 All right, we have spent the last couple of videos pretty much talking about protons. We have also spent the last couple of videos talking about neutrons. We've been talking about the mass number of different types of atoms. We've been saying that the mass number is very close to the actual mass of an atom. And as far as we are concerned, the mass number, which is the number of protons plus the number of neutrons, is going to be about equal to the actual mass of an atom when we're talking about an atom. So let's keep that in mind. Let's also remember that the atomic number, this thing over here, is the number of protons that an atom has. So that being said, I'm going to ask a couple of questions and I'd like you to pause the video at various points and try to answer those questions. First question is, what type of atom is this one that I've drawn over here? Remember that a red circle is my cartoon version of a proton. So the way that you answer this question is you have to look up who has one proton in the periodic table. You have to look that up so you can pause, look it up. Hopefully you've looked it up and you will see that this is, sorry, this is a hydrogen atom because all hydrogen atoms have one proton. If you don't have one proton, it ain't a hydrogen. Next question I would ask is what's the approximate mass or what's the weight? What's the approximate weight of this atom? You can pause, but I'm just going to say the answer. It is approximately one atomic mass unit in weight or in mass. So we've got a hydrogen and if we wanted to write the mass number, which is just one, for the symbol, if we wanted to write the mass number for hydrogen, for this particular hydrogen we would write a little number one in the upper left relative to the symbol for hydrogen. So hopefully that's not too big of a problem. Next atom is shown here. I'm going to ask the same question. What kind of atom is it? You can pause, look up the information, unpause and hopefully you realize that this is the same type of atom. Type of atom because it still only has one red circle, it still only has one proton. And if you have one proton you are a hydrogen atom. Now this is a different hydrogen atom than the one on the left. This guy over here also has one neutron. So it's different than the one on the left. If you wanted to know its mass number, well the mass number is two because it's got one proton and one neutron and if you want to know the mass number you have to add those up. And so if I wanted to write the symbol for this particular version of hydrogen I would write a little two here instead of the little one that I wrote for the other guy on the left. So both of those cartoons are supposed to be hydrogen but they are different from each other. If I asked what the mass of this atom was you would say it's about two atomic mass units or two Dalton's and remember this one is about one atomic mass unit. So all I've done is drawn two different types of hydrogen atom. Third atom that I'm going to show you, same questions. What kind of atom is it and how much does it weigh roughly? Again this is a hydrogen atom because it has only one proton, one red circle. It's different than the other two because it also has two neutrons. Has even more stuff in the center of the atom. So it's a different type of hydrogen atom but it's still hydrogen because the only thing that controls what type of atom you have is the number of protons. That's it. So we have a hydrogen atom. First I asked you what the mass number was. Mass number equals three because we've got one proton and two neutrons and we add those together and we get a three. So if I wanted to write, describe this molecule without drawing or this atom I should say without drawing a fancy picture I can write the symbol for hydrogen excuse me and I can write, let's try that again, I can write the symbol for hydrogen and I can write the number three because this guy here has a mass number of three and if I wanted to know roughly how much it weighs you would say it weighs approximately three atomic mass units or approximately three dolens. So that's sort of a brief introduction to different versions of hydrogen. This one weighs the least. The middle one maybe if I was feeling informal which I usually am I would call this heavy hydrogen, this guy over here because it's certainly heavier than the one on the left. If I was still feeling informal I might call the one on the right even heavier hydrogen. So they're all hydrogens, all three atoms are hydrogens but they're different from each other in how much they weigh or what their mass is. There is a fancy name for these versions of hydrogen. They are all called isotopes of hydrogen. So every single atom that I've drawn here and let me remove some of the noise, every single atom or cartoon of an atom that I've drawn here is a hydrogen atom but they're different from each other and the name for different versions of atoms of these hydrogen atoms the fancy name is to call them isotopes of hydrogen. So I'm going to give you a definition of what an isotope is, an isotope, this is kind of my definition is a version, I'm putting version in quotes of an atom that's