 At this point we are going to talk about a lot more about electrons, we're going to talk about how they are arranged in the atom, so kind of we'll be talking about where they're located, although as you'll see or as I will confess to you in a little bit, pretty much everything that I tell you in the next video is a lie, but it can be a useful lie and even though I feel like I need to take a shower after I say all this stuff, it's still somewhat useful, so let's let's just get going and you'll sort of understand as we go. Here is some pretend atom, you can probably figure out by now two red circles means two protons, two protons means a helium atom, this helium atom has a mass number of four and it also has two electrons and so two negative charges, two positive charges and I'm going to, because of that it's electrically neutral, so we could put a zero here if we wanted to. What we're going to talk about now is the arrangement or the location of these electrons in different atoms, so the idea here is this is a pretty simplified situation, I've got a helium atom and it only has two electrons, but there are many atoms that can have many more than two electrons and so we're going to talk about where those electrons go or where they're located in the atom or another way of saying that is how they're arranged and what I want you to think of is that the electrons are arranged or located in different energy levels in an atom, depending on how many electrons the atom has, there may be many different energy levels, so for example, or just a, this is something you're going to have to just kind of memorize, the lowest energy level and I'm putting energy level in quotes here because it's a little bit of a simplification, the lowest energy level has a slot for two electrons, in other words the lowest energy level can hold two electrons. The way I want you to think about this is think of this as, think of the atom as being like concert hall, there's a stage here and there's seating, this is the first row that I'm circling here and the first row has two seats and which means that it can hold two electrons and this is the lowest energy level and then in the stage on the stage there are protons and there are neutrons as well, okay so the lowest energy level in an atom can hold two electrons, the second lowest energy level, think of that as the second row, as far as seating is concerned, can hold eight electrons and the same is true for the third lowest energy level, it can hold eight electrons as well, so first row holds two electrons, second and third rows hold eight electrons, we are not going to go beyond the third energy level or the third row, so keep that in mind, let's see, so if I have one proton like this proton that just showed up here, I have a hydrogen atom, if I wanted to make an electrically neutral hydrogen atom, I would need an electron and I need one electron because it has to balance out the positive charge of the one proton and what I'm going to tell you is that electron will go into the first energy level, so watch this awesome animation, everyone see that now there's an electron sitting in the first energy level and so this is a neutral hydrogen atom, imagine that I had a helium atom, heliums have two protons, so here are my two protons, if I was going to make a neutral helium atom, I would need two electrons, so where do you think that second electron is going to go, we've already got one electron here, is there place for the second electron in the first energy level, yes there is, there's the empty slot, so if we wanted to make an electrically neutral helium atom, we would add another electron or the electron would probably be located, or as far as we're concerned would definitely be located in the first energy level, so there it is, now I've got a helium atom that is electrically neutral because it has two negative charges and two positive charges and you can keep doing this, you can say what if I had a more complicated atom that was electrically neutral, imagine that I had a lithium atom, lithium has three protons, if I wanted to make it electrically neutral it needs three electrons as well because the electrons are negatively charged, the first two out of these three electrons will sit in the first energy level, but we have to add one more, if we want to make a lithium that is electrically neutral that has a charge of zero, where does that third electron have to go, can it go in the first energy level, no it can't because we've used up all two slots already, so the third electron, everyone see that awesome animation I'm going to show it to you again, there it is, the third electron now starts to fill up the second energy level and this is basically how electrons are arranged in more and more complicated atoms, in other words if your atom has a bunch of electrons the electrons will sit at the lowest energy level, you'll sit as many as you can in the lowest energy level, once you fill that up you'll start to fill up the second lowest energy level, once you fill the second lowest energy level you'll start to fill up the third and that keeps going on and on and on, there's a general idea that the lowest energy level, the electrons in the lowest energy level are closest to the center of the atom, so they're like in in the front row, the electrons in the second lowest energy level are a little bit further away, and the electrons in the third lowest energy level are a little bit further away still. Again, that's a simplification, but it's partly true. And it's a reasonable way to think of it, and it lets us draw these pretty little pictures with orbiting electrons. So you can think of these electrons as sort of moving around in a circle around the atom. Again, not really true, but as far as we're concerned, it's good enough to understand what's going on. And again, just in case this is not clear, these filled blue circles mean that there is an electron sitting in that position. The empty or the white circles means that there's no electron sitting there. That's an empty slot. OK, so I've basically described electron energy levels. We're only concerned with