 All right, we have spent a lot of time talking about protons and neutrons, which are two out of the three particles that atoms are made of. What we're going to talk about now for a fairly long time is the third particle, also known as the electron. Just a refresher, the protons I am going to draw as these big red circles. I said in an earlier video that they have something called an electrical charge, and the type of electrical charge they have is called a positive electrical charge, and I'm going to represent that with a little plus sign. Usually what people say is that the proton has an electrical charge of plus one, and the reason they pick one is because it's an easy number to remember, and they want to use that to contrast it with the electrical charge of the electron, and the electron has an electrical charge that is exactly the opposite of the charge of a proton, so the electrical charge of an electron is negative one. That's something that you should memorize. The other thing to point out is that if I have one proton, so this is my cartoon version of a proton, and I have one electron, and I'm too lazy to make this blue, but it has the opposite electrical charge. If I think of the two of these things being together as one little unit altogether, the total electrical charge of both of them together is zero because they cancel each other out. If I have extra protons, so imagine I had two protons and a single electron, and I treated this whole thing as one little group, then the electrical charge is not zero anymore because even though one of the protons and one of the electrons cancel each other out, I have an extra proton here or an extra plus charge, and because of that, this little group that I'm circling has an electrical charge of plus one, and you can do this the other way as well. If I had one proton and two electrons, and I treated this whole thing as a group, then the electrical charge of that entire group is negative one because only one proton and one electron cancel each other out, and I have an extra negative charge, and so my entire group that I'm circling here has an effective electrical charge of negative one. So keep that in mind. I'm going to ask a bunch of questions related to that idea going forward. Okay, so let's look back at this atom. This is some pretend atom. It probably doesn't really exist in real life. It looks really unstable, probably to the trained eye. First question I'm going to ask is, what type of atom is it? You're going to have to get your periodic table to answer that question, or you may have to get it. You may have already committed this to memory, but if you want to figure out what type of atom you have, you look at the number of protons here. We have two protons because we have two red circles, and the atom that has two protons in it is helium, so I'm going to write the symbol for helium. This is capital H, lowercase e. If I wanted to describe approximately how much this atom weighs, we would count the number of protons and the number of neutrons, so the red circles and the gray circles. It turns out that there's three of them. So this atom, this helium atom, has an approximate mass of about three atomic mass units or approximately three dolens. And if we wanted to write that somewhere next to the symbol, we would write it in the upper left. So this is just a rehash of some of the other stuff that we went over in the videos, but now let's start with something new. So let me change colors. Okay, we're going to talk about the electrons at this point and how they relate to the protons. So the first question is, what's the total electrical charge on this atom? And the way that you have to answer this question is you have to count up the number of positive charges. This proton has a plus charge, this proton has a plus charge. You also have to count up the negative charges. This one has a negative, this one has a negative. So we have three negatives and two positives. Three negatives, two positives, and so two of the negatives will cancel out, two of the positives, but we will have one extra negative charge. So what you say is that this atom, this helium atom, has an electrical charge of negative one, because it has one extra negative charge, because it has three electrons, one, two, three, and only two protons. So this atom here, this helium atom, it has an electrical charge. Another way of saying that is that it is not electrically neutral. If something is electrically neutral, that means it has no electrical charge or a charge of zero. But this guy has a charge of negative one. The place that we would write this when we were writing with symbols and putting numbers in different places around the symbol is we would write the electrical charge here. So if I wanted to, actually I think it's supposed to be the other way, people traditionally write the number one and a minus sign. And that tells people that we are talking about a helium atom that has a charge of negative one, even though it's written a little bit backwards there. And we're putting the three here that tells people the mass number or the approximate mass of our helium atom. Another thing that people do sometimes, if the charge is only one, either negative or positive, is they will skip the number. So I could have written HE3 because the mass number is three. And instead of writing one minus, I can just write a minus. Sometimes people will write it that way. And this is just another way of saying that my helium atom has an electrical charge of negative one. So keep that in mind. We're going to do a bunch of other examples with respect to this. And what we're basically doing is we are talking about the different electrical charges that atoms can acquire, or the different electrical charges that atoms can exist under. And it's going to turn out that this is important for controlling