So the atomic number is symbolized by Z and it refers to the number of protons in a nucleus. And you can find the atomic number on the periodic table. So we're going to talk about hydrogen in this video. So for hydrogen, hydrogen's atomic number is one. So it's right here, so there's one proton in the nucleus of a hydrogen atom. In a neutral atom, the number of protons is equal to the number of electrons, because in a neutral atom there's no overall charge and the positive charges of the protons completely balance with the negative charges of the electrons. So let's go ahead and draw an atom of hydrogen. We know the atomic number of hydrogen is one, so there's one proton in the nucleus. So there's my one proton in the nucleus, and we're talking about a neutral hydrogen atom, so there's one electron. I'm going to draw that one electron somewhere outside the nucleus and I'm going to use the oversimplified Bohr model. So this isn't actually what an atom looks like, but it's a very simply view that helps you get started. So this is one, this one version of hydrogen. This is one isotope of hydrogen. So this is called protium. Let me go ahead and write that here. So this is protium and let's talk about isotopes. An isotope, isotopes are atoms of a single element. So we're talking about hydrogen here. That differ in the number of neutrons in their nuclei. So let's talk about the next isotope of hydrogen. So this is called deuteriums. Let me go ahead and write deuterium here. Deuterium is hydrogen, so it must have one proton in the nucleus and it must have one electron outside the nucleus, but if you look at the definition for isotopes, atoms of a single element that differ in the number of neutrons, protium has zero neutrons in the nucleus. Deuterium has one. So let me go ahead and draw in deuterium's one neutron. I'll use blue here, so neutrons are going to be blue. So deuterium has one neutron and since neutrons have mass, deuterium has more mass than protium. So isotopes have different masses because they differ in terms of number of neutrons. Notice though, that they have the same atomic number, they have the same number of protons in the nucleus. Right, it's one proton in the nucleus. And that's important because if you change the number of protons, you're changing the element, and that's not what we're doing here. We're talking about atoms of a single element. Deuterium is still hydrogen, it's an isotope. Finally, our last isotope, which is tritium. So tritium has one proton in the nucleus, one electron outside the nucleus, and we draw that in here, and it must differ in terms of number of neutrons, so tritium has two neutrons. Let me go ahead and draw the two neutrons here in the nucleus. And so those are the isotopes of hydrogen. How do we distinguish between the different isotopes? Well we're going to write little symbols to represent these isotopes. And so the symbol that we'll draw here for protium is going to have the element symbol, which is, of course, hydrogen, and then down here we're going to write the atomic number. So the subscript is the atomic number which is one, because there's one proton in the nucleus, and then for the superscript, we're going to write in the mass number. So let me move down here so we can look at the definition for the mass number. The mass number is the combined number of protons and neutrons in a nucleus, so it's protons and neutrons, and it's symbolized by A. So A is the mass number, which is equal to the number of protons, that's the atomic number which we symbolized by Z, plus the number of neutrons. So A is equal to Z plus N. And for protium, let's look at protium here. So in the nucleus there's only one proton and zero neutrons, so one plus zero gives us a mass number of one. And I'll use red here for mass number so we can distinguish. Alright, so mass number is red and let me use a different color here for the atomic number. Let me use magenta here. So the subscript is the atomic number and that's Z, and the superscript is the mass number and that's A. So this symbol represents the protium isotope. Let's draw one for deuterium. So it's hydrogen so we put an H here. There is still one proton in the nucleus, right one proton in the nucleus, so we put an atomic number of one. The mass number is the superscript, it's the combined number of protons and neutrons. So we look in the nucleus here. There's one proton and one neutron. So one plus one is equal to two. So we put a two here for the superscript. And finally for tritium, it's still hydrogen. So we put hydrogen here. There's one proton in the nucleus, atomic number of one, so we put a one here. And then the combined numbers of protons and neutrons, that would be three. So one proton plus two neutrons gives us three. So there's the symbol for tritium. So here are the isotopes of hydrogen and using these symbols allows us to differentiate between them. So let's take what we've learned and do a few more practice problems here. So let's look at a symbol for carbon. So here we have carbon with subscript six, superscript 12. And if we want to know how many protons, electrons and neutrons there are. So let's first think about protons. Well we know that the subscript is the atomic number and the atomic number is equal to the number of protons. So there are six protons in this atom of carbon. And if it's a neutral atom of carbon, the number of electrons must be equal to the number of protons. So if there are six protons, there must also be six electrons. And finally, how do we figure out the number of neutrons? Well let's go ahead and write down the formula we discussed. The mass number is equal to the atomic number plus the number of neutrons. So the mass number was right here, that's 12. So we can put in a 12. The atomic number was six, right here. So we put in a six. Plus the number of neutrons. Plus the number of neutrons. So the number of neutrons is just equal to 12 minus six, which is, of course, six. So there are six neutrons. So just subtract the atomic number from the mass number and you'll get the number of neutrons in your atom. Let's do another one. This is carbon and this time we have a superscript of 13. The atomic number doesn't change when you're talking about an isotope. If you change the atomic number, you change the element. So there's still six protons in the nucleus of this atom and in a neutral atom, there must be the equal number of electrons. So six electrons and then finally, how many neutrons are there? Well just like we did before, we subtract the atomic number from the mass number. So we just have to 13 minus six to figure out the number of neutrons. So 13 minus six is, of course, seven. So there are seven neutrons in this atom. Another way to represent isotopes, let's say we wanted to represent this isotope in a different way, sometimes you'll see it where you write the name of the element. So this is carbon. And then you put a hyphen here and then you put the mass number. So carbon hyphen 13 refers to this isotope of carbon and this is called hyphen notation. So let me go ahead and write this hyphen notation. Alright, let's do one more example here. Let's do one that looks a little bit scarier. So let's do uranium. So U is uranium. The atomic number of uranium is 92. The mass number for this isotope is 235. So how many protons, electrons, and neutrons in this atom of uranium? So once again for protons, we look at the atomic number, that's 92. So there must be 92 protons. In a neutral atom, the number of electrons is equal to the number of protons. So there are 92 electrons and then finally, to figure out the number of neutrons, we subtract this number from the mass number. So we just need to do 235 minus 92. And that gives us 143. So there are 143 neutrons.