the first three energy levels. And again, to summarize, the first energy level can hold two electrons. Second and third can hold eight. We're not going beyond the third energy level. And you can keep filling these up. I can have a beryllium atom. It's got four positive charges. If I wanted to make an electrically neutral beryllium atom, I would need four electrons as well. And we can put two of those four electrons in the first energy level, but we still have two left over. So we have to put two in the second energy level. So that is sort of a brief introduction to how electrons are arranged in atoms that become more and more complicated. All right, everything that we talked about in the previous slide dealt with atoms that became more and more complicated and atoms specifically that were electrically neutral. So every time I showed you a new atom, I said pretend it's going to be electrically neutral, which means the number of protons equals the number of electrons. And I basically went over where the electrons were going to be located in different energy levels. So as far as we're concerned, there are only three energy levels. First level can hold two electrons. Second level can hold eight. Third level can also hold eight. What I want to talk about now are different terms and a weird rule about electron energy levels. First is the term. Electrons, if you're looking at an atom and you're looking at the electrons in the different energy levels, the electrons that are in the outermost energy level are called valence electrons. So before we go to this more complicated example, let me try and explain what I mean. If I have a simple atom and there's only one energy level and there's one electron sitting in that energy level, even though it could hold two, this is the outermost energy level for that atom. And because of that, this electron sitting there is called a valence electron. But it won't always be that simple. Here's a different situation. Imagine that I have a more complicated atom. I'm not going to draw the protons. I'm just drawing the electrons and the energy levels. Here's an electron, or I'm sorry, here's an atom with three electrons. Two of the electrons, this one and this one, go in the first energy level. The second, oh, let's say that it has four electrons. The second, the third, and the fourth electrons go in the second energy level. This first energy level is not the outermost level anymore. The second one is because of that, these two electrons that I'm circling here are in the outermost energy level. And these two guys are now called valence electrons. And these two that were sitting in the first energy level, they're not valence electrons. Just let me bear with me. Let's do a slightly more complicated one than that. Imagine that I have a more complicated atom. It has a whole bunch of electrons. Here's two electrons sitting in the first energy level. One, two, three, four, five, six, seven, eight, sitting in the second. I've got these lovely little streaks. One, two, three, four, five, six, seven, eight. And then I've got another third energy level. And maybe that energy level has, I don't know, five electrons sitting there. Again, this first energy level, the one that I drew that has two electrons in it, this one and this one, those are not valence electrons because they're not the outermost energy level anymore. Neither is the second energy level, also not outermost. This time, because our atom is more complicated, this third energy level is the outermost one. And the electrons that I just circled, those are the valence electrons for this particular atom. So a valence electron is an electron that is in the outermost energy level of your atom. And it's gonna be a different energy level for different atoms. Now let's go over here. This is some pretend atom. It has one, two, three, four, five, six, seven protons. I believe that is a nitrogen atom. Don't really need to know that. And this is an electrically neutral nitrogen atom. So it also has seven total electrons. It has two electrons. There's number one and there's number two sitting in the first energy level. Those are not valence electrons. And then the second energy level is the outermost one. And the electrons that are sitting there, number three, four, five, six and seven, those are called valence electrons for this particular nitrogen atom. So hopefully that's a reasonable introduction as to what a valence electron is. That definition is not quite correct, but as you'll see, it's good enough. And it's correct a lot of the time. Here comes the rule. The rule is that atoms don't like, and I put like in quotes because atoms don't really have feelings. They don't like to have partially empty energy levels. So this is supposed to be an empty slot for a nitrogen atom. This is supposed to be another empty slot. That's a third empty slot. So as far as this nitrogen atom is concerned, it's electrically neutral, but its second energy level is only partially filled. And what I'm gonna tell you is that in general, atoms don't like to be in that situation where the outermost energy level is partially filled. And if you give them the chance, they will actually do something to try to fix that situation. So they will either try to fill the empty slots. So imagine that I have my nitrogen atom. In this state it has three empty slots. It's not happy with that situation. One way that it can try to fix that situation is to fill those three empty spots. It can, one way to do that is to steal three electrons from somebody else. Imagine there's another atom bouncing around nearby with three electrons and this nitrogen just steals them. So that's one way of this nitrogen atom becoming happy. Obviously happy is very informal term, but basically this is the nitrogen atom trying to live by this rule that I'm circling here. Another way that this nitrogen atom can fix this situation is to interact with a nearby atom and basically ask, I know it can't really ask, but ask the other atom if the other atom could share three of its electrons with those empty slots part of the time. So that's another way of fixing the