 Okay, the previous video dealt with protons, which I was drawing as a red circle. This video is going to deal with neutrons and protons. The neutrons are going to be drawn with sort of a grayish-blackish circle, so like that. So get used to both of those things, they're going to show up a lot on this video. At this point I want to talk about the mass of an atom. I put mass in quotes for a couple of reasons. One because I told you that usually I'm going to refer to mass as weight, and mass and weight are not exactly the same thing, but they're close enough. The other thing is I'm cutting an additional corner that's not terribly important, but I'll reveal that to you in a little bit. So I want to focus on this column over here. Previously, I showed you this table, this entire table in a previous video, and I said the protons weigh very little, the neutrons weigh very little, the electrons weigh much less. So I want to talk more specifically about how much the different particles in an atom actually weigh or what their masses are. So before we get there, I want you to pause the video every so often when I ask a question and try to answer my question. So this is a cartoon version of an atom. What type of atom is it? You can pause and unpause. Since this atom has only one proton, we can look up in the periodic table which atom has one proton and we can see that it's a hydrogen atom. So this is a cartoon version of a hydrogen atom. It's got one proton. If I wanted to describe how much it weighs or what its mass is, its mass is about one and the unit is a little bit weird. It's called an AMU, and AMU stands for Atomic Mass Unit, so that's what AMU stands for. Sometimes a little more informally, people say the mass is one Dalton. This is named in honor of that dead guy that we talked about a couple of videos back, but basically the mass of a proton is approximately one AMU. The math abbreviation for approximately is this little squiggly line here. It's not exactly one AMU, but it's close enough as far as we are concerned. So this hydrogen atom, if I asked you what its mass was, you would probably say or you would hopefully say it's approximately one AMU in mass or approximately one Dalton in mass. So let's get that under our belts. Then I might ask you another question. I might show you a new atom I'd say. First of all, what type of atom is this atom? Second of all, what's its mass, roughly speaking, or what's its mass in Dalton's or what's its mass in AMU? You can pause the video here. If you unpause, you'll see that this thing has two protons. So if we look up in the periodic table which element has two protons, it's helium. So this is the abbreviation for helium. So this is a cartoon version of a helium atom. And if you want to know what its mass is, well, this thing had one proton and it had a mass of about one AMU. This thing has got two protons, so it's got a mass of about two AMU. Or if we want it to be a little less formal, we could say two Dalton's. A lot of times biologists like to speak in terms of Dalton's instead of AMU, but as far as I can tell, they're identical. All right, here's a different atom. Two questions again. What's its mass? Approximately. And what type of atom is it? So you can pause for a moment and try to figure that out and unpause and try not to get tripped up here as far as the type of atom is concerned, it still has two protons. Because of that, if we look up who has two protons in the periodic table, it's still a helium atom. Just like the one above, this is a helium atom that I'm circling. This is also a helium. They're different from each other. You can see that the difference is that the bottom one has two other things stuffed into the atom. These are two neutrons. But the neutrons don't control what type of atom it is. The protons do. So since this has two protons, two plus charges, this one has two protons, two plus. I'm writing a plus there because they have positive electrical charge. They're both helium atoms. Helium. The second question is, what's the mass of the atom on the bottom? This is also pretty tricky. A lot of times students don't answer this correctly at the beginning. A lot of times students will say, well, this has an approximate mass of two AMU as well. That is not correct because if you look up here, I am telling you that the protons and the neutrons have approximately the same mass. So if this red guy has a mass of about one AMU and this one has a mass of about one AMU, so do the gray ones. They also have masses of about one AMU. And because of that, this helium atom has a mass of about four AMU. This one up here had a mass of about two AMU. So what you can see, and this is going to come up later, is that different types of atoms can come in different weights. More formally speaking, they can come in different masses. But it's easier to understand weights. So I have a helium down here that weighs about four AMU. And I have a helium over here that weighs about two AMU. They're still heliums, but they're different from each other in certain ways, and in how much is they weigh. The other thing that I want to point out is that in these calculations that we're doing, the electrons weigh so little that we don't count them. Now they do weigh something, and if you're being very, very careful, people do count them when they're describing the mass of an atom. But for these types of calculations, they weigh so little that we don't count them. This weighs one AMU, this weighs two AMU, this weighs four AMU, and we're just not counting the electrons because they're so tiny. So I want to talk about something now a little more formally called the mass number of an atom, and that's actually what we were doing on the previous slide. The mass number of an atom is the number of protons plus the number of neutrons in an atom. So if we go back to this slide over here, this atom that I'm kind of clearing up, the number of protons is two, the number of neutrons is two. So it has a mass number equal to four. And what we were saying before is we were saying the mass of the atom is about four AMU. What I'm saying is the type of calculation that we just did has a fancier name. It's called the mass number of the atom. And to figure out the mass number, what you do is you count the number of protons, you count the number of neutrons, and that's called the mass number. If you want to think about it in a slightly different way, the mass number is also equal to the atomic number, but that atomic number is just the number of protons plus the number of neutrons in an atom. So again, just to go back, this guy has a mass number of four. This one over here has a mass number of two. And this one over here has a mass number of one. So let's go back to mass number. Over here, this is some pretend atom. I'm going to ask you a bunch of questions. I think there are three of them. What's the atomic number of the atom? What's the mass number of the atom? What type of atom is it? What type of element is it? So you can pause the video and answer those questions. Here are the answers. The atomic number, this is just the number of protons. So all we're doing is counting the red circles in my cartoon. One, two, three, four, five, six. So the atomic number is six. What's the mass number? Well, we're counting all the protons