here's a diagram of an atom it's not perfectly accurate but for what we need right now it's going to do just fine so in this lesson we're going to look at characteristics that we can use to describe atoms like this one you know if somebody asked you to describe a person chances are you'd use characteristics like the color of their hair the color of their eyes what their height is and so forth just like that there are characteristics we can use to describe atoms the first of these characteristics and maybe the most important is the atomic number atomic number is often abbreviated by a capital letter Z and what the atomic number is is it's the number of protons in the nucleus of an atom in my diagram here I'm using these red circles to represent protons so I have 1 2 three protons in this atom so the atomic number is going to be three for this atom here atomic number is particularly important because the number of protons in an atom tell us what kind of an atom it is and by that I mean is it a carbon atom is it an oxygen atom is it a sodium atom we're going to talk more about that in the video on Isotopes so you don't have to worry about it for right now the next important characteristic that we can use when describing atoms is the mass number often abbreviated by an uppercase a here mass number is the number of protons plus the number of neutrons in the nucleus of an atom people always get confused by mass number here's why atomic number is the number of protons so everybody always wants mass number to be the number of neutrons but it's not it's a number of protons and neutrons in fact there's not really a name for the number of neutrons just the number of neutrons in an atom we just don't use it that often so anyway for mass number we want to count all of these together 1 2 3 4 5 6 7 the mass number of our atom here is going to be seven now finally the third characteristic we're going to talk about here is probably the most complex and that's called net charge I don't have a good way to Define net charge I mean I don't have a good definition so what I'm going to say here is it's how protons and electrons balance each other out let's talk a little bit more about this so charge is about how protons and electrons balance each other out so protons as you know have a positive charge and electrons have a negative charge so that means that if we have the same number of protons and electrons in an atom like I have right here three protons and three electrons I'm not drawing the neutrons here because we're not worried about them here we have the same number of protons and electrons they're going to balance each other out this 3 minus is going to balance out the three positive so we're going to end up with an atom that has a net charge of zero because these two things balance out and we can call this atom neutral which means that it has no net charge now what would happen in an atom where we have four protons and three electrons we don't have the same number of electrons and protons anymore so they can't totally balance each other other out instead we have more protons than electrons so that we're going to have a net charge that's positive because there's going to be some of this positive charge in the protons that can't be balanced out by the electrons in this case we have one more proton than electron so this is going to give us a net charge of plus one since our atom has a net charge we give it a special name we call it an ion an ion is any atom that has a net charge neutral is what you call it if you don't have a charge ion is what you call it if you have any charge okay this is where we have one more proton than electron what about this case here what if we had five protons and three electrons in this case the charges don't Balan out either but now I have two more protons than electrons I've got two more protons that aren't getting balanced out by these electrons so that means that I'm going to have a net charge of two plus for these two protons and again this is going to be an ion because it's got a charge now the opposite can happen as well here is an example where an atom has four electrons and three protons so now it's got more electrons and protons these three protons can only balance out three of the electrons so I have one more electron that's not getting balanced out that means that this total charge is going to be minus one that's a charge of this one more electron that I have in the the proton and if I had a case in which I had five electrons oh and I should have said we call this an ion because it has a charge just like this negative charge positive charge it doesn't matter if it's got a charge we call it an ion okay so anyway we have a case where we have five electrons and three protons now there are two more electrons and protons so I have two negative charges that aren't getting balanced out by the proton which means that I have a net charge of minus 2 and this also is an ion so that's how you can figure out net charge so what's the net charge for this atom well we have 1 2 3 four electrons and we have 1 2 3 protons since they're different numbers the protons and electrons aren't going to be balancing each other out we have one more electron than proton so that means that we have a net negative charge and since there is one more electron than proton this atom here is going to have a net charge of minus1 so now that we've talked about these three characteristics you can use to describe an atom let's look at the practice Problem video so that you can go over this and a little bit more depth and try a bunch of examples