situation is to basically broker a deal with another atom that has three electrons and say, hey, could you please share your three electrons with me at least part of the time so I can partially fill these empty slots and then I will be more stable. Those are two possible ways that atoms can sort of fix this rule if they're not obeying this rule. The other way is to be completely empty. In other words, completely empty the outermost shell of electrons. That means get rid of all of these electrons that are sitting there so that I have eight empty slots instead of three. In general, either situation works to satisfy this particular rule that I'm talking about. In general, however, the idea is that the atom will make a change that requires the smallest number of addition or removal of electrons. So in this case, this nitrogen atom, the easiest thing for it to do is to steal three electrons or share three electrons compared to losing five. So the idea or the rule of thumb is that as far as this atom is concerned, it's easier to steal three than to lose five. For other atoms, it might be easier to lose electrons. So it'll be a different situation as to how different atoms satisfy this rule. Sometimes some atoms have a tendency to steal, some atoms have a tendency to share, and some atoms have a tendency to get rid of the extra electrons in the outermost energy level. So before we go on, let's ask how many electrons are in this nitrogen atom and the way that it's drawn? Well, the valence electrons are the electrons in the outermost energy level. So here's the outermost energy level, this outer circle, and we count them, one, two, three, four, five. That's a horrible four. Okay, so the answer to that question is that there are five valence electrons in our neutral nitrogen atom. That's a different question than how many electrons because if I don't use the word valence and I just say how many electrons, then we have to count all of them. We even have to count the ones in the first energy level. So the answer to the second question is that there are seven total electrons. It just so happens that only five of them are in the outermost energy level. So try to understand the difference between those two questions. Okay, here comes the confession. The rule, actually it was only one rule, so let's just say rule, that you just read is a massive oversimplification. It's basically a lie, but it is a good enough lie. It's true enough of the time that this rule is useful to allow us to make certain predictions that we will go over in the upcoming videos. The reason that we tell you, or the reason that I'm telling you this horrible lie is that if you really wanna understand how electrons are arranged in atoms, it requires at least two more years of college math. That's a lot of calculus and actually math beyond calculus. So most people probably don't want to do that. So that's sort of why I am confessing to you that the rule that I'm telling you is not quite correct. It works a lot of the time, it's useful. I'll show you what it works for in a minute. But it doesn't work all of the time, so keep that in mind. I've also brought this up in the past with other classes. And then I mention it as we progress in the course. And as the course progresses and students forget about this confession, a lot of times when I bring it up and I say, well, yeah, that rule about the atoms don't like to have empty slots in their outer most shell, it's a little bit of a lie, a lot of times students look at me and they act as though I just told them that Santa doesn't exist. And so what I'm going to tell you is that Santa actually does not exist. And this rule is a little bit of a lie, but if you behave and you're good, sometimes you still get presents on Christmas. And if you follow this rule, sometimes you can still learn useful things. Now, if the rules and the terms that we used are not quite correct, what's the benefit of doing this? The general benefit is that you can learn things about atoms and what electrical charges they like to have. The thing to realize here is that people usually look at the atoms in the periodic table, these and these, and they think that all atoms like to be electrically neutral. That it turns out that that isn't really true. And if you follow the rule that I mentioned a couple of slides back, you can actually figure out what electrical charge different atoms prefer to have under many circumstances. So what you'll see is that nitrogen very often does not like to be electrically neutral and it will pick up three extra electrons and become charged at negative three. That is something that you can figure out just by following the rule that we went over a couple of slides back. And I'll show you some examples going forward. But the idea is that you will learn a rule that at the very least works for figuring out the preferred electrical charge for many of the atoms in the first three rows of the periodic table. This row, this row, and this row. And sometimes even into the fourth row, this works. After that, the rule kind of falls apart, but I'm not going to ever ask you any questions about valence electrons or electrical charge of atoms beyond these three, maybe four rows. We will talk about other atoms in the periodic table, the ones beneath the first three rows, but we won't be talking about electron arrangement in those cases. And so that's the benefit that it lets you figure out what charges many different atoms, especially in the first three rows, which is where a lot of chemistry takes place, what charge those atoms tend to end up with. And if you can figure out what type of electrical charge different atoms have, that actually lets you figure out a lot of things about how atoms stick to each other, which is where we're going. We're trying to figure out how atoms stick to each other and describe that to other people. And the first step in getting there in figuring out how atoms stick to each other is figuring out what electrical charges different atoms have or different atoms prefer to have. Here's a situation. I'm going to ask you four questions. Here are the