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Writing Ionic Formulas: Introduction

Feb 22, 2020
Let's talk about how to write chemical

formulas

for

ionic

compounds. So what this means is that we're going to start with the chemical name like magnesium chloride and we're going to learn the steps we need to take to take this chemical name and use it to write a chemical formula like MgCl2, okay? Let's start with the first example, lithium oxide. So when I solve these types of problems, the first thing I want to do is find both elements on the periodic table. I'm using this kind of weird version of the periodic table that I just wrote.
writing ionic formulas introduction
I left out a lot of the elements because they're not important to what we're doing here and I thought they were a little distracting, but don't be confused. This is actually no different from the periodic table you probably have in your book, it's just missing a lot of elements. Anyway, Lithium. . . where do I 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 and I'll talk a little more about that later. Anyway, Rust is just another word for Oxygen and Oxygen is over here.
writing ionic formulas introduction

More Interesting Facts About,

writing ionic formulas introduction...

They are on opposite sides of the periodic table and check this out too. There is a large, thick ladder that separates the lithium from the oxygen. What does a ladder do? If you remember, the ladder 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 to each other and that means we are dealing with an

ionic

compound. Ionic compounds are always metals and non-metals. So we have metals and non-metals forming an ionic compound, that means the atoms in that compound have a charge, so I want to know what the charge is on those atoms and I can do that by looking at where they live, what column they live in on the periodic table.
writing ionic formulas introduction
This is what I mean. So the lithium lives inside this column. Everything in this column has a charge plus one, so I'm going to write this right here. . . Li+1. . . everything in this column has a charge of two plus (+2), three plus (+3), you may want to write this down on your own periodic table that you have for reference. Everything here has a negative charge of 3 (-3) and the oxygen found in this column has a negative charge of two (-2), so I'm going to write it right here. So Li+1, O2-. Okay, now I'm wondering: do the positive charge (+) and the negative charge (-) balance when I have an atom of both?
writing ionic formulas introduction
The answer is no because I have one positive charge but I have two negative charges so the charges do not balance out. But I want to figure out how to balance them. I need to balance them so I can write the chemical formula, so what I can do is add more lithium atoms, I can add more oxygen atoms, or I can add both until the charges balance. I have two minus (2-) here and just one plus (+1) here, so what I'm going to do is add another lithium atom, Li+, so now I have two positives to balance my two negatives and now they balance out .
So to balance the charges, I have to have two Lithium and one Oxide or one Oxygen. Now when I write down the chemical formula, I will literally say how many of each of these atoms I need for the charges to balance. We've said it's two lithium atoms, so I'm going to write "Li" and then the next two indicating that I have two lithium atoms and then an "O" to show that I have an oxygen and I'm not going to write anything after that. . If you have a card with nothing after it, it means you only have one of them.
So lithium oxide, two lithiums and one oxygen, is how we get the charges on this compound to balance. Here's the next one, Potassium Nitride. Like before, I'll pull out the periodic table and find these two guys on it. Potassium is K, it's right over here and nitride is what we call nitrogen when it has a charge, so nitrogen is over here. Check it out! Metal and non-metal, separated by this big thick ladder, that's why we know we're dealing with an ionic compound and whenever we think of an ionic compound. . . metals and non-metals. . . We want to think about charges, what charges do atoms have?
Potassium lives in this column here, the same column as lithium, so it has a charge one plus (+1), so I'm going to write that down here, K+. And nitrogen lives in this column where everything has a charge except three (-3), so here we have N3-. When I have a K+ atom and an N3- atom, do the charges balance here? Do the plus and minus balance each other? It's not like that, I have one more and three less, that doesn't add up well. But you could add more atoms of any type to balance the charges. So since I don't have as many pros as cons, I'll add a lot more potassium.
Here I add another one, now I have 2+, and now I'm going to add one more and I have 3+. . . The potassium 3+ on this side balances the nitride 3 on this side. Now, when I write my chemical formula, I literally mean how many of each type of atoms I need for the charges to balance. I'm going to need three Potassium so I'm going to write "K3" and I only need one Nitrogen or one Nitride so I'm going to write "N" with nothing after it and if I don't write anything after it, it means one. So three potassium, one nitrogen, the correct formula for this is K3N.
Well, here's another one. Sodium Chloride, which is what common table salt is called. Once again, I will find them both on the periodic table. The sodium here is in the one plus (+1) column and the chlorine or chloride is in this column here. We have metals and non-metals, so we are dealing with ionic compounds, which means the atoms have charges. Now I want to know what the charges are. Like I said before, Na is at one plus (+1), so a sodium atom has a charge of one plus (+1) here and then chloride is what we call chlorine when it has a charge and chlorine lives in this column, so it has a charge of one minus (-1), so Cl-.
Okay, so I have one sodium atom plus one (Na+) and I have one chloride atom minus one (Cl-). When I have an atom of both, do the positive and negative balance each other? That's right, I don't need to add more of any 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. I'm just going to say NaCl. I don't put any number after it because if I don't put a number it means I only have one.
So I have one sodium and one chloride, okay? You may already be understanding this. I want to try to convince you to look at this last example because it is complicated and 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 column of three plus (+3), an atom or ion of Aluminum, an ion is what we call an atom that has a charge. An aluminum ion has a charge of three plus (+3) and an oxygen or oxide atom here has a charge of two minus (-2), Al2+ O2-.
Now I need to calculate how many of each of these atoms I need. so that the charges balance out and this example is a little more complicated than the previous ones. How are we going to be able to add atoms to obtain the same amount of positive and negative charge? Let me give you a hint. We'll start by adding another aluminum atom here or aluminum ion to be precise. So now I have three more here and three more here, each aluminum atom has a charge of three more (+3), so I have a total of six more (+6). Now I can balance the six plus by adding more oxygen.
I add one more Oxygen and I have a total of four less and if I add one more I have a total of six less. This is how we balance the charges in aluminum oxide with two aluminum ions with three plus (+3) each, a total of six plus (+6) and three oxygen ions with a total of two minus (-2) each one, which gives us minus six (-6). Now I'm going to write down the chemical formula to show how many of each I need to have. They will be Al2 and O3. I have two Aluminums and three Oxygens. Okay, now let me tell you what I'm going to do.
If you're still a little shaky on this, I'll do some more practice problems in a minute. If you feel really good about this, you might want to turn off the video and go watch videos I have on how to write

formulas

for compounds with transition metals and compounds with polyatomic ions. But before we do some more practice problems, I want to talk about two common mistakes that a lot of people make when

writing

these formulas, so you might want to stick around and look at them. These are the two mistakes that people make. Suppose you are asked to write down the chemical formula for lithium oxide.
As you do so, you find that you want to have two lithiums with one more each to balance the oxygen charge, okay? Here's a mistake people make. They say, okay, I have two lithium and one oxygen, so I'll write LiO1. They say yes, yes, yes, there is only one oxygen, so it doesn't hurt to add the O1 here, right? It can't hurt. Hurts! It's just not right to do it, you never want to put a one after an element. So Li2O1 is wrong. Instead you want to write Li2O without numbers after this and this is the correct way to do it.
Here's another mistake people often make. They don't remember to get rid of the charges when they write the chemical formula. So they end up

writing

something like Li+2O2- where the charges that were on the atoms here remain when they write the chemical formula. Unfortunately, this is also incorrect. When you write the chemical formula you want to get rid of all charges, so you simply write Li2O again. Make sure there are no charges in the chemical formula when you finally correct it, okay?

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