heavier or lighter than other versions of the same element. And so you can look at all of these, this is an isotope of hydrogen, we can write it H with the number one, the reason it's an isotope is because it's lighter than hydrogen number two over here and hydrogen number three over here. This middle one, excuse me, this middle one here is also an isotope of hydrogen because it's heavier than the first one and it's lighter than the second one. And then finally this third version of hydrogen is an isotope of hydrogen because it's heavier than the first two that I've drawn and there are other isotopes of hydrogen but I want you to sort of understand what this word means. It just means a version or even more informally a flavor of an atom that is either heavier or lighter than other flavors or versions of an atom. So and the way that we distinguish between them if we don't want to draw these fancy cartoons is we basically write their mass number. We write the symbol and we write the mass number. The middle one is H with a two in the upper left and the right one is H with a three in the upper left. So instead of drawing this cartoon, I can draw H with a one in the upper left. If I do that, I am basically saying, hey look, I am only talking about hydrogens that look like this one. Only hydrogens that have one proton, no neutrons. If I write an H with a two, that means I'm only talking about hydrogen atoms that have a mass number of two. Here, only hydrogens that have a mass number of three and these again are called isotopes. It's not just hydrogen atoms that can have isotopes. All elements can have isotopes, including whatever this atom is here. So I want you to pause the video in a moment and answer these three questions. In other words, what type of element is it? What's the atomic number? And really this should say what's the mass number or what's the approximate atomic mass? So you can pause the video and on pause. We're going to skip number one for the moment because it's easier to answer once we've answered the second question. What's the atomic number? Well, atomic number is a number of protons and this guy over here has one, two, three, four, five, six protons. Six protons, so the atomic number for whatever this is, is six. If I want to know the mass number, I have to count up the protons and the neutrons. There's six gray ones, one, two, three, four, five, six. Neutrons as well. Mass number is 12. And if I want to know what type of element it is, again, the only thing we need to know is how many protons it has. That's the only thing that will tell us what type of atom it is. If you, and you have to use the periodic table to do this unless you have the periodic table memorized, in which case you are an even bigger nerd than I am, if you look up who's got six protons in the periodic table, it's carbon. So this is a cartoon version of a carbon atom with a mass number of 12. Same type, same collection of questions for this other thing on the right. What element is it? What's its atomic number? What's its mass number? And what you will see, you can pause the video if you want and unpause. The atomic number is a number of protons. Well, it still has six protons, so the atomic number is still six. What that means is this is still a carbon atom, but it's different than the one on the left because the mass number, it's got six protons. If we count the gray circles, which are supposed to be neutrons, one, two, three, four, five, six, seven, eight, eight neutrons. So this atom on the right, it's a carbon atom, but its mass number is 14 because six and eight is 14. So this is an isotope of carbon, and so is the one on the left. That's just another isotope of carbon. And so again, part of the purpose of the slide is to show you that other elements can have isotopes as well. It's not just hydrogen that does this. If I wanted to describe to people this isotope of hydrogen, the one that I'm circling exclusively, what I would do, I'm sorry, this isotope of carbon that I'm circling exclusively, I would write the symbol for carbon, which is a capital C, and I would write a 12. And if I wanted to describe this one to other people without resorting to this elaborate picture, I would write the symbol for carbon, and I would write a 14. So I am basically telling people when I write this that I'm only talking about carbon atoms with a mass number of 12, with an approximate mass of 12 atomic mass units, and if I write this, I'm only talking about the carbons that have a mass number of 14. Again, there are other versions of, there are other isotopes of carbon, there's not just the one, the two that I've drawn here, there are many other weights that carbon can come in. Now, if you were going to speak these atoms out loud to someone else, the way that you would say this is you would call it, I think I have it written down here. Well, I have the other one written. If you were going to talk about this atom or this isotope exclusively, the way that you would say it out loud to other people is you would call it carbon 14. And as you might guess, if you were going to talk about this isotope exclusively, you would call it carbon 12. So that