questions. We're going to be referring to this atom. First question is what element is it? How many valence electrons does it have? Is its valence level filled and how can it fix its problem? So you can pause the video, try to answer those questions. I'm going to unpause at this point. And so this guy has a single proton. If you look up who has a single proton in the periodic table, it's hydrogen, whose symbol is capital letter H. How many valence electrons does it have? So valence electrons are the ones in the outermost electron energy level. This guy only has one energy level, so that has to be the outermost one. And it has one electron there. So the answer to the second question is one. Our guy has one valence electron. Is its valence level filled? The answer is no, because the first energy level can hold two electrons. And we have an empty slot there, so no. And so what you have to do at this point is you have to think back to the rule that we just discussed a couple of slides back. And the rule is that the atoms don't prefer to have these partially empty energy levels. So you can think of this hydrogen atom as being maybe sad or unhappy. There's our sad hydrogen atom. How can it fix its problem? Well, it can do it in one of two ways. And this is one of the situations where it's not obvious what hydrogen's gonna do. Hydrogen can either completely get rid of that electron and then make the first energy level completely empty, or it can steal or share another electron to completely fill the first energy level. What I'm gonna tell you, and you wouldn't necessarily be able to predict this, so I would not quiz you on it, is that hydrogen has a tendency to get rid of this guy, to satisfy that rule that I mentioned a couple of slides back. But that being said, if our hydrogen got rid of this extra electron, pretend it went away, it's gone, then our hydrogen atom would have one positive charge from the proton, one plus and zero negative charges. Because of that, our hydrogen atom would have a total electrical charge of plus one. And what I am going to tell you is that hydrogen atoms do not like to be in the situation that I've drawn here, where they have one electron and one proton. They much prefer, and if given the opportunity, they will turn into hydrogen ions, and if you remember an ion is some atom or group of atoms that's charged, they will turn into hydrogens that have a charge of plus one. So we've just figured out a way, or at least a little bit of a way, to determine that hydrogen doesn't like to be electrically neutral, and it prefers to have an electrical charge of plus one. So this is how you use the rule that I described a couple of slides back. You basically pretend, let's see, pretend you have an electrically neutral atom. That's how you start. You look at the valence electrons, and you ask if the valence electron level is filled or partially filled. If it's partially filled like it is over here, then you say that your atom is sad. I know that's a huge oversimplification, but you gotta live with it, and then you say how can it fix its problem, and you pick the easiest way to fix it, and if you do that, you can figure out what the electrical charge is that is generally preferred for many of the atoms in the first three rows of the periodic table. We just did it for hydrogen, but we're gonna do it for a couple of other examples as well. Here's a different atom. Same four questions though. What kind of element is it? How many valence electrons is a valence level filled? How can it fix the problem? You can pause the video, try to answer those questions. On pausing, this guy has two protons, two plus charges. And if you look up who has two positive charges in the periodic table, you'll see that it's helium. This is the symbol for helium. If we wanna get fancy, we can count the protons and the neutrons and figure out that it has a mass number of four. So we've answered the first question and then some. Where's the outermost electron energy level? Well, only one level, again. So it's easy. That has to be the outermost one. How many valence electrons? In other words, how many electrons in the outermost energy level? One and two. So the answer to question number two for this helium atom is that there are two valence electrons. This is an electrically neutral helium atom because it has two plus charges and it has two negative charges from the electron. So if we want to, we could put a zero there. Is its valence level filled? Yes, it is because the first electron energy level can hold a maximum of two and that's how many electrons are sitting there at the moment. So yes, filled. How can it fix its problem? This is a trick question. There is no problem because the rule is that atoms prefer not to have partially filled electron energy levels. And this one is completely filled. So not a problem. And because of that, we can basically learn that helium atoms prefer not to have an electrical charge. Helium atoms are generally perfectly stable being electrically neutral or having a charge of zero. So we just use that rule again to figure out that heliums don't like to have an electrical charge. Third atom, same questions. What element is it? How many valence electrons is the valence level filled? How can it fix the problem? You can pause the video again, try to answer these questions on pausing. This guy has one, two, three, four, five, six, seven, eight, nine protons, so nine plus charges. If you look up in the periodic table, what atom has nine positive charges? It's a fluorine. The symbol for fluorine is a capital letter F. If we wanted to know the mass number, it's one, two, three, four, five, six, seven, eight, nine, 10, 10 neutrons plus nine protons makes a total mass number of 19. This atom, the way that I have it drawn, it has nine total electrons or nine negative charges. You can count them up. It's got two in the first level and level one. And it's got one, two, three, four, five, six, seven electrons in level two, which makes a total of nine electrons and so nine negative charges, nine positive charges. Right now our fluorine atom is electrically neutral. How