. When I have one atom of K plus and one atom of N3 minus, do the charges balance out here? The plus and minuses balance out? It doesn't.
I've got one plus and I've got three minus. That doesn't balance right. But I can add more atoms of either type in order to get the charges to balance.
So since I don't have as much plus as I have minus, I'm going to add a bunch more potassiums. Here I add another, now I have 2 plus, now I'm going to add one more, now I have 3 plus, the 3 plus potassium on this side balances out the 3 minus nitride on this side. Now when I write my chemical formula, I want to literally say, how many atoms of each type do I need for the charges to balance? I'm going to need 3 potassiums, so I'm going to write K3, and I only need 1 nitrogen, or 1 nitride.
So I'm going to write N with nothing after it. If I don't write anything after it, it means 1. So 3 potassiums, 1 nitrogen. The correct formula for this is K3N.
Okay, here's another one. Sodium chloride, which is the name of common table salt. Once again, I'm going to find them both on the periodic table. One right here is in the 1 plus column and chlorine.
chlorine or chloride is in this column right here. We've got metal and non-metal. So we're dealing with ionic compounds which means the atoms have charges.
Now I want to figure out what the charges are. As I said before, Na is in the 1 plus column. So an atom of sodium has a 1 plus charge here.
And then chloride is what we call chlorine when it has a charge. And chlorine lives in this column so it has a 1 minus charge. So Cl.
Okay, so now I have one atom of sodium plus one and I have one atom of chloride minus one. When I have an atom of both of these, if it's positive and negative, does it balance out? It does. I don't need to add more of either one of these atoms.
I'm totally set. So to write the chemical formula for sodium chloride, I want to say again how many of each I need for the charges to balance out. I'm just going to say NaCl.
I don't put any numbers after them. them because if I don't put a number it means that I just have one of them. So I have one sodium and one chloride. You might be getting the hang of this. I want to try to convince you to watch this one last example because it's a tricky one that confuses a lot of people.
Aluminum oxide. Once again periodic table, aluminum here, oxygen here. Metal non-metal, it's ionic which means charges are important.
So aluminum is in the 3 plus column, one atom. or an ion of aluminum. An ion is just what we call an atom that has a charge.
An ion of aluminum has a 3 plus charge and an atom of oxygen, oxide here, has a 2 minus charge. Al3 plus, O2 minus. Now I need to figure out how many of each of these atoms I need for the charges to balance out.
And this example is a little bit trickier than the ones before. How are we going to be able to add atoms to get the same number of positive and negative charge? Let me give you a hint.
We're going to start by adding another aluminum atom in here, another aluminum ion to be precise. So now I have 3 plus here and 3 plus here. Each aluminum atom has a charge of 3 plus. So I have a total of 6 plus.
Now I can balance out the 6 plus by adding... more of the oxygen. I add one more oxygen, I have a total of 4 minus and I add one more, I have a total of 6 minus. So this is how we balance out the charges in aluminum oxide with two aluminum ions with 3 plus each, total of 6 plus and three oxygen ions with a total of 2. minus each which gives us minus 6. Now I'm going to write the chemical formula to show how many of each I need to have. That's going to be Al12 and Oxygen123.
I have two aluminums and three oxygens. So now let me tell you what I'm going to do. If you're still a little bit shaky on this, I'm going to do a few more practice problems in a minute. If you feel real. Good about this, you might want to just turn the video off and go and watch the videos that I have on writing formulas for compounds with transition metals and for compounds with polyatomic ions.
But before I do a few more practice problems, I want to talk about two commonly made mistakes so many people make when they're writing these formulas. So you might just want to stick around to watch that. Here are the two mistakes people make.
Let's assume that you're asked to write the chemical formula for lithium oxide and you do this you find out that you won't have. two lithiums with one plus each to balance out the charge on oxygen. Here is one mistake that people make. They say, okay, I have two lithiums and one oxygen so I'll write Li2O1.
They say, yeah, yeah, yeah, there's only one oxygen so it can't hurt to just add the O1 here, can it? It can't hurt. It does hurt. It's just not right to do. You never want to put a one after an element.
So Li2O1 is wrong. It's not right. Instead you want to write Li2O without any numbers after this and this is the right way to do it. Here's another mistake that people often do. They don't remember to get rid of the charges when they write the chemical formula.
So they end up writing something like Li2O2-where the charges that were on the atoms here stay when they write the chemical formula. This also unfortunately is wrong. When you write the chemical formula, you want to get rid of all of the charges.
So you just write again Li2O. Make sure there aren't any charges in the chemical formula when you finally write it. Okay?