should be relatively easy to understand where those names are coming from. All right, so another game, let's play how many. How many protons does oxygen 17 have? So that's how I pronounce this. This is an isotope of oxygen. The second question is how many neutrons does oxygen 17 have? The way that you do this is you have to use your periodic table. And the periodic table will answer the first question. If you look up in the periodic table, oxygen atoms doesn't matter what kind. If it's an oxygen atom, it will have eight protons per atom. So that gets us halfway there. We figured out that here it's got eight protons. Second question is how many neutrons does it have? Well, we know that this number, 17, is the mass number. And we know that part of the mass number is derived from the number of protons. And the rest of the mass number is derived from the number of neutrons. So if we want to figure out the neutrons, we basically say, look, this oxygen weighs about 17 AMU. Eight of the weight comes from the protons. So the rest have to come from the neutrons. So if we want to figure out the number of neutrons, it's going to be 17 minus eight, which is nine neutrons. Hopefully I did that math correctly. Sounds looks good. So the answer to the next, the last question is nine neutrons. And again, a little bit of the purpose of the slide is to get you accustomed to reading isotopes, the way that they are typically written with the mass number in the upper left relative to the symbol. Okay. Now I want to talk about something related to isotopes called average atomic mass. It has many other names. It has many other names. This is the one that I tend to use and probably a lot of people tend to use it. So here we go. This is a thought experiment. Mostly what I'm going to tell you is not true, but it's just useful to think of. Imagine that I live in a universe where all the hydrogens come looking like this. All of the hydrogens have a mass of about one AMU. There's no other isotopes of hydrogen. They don't come in two AMU or three AMU or anything else, just one. If I asked you what the average mass of a hydrogen atom was in this universe, you would say, hey, it's about one AMU because they only come in this flavor. They only come in this weight. Now this is not the universe that we live in, but I just want you to think about that for a second. Let's pretend we live in a different universe where 50% of the hydrogens come weighing one AMU and 50% of the hydrogens come weighing about two AMU. There's no three AMU, there's no four AMU. They just come in ones and twos and it's 50-50. If I asked you what the average mass of hydrogen was, you would say, well, half of them come as one AMU, half of them come as two AMU. And to take the average, you add up the weights and you divide by the number of atoms. So one plus two is three divided by two. So you would say the average mass of a hydrogen atom in this universe is about one and a half AMU or 1.5 AMU. And again, I would tell you that you've done the calculation correctly in our pretend universe. That's what the average mass of hydrogen is. Again, we don't live in this universe, but I want you to think of, if you had different isotopes of hydrogen, what the average mass of the hydrogen atom would be because they come in two different weights, then you have to average them. Again, this is not the universe we live in. The universe we live in, almost all of the hydrogens come weighing one AMU, but not all of them. Every once in a while, there's a hydrogen weighing two AMU. Every once in a while, there's some weighing three AMU, roughly. And I think that there are even ones that are heavier than that, but they're all pretty rare, except for the first one. If we live in this universe, which we do, the first question I would ask is, is the average atomic mass of hydrogen exactly equal to one, approximately one AMU? Or is the average mass number of hydrogen equal to exactly one? And you'd have to say no to this. The reason you'd say no is because even though most of the hydrogens come with a mass number of one, every once in a while, there's one with a mass number of two. Every once in a while, there's one with a mass number of three, and probably even higher than that. And because of that, the average mass, excuse me, is not going to exactly equal one. It's gonna be a little bit more than one. And if you look in a more detailed periodic table, and I'm gonna show you more detailed periodic table in a minute, what you will see is you'll see more information that tells you the average mass of hydrogen atoms taking into account the fact that there are many different isotopes, taking into account the fact that there are many different weights that hydrogen atoms come in. So here's an example of a much more detailed periodic table. This is only a piece of the periodic table, but I want you to focus on the hydrogen here. This number one is the atomic number for hydrogen. And you remember that the atomic number is the number of protons. There's a lot of extra information in this particular periodic table. I want you to ignore most of it. You can ignore this top number, you can ignore this top right