many valence electrons does it have? Well, it's got seven because level two is the outermost level. And so the answer to that is seven. Is the valence level filled? No, it's not. If you remember, second energy level can hold a maximum of eight electrons and so we have an empty slot here. So no, almost filled but not quite. How can it fix its problem? Well, you can think of two possible ways. One, it could get rid of seven electrons, could get rid of the seven in the outermost shell. But the idea is that is not as easy as it would be to actually just steal one. And so fluorine has a very strong tendency to just steal one electron from an atom that might be bouncing around nearby. If it steals one electron like that, let's say steal an electron, it's not electrically neutral anymore because it used to have nine negative charges, but if it steals one, it's gonna end up with 10. But it still only has nine positive charges. So it has one extra electron or one extra negative charge. And because of that, what you can reason, what you can figure out through this rule that we've been using over and over again is that fluorine atoms do not like to be electrically neutral. Fluorine atoms have a tendency to have an electrical charge of negative one. So we've used this simple rule over and over again three different times to figure out that some atoms like to be positively charged, like the hydrogen, and I said it, likes to be charged at plus one, some atoms like to be neutral, like the helium, and some atoms like to be negatively charged, like fluorine, which has a tendency to have a charge of negative one. So you can do this for many of the atoms in the first three rows and even a little bit beyond the first three rows of the periodic table. If you follow this rule, in many cases, you can learn certain things about the shape of the periodic table. A couple of slides back, I told you that hydrogen has a tendency to be charged at plus one, and you can figure that out from this rule. But it turns out that everybody in this column pretty much has a tendency to acquire an electrical charge of plus one. If you follow the rule that I described that we've just been going over, and you use it on beryllium and magnesium, and calcium, and you can even go further down, you will see that these guys, if you follow that rule, have a tendency to be charged at plus two. Over here, in this general area, the rule breaks down. And so we're never gonna use it in that area. We will talk about electrical charge, a lot of the atoms here, but it won't be in relation to that rule, and we won't use it here either. The rule tends to work on the edges. So a couple of slides back, I said, we reasoned through the fact that helium likes to have an electrical charge of zero, another way of saying that is helium likes to be electrically neutral, and this is true for all of the atoms in this column. They prefer to be neutral. We said that fluorine, by following this rule, prefers to have an electrical charge of negative one. That is horrible negative one, huh? There. And if you follow this rule for chlorine, and many of the others, you can figure out that the atoms in this column have a tendency to be charged at negative one. Here, negative two, it works there. And here, negative three, although this starts to fall apart as well in that general area. But, again, that's why it's a rule and not an absolute law or anything like that, but you can figure out by following the rule that we've been going over that the atoms in this column prefer a charge of plus one, plus two, minus three, minus two, minus one, and electrically neutral. So there's two ways you can go about this because you kind of need to be able to do this. You can either memorize this. You can look at this chart and say, all right, it's plus one, leftmost column plus two, then big wasteland in the middle, minus three, minus two, minus one, zero. Or you can memorize the rule and then you can reason through it for most of the atoms. And I will only ever ask you, what's the preferred electrical charge of an atom where you can actually figure this out correctly? I won't ask you any of the weird situations. Now, why is this important? It isn't really. Much of what I teach in this course is not going to be relevant in the real world. In particular, I think many of the people who are listening to this are trying to get into the healthcare field. And there is probably never going to be a situation where someone turns to you and says, quick, how many valence electrons does an electrically neutral nitrogen atom have? That's probably not gonna come up. So at some level, a lot of this is not relevant. What lets me sleep at night is that I feel like this course is a test to see whether people can think logically about weird things that they may never have encountered. So you may never encounter this, but you will somewhere in your careers encounter weird things that you have to think logically about. And so this course, at least at some level, is a test of how well you can think logically about unusual things. Maybe in a more practical sense, so I'm gonna get off of my soapbox now. Why is this important? It's important to understand other parts of the course. And as I mentioned, if you can figure out what preferred electrical charges different atoms tend to have, it can help to explain how different atoms attach to each other. So for example, I think most of you know that the formula for water is two hydrogens stuck to an oxygen. If you follow these rules, and we will do this in the next chapter, you can figure out why it's always two hydrogens stuck to an oxygen for water and not, let's say, 12 hydrogens stuck to 84 oxygens. So this doesn't really exist, but if you follow the rules that we've been going over, you can figure out that water, or when you're merging hydrogen and oxygen atoms, it will very often be two hydrogens to one oxygen. So it helps you figure out how molecules are assembled, let's say. So that's it. There was a lot of stuff, a lot of confessions. I'm gonna go clean myself off because I feel horrible about lying to you, but that's it.