how different atoms can stick to each other. So here, here's a new atom. I'm going to ask a bunch of questions and you can pause the video and try to answer these questions. First, before we go on to this question down here, is what type of atom is this? Is it what's the mass number? And then what's the total electrical charge of this atom? So you can pause the video. You're going to need a periodic table if you haven't committed this stuff to memory. All right, unpausing. This guy has two protons, two red circles. So it's got to be a helium. The answer to the first question is helium. What's the mass number of this atom? Well, it's got two protons. It also has two neutrons. And the mass number is just the number of protons plus the number of neutrons. So the mass number is four. So our atom here weighs about 4 AMU. And we can put a little four in the upper left with respect to the symbol. Last question is, what's the total electrical charge on this atom? Well, we've got two positive charges. We also have two negative charges. So two pluses, two minuses, those exactly cancel each other out. So the charge, the electrical charge of this atom, is zero. Another way of saying that is our helium, can I spell helium? Apparently not. Atom is electrically neutral. So I could put a zero up here to tell people that it's neutral. Usually when the electrical charge is zero, people don't write anything. So if you don't see anything written up there, the implication is that the atom has an electrical charge of zero. So that's just one other exercise. You can look at these cartoons and you can figure out what the electrical charge of the atom is. Let's see. What about this atom here? You can see that this is still a helium, two protons. It still has a mass number of four. So it weighs about 4 AMU. And what's the total electrical charge here? You can pause the video, I guess. It's got two plus charges and it's got one minus charge. So two pluses, one negative charge. So they don't cancel each other out perfectly. We have one extra plus charge. And because of that, our helium atom has an electrical charge of plus one. And the way that people write this, there's usually two different ways that people write this. They will either write one plus in the upper right of the symbol or you can just write a plus sign if the electrical charge is only plus one. This is only true if the charge is plus one or minus one. If the charge is plus two or plus three or minus two or minus three, then you actually have to write the number as well. So that's the electrical charge of that helium atom. Here's a different one. This time I took away the other electron. Now this helium, it's still a helium because it's got two protons, has a mass number of four. But now it doesn't have any electrons. Zero electrons, I want to know what the electrical charge is. Well, we've got two pluses and zero negatives. What that means is we have two extra positive charges. So our helium atom has an electrical charge of plus two. And the way that that would be written is two with a plus sign. And this time you can't just write a plus because you have to tell people that there's two. So this is a simplified way of telling people what this cartoon is telling you. Our helium atom has an electrical charge of plus two. So now you should have a reasonable sense of how to figure out the electrical charge of different atoms if you know how many protons and how many electrons they have. And just to summarize that, you can figure out the electrical charge of an atom by counting the protons, counting the electrons, and taking the difference. So sometimes atoms can be positively charged. And sometimes they can be neutral. And sometimes they can be negatively charged. And the only way that you can figure that out is by figuring out how many protons and electrons they have. Let's see. There's a special term that you need to know. This actually gets used a lot out there in biology, I guess. Any atom or group of atoms that are stuck to each other that has an electrical charge is called an ion. So you definitely need to know this word. So if I have helium four and the charge is plus two, this thing is an ion because it's an atom that has an electrical charge. And so that's the definition of an atom. Our helium, an earlier helium, I think it had a mass number of three, one of the heliums that we did, we said it had no electrical charge. This is not an ion because it doesn't have an electrical charge. You either have to have a charge, a positive electrical charge, or a negative electrical charge. And then you can call that atom an ion. If you don't have any electrical charge, if you're neutral, then you're not an ion. Keep that in mind. Then there's two other terms that are coming up. If you have an atom with a positive electrical charge or a group of atoms, it's called a cation. So this guy over here that I'm circling, that's a cation because it has a charge of plus, of positive two. Then the other type, if you have an atom or group of atoms that have a negative electrical charge, they're called anions. So if I had a helium and the charge was minus four, I'm not sure. Yeah, I guess that's possible. Probably not realistic. But if I had a helium atom with an electrical charge of minus four, it's going to be called an anion. So there are ions. Those are atoms or groups of atoms that are charged. And then you can break them into two categories, cations, which have positive charge, and anions, which have negative charge. You should commit those three terms to memory, ions, cations, and anions. If it helps you at all, a long time ago, somebody told me the way to remember cations is the T in cation looks like a plus. So if that helps you remember it, good for you. But that is the beginning of the introduction of electrons and charged atoms, which are called ions.