 Okay, I think this should be a relatively short video, but it's important to get this information under your belt before we move forward. I want to talk about describing molecules and describing atoms. So not just molecules, atoms and molecules. And using the symbols for the atoms for describing molecules to other people. So this capital letter X here is supposed to be a generic symbol. So there is no element with the symbol X, so I'm just using that. What I want to tell you is that when you write the symbol for an atom, like we have right here, there are four places where you can put a number next to the symbol. I'm writing four boxes for the four places. You can put a number in the upper left, in the lower left, in the upper right, in the lower right. And those positions, if you put a number in one of those positions, they mean different things. So for example, if you put a number over here, we're going to talk about this in more detail, it more or less means how much the atom weighs. More formally it's called the mass number of the atom. I'm cutting a big corner there, but more or less, if you put a number in the upper left of the symbol of an atom, it's roughly telling you how heavy or how light the atom is. We're not going to talk about this number until later in the course, so you can ignore it for the moment. Atoms can have electrical charges, which we will talk about later this week. And if you want to tell people what the electrical charge of your atom is, you write a number in the upper right. The one that I'm most concerned about now is the one in the lower right. If you put a number here, this number tells you how many of the X atom are stuck to each other, attached to each other. And I'm going to go into a little bit more detail on the next page about what I mean by that. So as an example, I want you to look at this little picture here. This is supposed to be the, let's call it the molecular formula for some molecule. And what it's supposed to tell you is it's supposed to tell you what the different types of atoms there are in this molecule and how many of each one of each type of atom are stuck to each other. So these are all more or less these are pretend names. But I want you to basically, in a moment, I want you to pause the video and tell me what types of atoms there are and then how many of each type you can pause right now. Okay, so the types of atoms that there are here are, there's a Y atom, whatever Y is, we might talk about that later. There's an X atom, there's also a Z atom. So those are all capital letters, which means these are all individual symbols for different types of atoms. And if you want to figure out how many of each one there is, you have to look in the lower right next to each symbol. Now in the lower right next to Z, there's a four. That means there's four Z atoms in this molecule. If you look at the X symbol in the lower right there, there's a two. That means there's two X atoms. And if you look at the symbol for Y, there's no number, no number. If no number is written, the implication is that there's only one of that atom. So there's one Y atom in this molecule. And basically what it means when you smush all of the symbols together and put a number, maybe, in the lower right is that all of these atoms are stuck to each other. So there's one Y atom. There's two X atoms. And there are four Z atoms all stuck to each other in this molecule. This is called a molecular formula. You should understand the basics of how to read it. This does not tell you how the order that the atoms are attached to each other in. All it tells you is that they are stuck to each other. But it doesn't give you enough detail about explaining how they're attached to each other. So keep that in mind. All this tells you is the different types and how many of each different type there exists. So if you don't understand that, here's a slightly different example. This is the molecular formula. This is the molecular formula for water over here. Many of you are probably familiar with this. You might have heard the phrase H2O, meaning water. But if you look at it, there's the symbol O. That is the abbreviation for oxygen. And there's no number in the lower right, no number, which means there's a 1 implied to be there. And if you look at the other symbol, it's a capital letter H, which stands for hydrogen. There's a 2 in the lower right with respect to the capital H. That means there's two hydrogen atoms. So if you see the formula H2O, it means two hydrogen atoms plus one oxygen atom stuck to each other. It doesn't tell you how they're stuck to each other. But all it really does is tell you how many there are and that they are stuck to each other. So keep that in mind. Here's a different practice problem. I want you to look at this. This is a formula of some pretend molecule. It doesn't really exist. I believe all of the symbols are fake. And what I want you to do is I want you to pause the video and basically tell me what types of atoms there are and how many of each type. This is a little tricky. So keep that in mind. You may be stumped. So all right, pause, unpause. Here are the different types. There is usually the way that I ask this to the on-ground students as I say, how many y atoms are there in this molecule? And this is actually a trick question, because there are no y atoms in this molecule. There are y a atoms, however. And the reason why I want you to realize this is just because you see a capital letter y, you have to look to the right and see if there's also a lower case letter. And this is a lower case letter a. So what this is saying is there are no y atoms, because you can't have another atom that starts with a lower case. So the y and the lower case a are stuck to each other. They are the symbol for some atom that has a symbol y a. And if we're counting the y a atoms in this molecule, there are three of them, three atoms. Then I might say how many x atoms are there? Here's our x. There are two of them, so two x atoms. How many z atoms are there? Well, this is a capital Z. There's no lower case letter immediately to the right of the z. So it's just a z atom, whatever z is. There's no number immediately to the right. So there's one. This should say two atoms for x, one atom for z. And then finally, there are r atoms in our molecule. And how many r atoms? There are four of them. So the reason why this slide exists is I want you to realize that if you see a lower case letter in a formula, that means that that lower case letter is part of a larger symbol. So there isn't a y atom in this molecule. There is a y a atom in this molecule, and there happen to be three of them. So in my pretend molecule, there's three y a atoms stuck to two x atoms, also stuck to one z atom, also stuck to four r atoms. So keep that in mind. You should be able to read molecular formulas like this. Oh.