 Okay, this unit marks the beginning of where the course becomes more difficult, more difficult than it has been. For some of you listening, it still may be very easy, it may be trivial, for others of you it may seem as though it gets extremely difficult. So usually in the on-ground class I have this slide written, it just sort of says pay attention, this is where it starts to get hard, this is one of my favorite quotes in math, you don't understand things, you just get used to them, so if it sounds as though I'm speaking a foreign language that you don't understand, it's possible that if you stick around long enough you might get used to it, so keep that in mind. I'm going to do a very brief review of some of the information that we've covered in the previous units. Starting off with roughly speaking the mass or the weight of an atom, different types of atoms, so that protons and neutrons weigh about the same, they weigh about one Dalton, the electrons don't weigh enough to count. This was a hydrogen atom, because in my cartoons the red circle means proton, and this thing has one proton, and if you look up who's got one proton in the periodic table, it's hydrogen, and this guy weighs about one Dalton, or one AMU. This is a helium atom, because it's got two protons, and only heliums have two protons, and it weighs about two AMU or two Dalton's, and then this was another helium atom, because it still only has two red circles, which means two protons, two protons is helium, but the red circles and the gray circles, they weigh roughly the same, so this particular helium weighs about four AMU or four Dalton's. So you can have heliums or any other type of atom that come in different weights, and those different weights are called isotopes. Again, this is more review, I said remember atomic number is number of protons and an atom, and I showed you three different versions of hydrogen atoms, these are all hydrogens because they only have one red circle, one red circle in my universe means one proton, and proton means hydrogen atom, but this one weighs about one AMU, this one weighs about two AMU, this one weighs about three AMU, and just like I was saying in the previous slide, these are called isotopes of hydrogen, they're just different weights, different possible weights that hydrogen can come in, and hydrogen can come in heavier weights than three AMU as well, I'm just showing you three out of many other possibilities. Then we're continuing with the review, this is just a piece of the periodic table, I want you to be able to read it at this level, I want you to be able to look, find the name of the element in the box, you can guess what this is, this is the atomic number, the atomic number will always be a whole number, this is the symbol or the abbreviation of the atom, and this number here is the average atomic mass, so what this means is none of the hydrogens come weighing 1.0079 AMU, but that's the average, some come weighing about one AMU, every once in a while there's a hydrogen that weighs two AMU, every once in a while there's one that weighs three AMU, etc, etc, but if you take the average weight, it's right around here, if you take the average weight of lithium atoms, they weigh about the average is 6.941 AMU, again this is a little bit of, can be a confusing point to students, they think that you can't have a lithium that weighs seven or eight or nine, or even five AMU, because they look at this number and they think well all lithiums have to weigh this, this is the average, just like if the average on the exam is 81%, that does not mean that everyone scored 81%, some people scored above, some people scored below, but the average is 81%, here some lithiums weigh more than 6.9, some weigh less than 6.9, but if you average their weights they come out to this, that's what this number means. I'll talk briefly about oxygen just as an example, the average oxygen, if you look in the periodic table, has the average atomic mass of oxygen is about 16 AMU, it's something like 15.999, but for the purposes of this unit I'm pretty much going to round the average weights to the nearest whole number, so we're going to say the oxygen weighs about 16 AMU. Hydrogen, the average weight of hydrogen is about 1 AMU, on the previous slide you saw it was about 1.0079, but again for the purposes of this unit, so that we don't get bogged down in the details of these decimal points and that nonsense, we're just going to call, we're going to round to the nearest whole number, so hydrogen has an average weight of about 1. Taking both pieces of information into account, this means that your average oxygen is about 16 times heavier than your average hydrogen, and I can show you that visually, here's your average oxygen atom, or an isotope of oxygen that weighs 16 AMU, here's your typical hydrogen atom that weighs about 1 AMU, and you can see visually this guy over here is about 16 times heavier, so again this is, should all be review, but this is to put you in the right frame of mind for what's coming up. This slide is not necessarily a review, however you have enough information to do what I want you to be able to do in this slide already. You can look at the periodic table and compare the average weight of two different elements, so I can say well how much heavier is carbon, how many times heavier is carbon than helium, and again just rounding to the nearest whole number, carbon's weigh about 12 AMU, helium's weigh about 4 AMU, so I would say that carbon is about three times heavier than helium, because well 12 divided by 4 is 3, so carbon's weigh about 12 AMU, helium's weigh about 4, so your average carbon is about 3 times heavier. You should be able to do that for any of the comparing any two elements in the periodic table. Nitrogen is about 14 times heavier than hydrogen, et cetera, et cetera. You can do this for any of them. It won't always come out to be a whole number. It won't always be as simple as I just showed you, but you can do it. You take the, you know, you just comparing their weights. Again, this is a little bit of a review. Molecule, we defined very early in the course two or more atoms attached to each other. If I just write the letter H all by itself, that's not a molecule, because you have to have two or more atoms stuck to each other. This is a hydrogen atom, not a molecule. If, however, I write H with a subscript of 2 to the right, this 2 means two hydrogens stuck to each other. So this whole thing is a hydrogen molecule. This was a hydrogen atom. This is a hydrogen molecule. Sometimes students get confused by this because they have the same front part of the name. They're both called hydrogen. One, however, is an atom. One is a molecule. If you wanted to draw the molecule out for hydrogen more clearly, you would write H and then a solid line and an H. And this solid line is called a covalent bond. But what it means is there are two hydrogens attached to each other. And what I'm circling here, this little egg that I'm making, that is one hydrogen molecule because they are stuck together and they're considered to be one thing. This thing that I'm circling over here is just one hydrogen atom. It's not stuck to anything. It's floating around all by itself. So you should be able to understand the difference between those two types of things. If I write H and then a space, here's my space, another H, another space in the letter O, all I have is some atoms. I have two hydrogen atoms and an oxygen atom all floating around all by themselves. They're not attached to each other. However, if I remove the spaces and I write H with a subscript of two and an O means everybody here is attached to each other in some way. We don't necessarily know how they're attached, but we know they are attached. And this is called one molecule. In this case it's one water molecule, but I just want you to get the sense of how to read a molecular formula. So that's it for the review. There's going to be new stuff coming up, but that is again just a brief review for this unit.