number. This is the symbol. This is the name of the atom, obviously. And then underneath, there's these two numbers, 1.00. Let's just stick with the first one, 1.00784. What that number is, is the average atomic mass for hydrogen atoms. So people have looked very carefully at this, much more carefully than I would ever want to. And they've basically figured out what the average weight of a hydrogen atom is. And they know that it's somewhere in between 1.00784 and 1.00811. That's where this is coming from here. Depends on where you dig and pull up your hydrogen atoms as to where the number is. But it seems to be somewhere in between these two. It seems to be in this range. And you'll notice that these numbers are a little bit bigger than one. So what that's telling you is that most of the hydrogens come with a mass number of one. But every once in a while, there's one with a mass number of two. And every once in a while, there's one with a mass number of three. And those twos and threes and maybe the fours and fives, they pull the average up a little bit more than one, a little bit over one. And when you look at a more detailed periodic table, you will usually see an atomic number number, which is always a whole number. Here's another one, here's another one, et cetera. And then you'll see this thing that is usually some decimal number. And this is the average mass or the average weight of your atom. Hydrogen weighs a little bit more than one. And what I want you to realize, and this sometimes confuses the hell out of students, is that this does not mean all of the hydrogens weigh 1.00, weigh somewhere in between these two. This is an average. If you take a test in a class, and I tell you that the class average is an 80, that does not mean that everybody scored an 80. If I tell you the average atomic mass is 1.007A4, that does not mean all the hydrogens come weighing 1.007A4. It means that there are some hydrogens that weigh less than this. There are some hydrogens that weigh more than this. But if you take the average, it comes out to somewhere in this range. And you will see this in this particular periodic table, the average atomic mass is written at the bottom of the box. It doesn't really have to be there. That's just the way that it's done here. You can put them in any different place inside of the box. Usually you have to refer to a key. There's a key here that's supposed to tell you what everything means. So this letter Z, if you look it up, it says Z is the atomic number. You can ignore this. You can ignore this. And then SAW stands for Standard Atomic Weight, which is a weird way of saying average atomic mass. But you can see that this average atomic mass is almost never a whole number. So for beryllium, it's 9.01, blah, blah, blah, blah, blah. Don't worry about what this parentheses means and that number in there. But basically what you should think is that most of the berylliums probably come with a mass number of about nine. But there are some that are heavier than nine. There are some that are lighter than nine. But if you average them all up, the average mass is about 9.012. Magnesium has a range because they don't know exactly where it falls. Scandium, you know, 44.955. So what you'll see is this number that is almost always a decimal is telling you what the average mass of your atom is. So eventually you're going to have to use a more detailed periodic table. You can use this one if you want. I will send it to you. You can send me an email and I will send it to you. Believe it or not, I am such a massive nerd that I made this periodic table. So I don't know, I guess I should be embarrassed about that and I probably am a little bit. But you can ignore this number, this number. The other ones you should know. You can also use the periodic table in your textbook. That's pretty good. It doesn't have as much information in it, which means in my opinion that it's probably less confusing. But I tried to be super anal retentive when making this, so probably may be overkill for most purposes. What I want you to know is that these numbers here, these decimal numbers, are the average mass of different elements. So to summarize, we've spent a lot of time talking about protons and neutrons. So here's a very brief summary of what you should know. You should know what atomic mass means. That is roughly equal to how much an atom weighs. And I'm going to put weighs in quotes. You should know what atomic number means. That is the, that's the number of protons. You should know the electrical charge of protons, neutrons, and electrons. Protons have a positive electrical charge. Neutrons have no electrical charge. Electrons have a negative electrical charge, which I'm going to write with a minus sign. Positive electrical charge I'm going to write with a plus sign. You should know the relative weights of these particles. So protons weigh about 1 AMU. Electrons weigh about zero. And neutrons weigh about 1 AMU. Then you should know how to use protons and neutrons to figure out approximate mass and mass number. And that's it. Don't forget about isotopes and average atomic mass, but that is it. Next, I think we go on to electrons.