 Okay, this section is going to talk about molecular formulas. I know that we've talked about molecular formulas before, so some of this, even some of this is a review, but there will be a little bit of new information. So again, let me just try to put you in the right frame of mind. Sometimes you need to describe a molecule to other people, maybe not you, but pretend you're in a situation where you actually do need to. Suppose you needed to describe water to somebody else. You could draw this beautiful picture and you could tell them, look, I've got an oxygen atom. It's in the middle. It's bigger than the hydrogen atoms that it's attached to. There are these covalent bonds, which I'm showing by solid lines. Not only all of that, but the molecule is bent. You can see that the hydrogen to oxygen to hydrogen is not a straight line. It's actually bent. This is a very descriptive, detailed, sort of description for other people, for a water molecule. However, almost always, this is overkill. Not every single time, but a lot of the time this is overkill. You want a simpler way of describing molecules to other people, and this is how you would do it. You say, you write H with a subscript of 2 and an O. That is telling people, look, there's two hydrogens and an oxygen stuck to each other. I'm not telling you exactly how they're stuck to each other. I'm not telling you how big the atoms are or whether the molecule is bent or anything like that. I'm just telling you who's there and how many are there. This is usually good enough. This is the type of thing that you need to get under your belt as far as reading formulas is concerned, or reading molecular descriptions is concerned. This is called a molecular formula. Again, you've seen this earlier. So you can ignore most of the stuff above. I have already said this in earlier units. This is the formula for water. Could I have written H2O1? Sure, you absolutely could have, but this is considered a little bit amateurish. No one really writes the one here because if you did that for this molecule, you would be writing ones all day long if you had to write molecular formulas. So people, just because they're lazy and they want an easier way of writing formulas out, they just omit the one. If there's no number there, the one is implied. But you could put it there if you wanted to. This is the formula for carbon dioxide. Again, I could have written C102, but nobody does that. So what I'm emphasizing here is you have to get used to looking at these things and realizing that there's a one implied to be there because it almost never will be written if there's only one of that atom. This is the formula for carbon dioxide. That's the stuff that we breathe out as waste. And so now I'm just going to ask a series of questions. You can pause the video and answer the questions, sort of play along at home if you want. So I'm going to ask questions. How many carbons? How many hydrogens? And how many oxygens are in this molecule? You can pause, unpause. There are six carbons because there's a subscript of six there. 12 hydrogens, six oxygens. Next question, how many sulfurs are in this molecule? It's a trick question. There are no sulfurs, no sulfurs. So if the symbol is not there, it's not in the molecule. Next one, pretty straightforward. How many nitrogen atoms are in this molecule? Well, there are two because there's a subscript of two next to the symbol for nitrogen. Again, you can pause. How many carbons hydrogens and oxygens are in this molecule? And unpausing, there are two carbons. Some of you may have been fooled by this, but they're spread out. Here's one carbon, here's another. There are four hydrogens. Three here, one here. And there are two oxygens. And for those of you who didn't notice this, and you're not alone, a lot of people don't notice this, you're probably furious because, well, you're probably not that furious, this is just chemistry. But you're probably wondering, who on earth would do this? Why would you do this? Why didn't you just write C2H4O2? Isn't this the best way to do it? The short answer is that you'll realize much later in the course why people would bother to spread out the atoms the way that they're spread out here. The sort of slightly more detailed reasoning is that sometimes spreading the atoms out like this tells a chemist who has been properly trained a little bit more about how the atoms are attached in the molecule. So sometimes they're just going to do this to you, and you have to be careful to look across the entire molecule. If I say how many carbons there are, don't just stop here. Make sure you keep looking to see if there are any other capital Cs all by themselves alone in your molecule. And there are, in this case, there's 1 and 2. And the same with the hydrogens and the same with the oxygens. So again, you could write this as the formula for this molecule. But this, actually, to a trained chemist does not give as much information as this does. So sometimes you're going to see it this way. This is the molecular formula for something called acetic acid. Acetic acid is basically the main ingredient in vinegar. So this is the stuff that makes your salad dressing taste tart. OK, this one sometimes gives people trouble again. You can pause the video after I ask the questions. How many calcium, how many nitrogens, and how many oxygens are there in this molecule? You can pause and unpause. There's one calcium. There are two nitrogens, and there are six oxygen atoms. The reason is because this 2 here, this subscript of 2, outside of the parentheses, means that there are two sets of everything inside of the parentheses. So most of the time people will draw the formula for this molecule or they'll write the formula for this molecule this way. But you could have written it CA NO3 NO3. And how many sets of NO3s are there? There are two of them. So people get lazy. They don't want to write the NO3 twice. They write CA NO3, parentheses around the NO3, and we have two of them. But if you write it out this way, the way that nobody actually writes it out, because it's