 All right everybody, this section deals with matter and energy. So we're getting closer to what you would probably consider to be real chemistry. So way back earlier in one of the videos I said that chemistry studies materials or matter, material objects. This is a definition for matter that gets used a lot. Anything that has mass, in other words, anything that weighs something and takes up a certain amount of space. It's a pretty informal definition of what matter is, but it's good enough for us. I have an even more informal definition and that is matter or material objects is stuff. Things that you can touch and feel. So chemistry is interested in learning as much as it can about things in the physical world. Things that you can touch and feel. This is probably not news to anybody, but all material objects are made of atoms. Atoms are small, so again probably not earth-shattering news to anyone here. There are many different types of atoms. They come in different types. Some examples, there are hydrogen atoms, oxygen atoms, carbon, copper atoms. We will talk about all of those throughout the course, but you just should be aware that atoms come in different types. An example, this penny is made almost exclusively of copper atoms. There are probably some other non-copper atoms in there, but pretty much it's only made of one type of atom. Let's see. There are currently about 118 different types of atoms. I put 118 in quotes because the ones that are shown in gray here in particular are, they don't last very long and there's occasionally disputes as to whether anyone has actually been able to make them. But let's just say, for the sake of argument, there's about 118 different types of atoms. These are also called elements. Basically each box in this kind of silly looking chart represents a little bit of information for one of the 118 different types of atoms. So there should be about 118 different types of boxes or 118 different boxes in this picture. There are actually a few more than 118. These two over here have little asterisks in them. Basically what they mean, what this one with the first asterisk means is that all of these boxes actually should be jammed in there. And this double asterisk over here means all of these boxes that I'm circling here should be jammed into that area. The problem is if you do that, then it makes this whole periodic table extremely wide and it's hard to see everything. So traditionally people have yanked these two rows out and just said, look, they actually belong in this spot and this spot, but we're not going to put them there because it makes things look ugly. So 118 different types of atoms. Let's see. And again, just to get back to the fact that we are talking about what material objects are made of, there are different types of material objects. The first thing that we're going to talk about are material objects that are called pure substances. The first type of pure substance is if you have a material that is only made of one element. So if you have, as an example again, this is a copper penny. So presumably it is made exclusively of copper atoms. Now there might be a couple of extra different types of atoms thrown in there, but mostly it's copper atoms. So if these circles, these gray circles, are going to be my cartoon representation of an atom, at least for the moment. So if you could zoom in and look at the atoms in this penny, what you would probably see is roughly the same type of atom, pretend these are all copper atoms, just next to each other over and over and over again. So this material, this penny, which is a material, it is a relatively pure substance because it's made of the same type of atom over and over and over again and pretty much nothing else is contaminating it. And because of that it's called a pure material, sometimes it's called a pure element. That's one type of matter. There's another type of pure material called a compound. A compound is a material that is made of two or more types of atoms. There have to be at least two different types, but they're always attached to each other in the same way. Over here, this is just one type of atom. So it's not a compound because it's just made of copper, copper, copper, copper, copper atoms, all sort of laid end to end and on top of each other. Compound has to have at least two different types. This is a cartoon representation of a compound. It doesn't really matter what the atoms are, but there's a big gray one and a little purple one, big gray one, little purple one. So it's made of two different types of atoms, a gray one and a purple one, or a big and a small one if you want. And they're always attached to each other in the same way. It always goes big, small, big, small. So if you have that, then you have something, a pure material called a compound. This is, let's see, one, two, three, four things. These are also a pure compound. Why? Because they all, one, two, three, four of them have two different types of atoms and the atoms are always attached to each other in the same way. There's a blue one and there's two red ones and they're always attached to red ones to a blue one. So this is a pure compound. There's nothing else there except always two reds attached to a blue in the same way. This over here, you can ask yourself whether it is, is it a pure compound? And the answer is no for a couple of reasons. One, this thing right here is not a compound at all. This is just a cartoon version of a lonely old blue atom. Then even if this one was gone, this is a compound. This is the same compound. But then these two things that I'm circling last, they're also compounds but they're not the same as the first two things that I circled. Because of that, this thing is called a mixture. So a mixture is when you have elements or compounds but they're not all identical to each other and they're mixed up together and that's why it's called a mixture. So not a compound, it's called a mixture. There's a different type of pure material that is similar and maybe related to a compound. It's called a molecule. This is two or more atoms stuck to each other in the same way. So as an example, this I showed you on the previous slide. This is a cartoon version of what I said was four identical compounds. This is a compound number two, number three, number four. They are identical compounds. They are also four identical molecules. The difference between a molecule and a compound is the compound has to have at least two different types of atoms, types of atoms. And so we've got a red one and a blue one and two red ones always attached to each other. So these are cartoon versions of compounds. The molecule, you have to have two atoms but they can be identical. They don't have to be different. That's the difference between a molecule and a compound. The compound has to have at least two different types. Molecule can have two atoms but they can be identical. So this is sort of both four identical or pure compounds and it's also four identical molecules. Hopefully that makes sense. Let's see. This is, again, this is an atom and these are also molecules but they're not identical. So this is a mixture again. Let me erase a lot of this junk just in case you can't see that. So this is atom. These three things that I'm circling here, they are both molecules and compounds. You can call them either thing and you're going to be correct. So again, what's the difference between a compound and a molecule? Compound has to have two or more types. Molecule they can be identical. You need two but they can be identical. This thing, this cartoon that I've been drawing on the past couple of slides, it's a molecule and it's also a compound because it's got two or more atoms so it's a molecule and it's got two or more types of atoms so it's also a compound. This thing here, this cartoon thing, is a molecule because it's got two or more atoms stuck to each other but they're not different from each other. They're two little red ones and they're supposed to be identical. So this thing is not a compound. So there's a subtle difference between compound and molecule. Sometimes people loosely use them, the words interchangeably. It's not going to really bother me if you use the words interchangeably but you should know that there is a subtle difference. Let's see. So you should know the definition of molecule, compound, what a mixture is, what a pure material is, which I didn't write down here. And you should know that all materials are made of atoms.