plus all of the neutrons. So we already said there were six red circles. Let's count the gray ones. One, two, three, four, five, six. So six neutrons, six protons. Mass number is 12 for whatever this atom is. If we want to speak a little more informally, we can say that the mass of the atom is approximately, that's what that squiggly line means, approximately 12 amu or 12 atomic mass units. And then finally, I can ask, what type of atom is it? The type of atom it is just depends on the protons. So let's go back. We said that it has six protons. We can look up in the periodic table. And if you look up in the periodic table, which atom has six protons, it's C for carbon. So this is a cartoon version of a carbon atom with mass number equal to 12. And so what we're doing at this point is we're describing, roughly speaking, how much different types of atoms weigh. So this atom weighs approximately 12 amu. More formally speaking, it has a mass number of 12. Now, I want to talk about where to write this mass number. Much earlier in the course, I showed you a pretend symbol. So this is a pretend atomic symbol. And I said that there were four places where you could write a number, upper left, upper right, lower left, lower right. So if you wrote a number here, this is how many x atoms are stuck together in some molecule. This one I may have mentioned, but we're not going to talk about it until the next video or the one after that. This one I said we can ignore for a while. This place is where you put the mass number. So whenever you see a number in the upper left of a symbol, that is telling you, roughly speaking, how much that particular atom weighs. So as an example, let's clear up some of the noise here. We said this is a carbon atom with a mass number of 12. The symbol for carbon is a capital letter C. So if I wanted to tell people the specific type of carbon atom that I'm working with, I wanted to tell them roughly how much it weighed, I would put a 12 in the upper left. And that tells people, look, I'm only talking about carbon atoms that have six protons and six neutrons. So that is, again, a new piece of information. Whenever you want to write the mass, the approximate mass, or the mass number of an atom, you write the symbol. And then in the upper left, you write the mass number. Here is a piece of information that may not have been clear in the earlier part of this video. When I was showing you an atom with, let's say, this was a hydrogen atom because it has one proton. And I said that it was approximately one AMU in mass. And then I said the mass number equals one. There's a slight difference. The mass or how much the atom weighs is not exactly the same as the mass number. They're always very close, but I want to show you that they're different slightly. So if I write the capital letter H, which is the symbol for hydrogen, and I write a one in the upper left, that means I'm talking about hydrogen atoms that have a mass number equal to one. And however, if you look at their actual mass, it's not exactly equal to one. It's 1.00782503207, and then a couple of extra pieces of information there. It's almost one, but it's not exactly one. Here I'm going to write the mass number is exactly one. But the mass will be close, but never exactly that. So I want you to realize we are only going to be concerned with mass number. We're not going to be concerned with this thing called isotopic mass, which is just the mass of the atom. U is another abbreviation for atomic mass units. And this data, this chunk of information here is taken from Wikipedia. Again, this is a different version of hydrogen. It has a mass number of two. What that means if we wanted to draw a cartoon of it, it has one proton and it has one neutron, which means it has a mass number equal to two. And we would write a number two in the upper left of the symbol there, and its mass number is two. So as far as we're concerned, its weight or its mass, the weight of this one or the mass of this one, is about 2 AMU. But if you look very carefully, it's actually 2.014, blah, blah, blah, blah, blah. As far as we're concerned, ignore this. I just want you to realize that when we're saying mass atom, a lot of times I'm going to roughly say that means the mass, even though they're not quite exactly the same. And this is true all the time for different versions of hydrogen and different versions of all of the elements. The mass number and the masses will not always, they won't be identical to each other. They'll be very close, but they won't be identical. So again, mass number of three, its mass is almost three, et cetera, et cetera. So keep that in mind. Mass of an atom is close to but not exactly the same as the mass number of an atom, and all we are going to talk about is the mass number in this course. OK, here's a problem to work on. You have an oxygen atom that has a mass number of 15. Now you're going to need the periodic table here. So break one out. The question is how many neutrons does this oxygen atom have? You can pause the video, you can unpause. The way that we can do this is if we look up oxygen, well, first of all, this thing has an atomic, a mass number of 15. What that means is it has to have 15 protons plus neutrons in the atom. Now at the moment we don't know how many protons and how many neutrons we have, just that the numbers have to add up to 15. But if we look up oxygen in the periodic table, you will see that oxygen has eight protons, or the atomic number of oxygen equals eight. What that means is we can color eight of the little circles in my cartoon as being red. So these two rows are red, which means in my world they're protons. And so if it didn't have eight protons, it wouldn't be oxygen, and the question says that they're oxygen. So there have to be eight protons or eight red circles. What that means is the rest of them have to be neutrons. And so I'm going to color them gray. Now all we have to do is count them up. One, two, three, four, five, six, seven. So the answer to the question is that our oxygen atom has seven neutrons. If we wanted to write this particular version of oxygen, an oxygen that has a mass number of 15, we would write the symbol for oxygen, which is a capital letter O, and we would write a 15 in the upper left. And that tells people I am only talking about oxygens that have a mass number of 15, or that have a mass of approximately 15 AMU. If I can simplify this, if somebody ever asks you how many neutrons does some version of an atom has, you basically take the mass number. They have to give you the mass number. You subtract away the number of protons, and that equals the number of neutrons, which is basically what we just did. We knew the mass number was 15. We knew the number of protons was eight. Therefore, the number of neutrons has to be seven. So that's all we're doing. And basically, this whole video is designed to make you comfortable with, to get you used to thinking about protons and neutrons contributing to the weight of an atom, or more formally contributing to the mass of an atom. So that's it for this video. We are going to talk about mass number and mass of an atom in more detail in the next video, because it gets a little more complicated. And then she said, just because you become a young man, it's still something.