tedious, you can see that there's two nitrogens and there's six oxygens. So another way of doing this is you should realize that there's a one implied to be to the right of the symbol for nitrogen. And then you can do 2 times 1 is 2, so two nitrogens. And you can do 2 times 3 is 6, or six oxygens. So that's another way of counting these sort of more complicated formulas. And I mentioned this before, I think in an earlier video. I said, couldn't you just write the formula as CA N206? And again, you can. That's perfectly valid. However, it's usually written this way because chemists who are in the know, chemists who have been trained, will get more information out of this formula. They will realize that the NO3s travel in little packs and I have two of those packs in my molecule. Over here, I don't know who's traveling as a pack and who isn't. So this is the more common way that you'll see a lot of formulas written. You cannot, however, and this is something that students like to do, you can't jam the number 2 in between the calcium and the parentheses. If you do that, what you are telling people is that this calcium and this stuff over here are no longer attached to each other. So because this is not a subscript anymore, it's separating the calcium and the NO3. So can't do that even though there will be a desire for some of you to do it, not allowed. That means if you start jamming numbers in between the symbols and it's not a subscript, and this one is not a subscript, then what you're saying is the symbols or the atoms are no longer attached to each other. So you can't write a formula that way. So a little bit of practice. How many hydrogens and how many oxygens here? Should be two hydrogens and one oxygen. Here, you can pause the video. How many carbons, hydrogens, oxygens, and sulfurs? Well, 17 carbons, 14 hydrogens, four oxygens, one sulfur. What does this mean? Two in front of the entire H2O. Now, this is different than what I warned you about on the previous slide. I didn't jam this number in between any of the symbols. I put it at the very front of the formula for the molecule. A lot of times, beginning students say, well, what it means is that I have two water molecules. Here's water molecule number one. Here's water molecule number two. And if I wanted to tell people that I had two of them, I would put a two in front of the formula. And if I didn't put any number in front of the formula, well, it would mean that a one was implied and that I would have one water molecule. And so this is, without a number, is code for saying one water molecule with numbers in front of the formula. It tells you how many you have, how many of the water molecules you have. What I'm going to tell you is that's kind of sort of right, but not quite. What this actually means is, well, let me back up. What this means, if there's no number here, this could mean one water molecule. The number one is definitely implied there. However, it doesn't necessarily mean one water molecule. It could mean one dozen water molecules. It could mean one billion water molecules. It could mean a trillion water molecules. And you have to know what question you're trying to answer as to what this number one actually means. Again, it could mean one, one dozen, one billion, one trillion depends on the situation. You can't necessarily know without other information. And if that's true, this two here, all it really means is twice as much as whatever I'm talking about in this situation here. So if this one meant one water molecule, then yeah. This two means two water molecules. But if this one means one dozen water molecules, then this two means two dozen. If this means one billion water molecules, this means two billion water molecules. So keep that in mind. There's a subtle difference. This does not always mean number of molecules. It just means twice as much as whatever I'm talking about up here. Because this only means twice as much as whatever I'm talking about in the top situation, then I am allowed to write fractions and decimal numbers in front of a molecule. I can't have 3.58 molecules because I can't sort of saw one of these water molecules in half. It's not really stable that way. However, I can have 3.58 billion water molecules. And then if so, it allows me to put fraction and decimal numbers in front of the molecule. And if that was true, if this was 3.58 billion, this would mean one billion. So keep that in mind. And again, another somewhat difficult one, how many magnesiums, chlorines, oxygens in this molecule? So you can pause and unpause. There's one magnesium because there's no number immediately to the right of the symbol for magnesium. There are two chlorines because there's a one implied here, but there's a two outside of the parentheses. So 2 times 1 is two chlorines. And there are eight oxygens because there are four inside of the parentheses. And you have two sets of whatever is inside of the parentheses. So 2 times 4 is 8. So one magnesium, two chlorines, and eight oxygens. So again, I just want to talk about where you can put numbers surrounding a symbol for an element. So this is some generic symbol. It can be any atom you want, any symbol of any atom you want. There are four places where you can put a number to tell people about your atom. This is how many are stuck to each other if you put a number there. This is the electrical charge. This is called the mass number, which is roughly equal to the weight of the atom. And this we haven't talked about. And we'll talk about it much later in the course, but right now not really necessary. So you just learned the fundamentals of how to read a molecular formula. It should become second nature to you. You should sort of be doing this enough in this class that you'll be able to look at a formula and figure out how many of each type of atom you have in a particular molecule by looking at the formula. So if I give you the formula of a compound, you should be able to tell me what the different types of atoms are and how many of each type there are. And that is it for reading molecular formulas.