0:02 here's a diagram of an atom it's not
0:05 perfectly accurate but for what we need
0:07 right now it's going to do just fine so
0:09 in this lesson we're going to look at
0:11 characteristics that we can use to
0:14 describe atoms like this one you know if
0:17 somebody asked you to describe a person
0:19 chances are you'd use characteristics
0:20 like the color of their hair the color
0:22 of their eyes what their height is and
0:25 so forth just like that there are
0:28 characteristics we can use to describe
0:31 atoms the first of these characteristics
0:33 and maybe the most important is the
0:36 atomic number atomic number is often
0:39 abbreviated by a capital letter Z and
0:41 what the atomic number is is it's the
0:44 number of protons in the nucleus of an
0:47 atom in my diagram here I'm using these
0:50 red circles to represent protons so I
0:54 have 1 2 three protons in this atom so
0:57 the atomic number is going to be three
1:00 for this atom here atomic number is
1:02 particularly important because the
1:05 number of protons in an atom tell us
1:07 what kind of an atom it is and by that I
1:09 mean is it a carbon atom is it an oxygen
1:13 atom is it a sodium atom we're going to
1:15 talk more about that in the video on
1:16 Isotopes so you don't have to worry
1:17 about it for right
1:20 now the next important characteristic
1:23 that we can use when describing atoms is
1:25 the mass number often abbreviated by an
1:28 uppercase a here mass number is the
1:30 number of protons
1:32 plus the number of neutrons in the
1:35 nucleus of an atom people always get
1:38 confused by mass number here's why
1:41 atomic number is the number of protons
1:44 so everybody always wants mass number to
1:47 be the number of neutrons but it's not
1:49 it's a number of protons and neutrons in
1:52 fact there's not really a name for the
1:54 number of neutrons just the number of
1:56 neutrons in an atom we just don't use it
1:58 that often so anyway for mass number we
2:02 want to count all of these together 1 2
2:06 3 4 5 6 7 the mass number of our atom
2:10 here is going to be seven now finally
2:11 the third characteristic we're going to
2:13 talk about here is probably the most
2:15 complex and that's called net
2:18 charge I don't have a good way to Define
2:20 net charge I mean I don't have a good
2:22 definition so what I'm going to say here
2:25 is it's how protons and electrons
2:28 balance each other out let's talk a
2:30 little bit more about this so charge is
2:33 about how protons and electrons balance
2:35 each other out so protons as you know
2:38 have a positive charge and electrons
2:41 have a negative charge so that means
2:43 that if we have the same number of
2:46 protons and electrons in an atom like I
2:49 have right here three protons and three
2:51 electrons I'm not drawing the neutrons
2:52 here because we're not worried about
2:55 them here we have the same number of
2:58 protons and electrons they're going to
3:02 balance each other out this 3 minus is
3:04 going to balance out the three positive
3:07 so we're going to end up with an atom
3:09 that has a net charge of zero because
3:11 these two things balance out and we can
3:14 call this atom neutral which means that
3:18 it has no net charge now what would
3:21 happen in an atom where we have four
3:24 protons and three electrons we don't
3:26 have the same number of electrons and
3:29 protons anymore so they can't totally
3:31 balance each other other out instead we
3:34 have more protons than electrons so that
3:36 we're going to have a net charge that's
3:37 positive because there's going to be
3:39 some of this positive charge in the
3:41 protons that can't be balanced out by
3:44 the electrons in this case we have one
3:46 more proton than electron so this is
3:50 going to give us a net charge of plus
3:53 one since our atom has a net charge we
3:56 give it a special name we call it an ion
3:59 an ion is any atom that has a net charge
4:00 neutral is what you call it if you don't
4:03 have a charge ion is what you call it if
4:06 you have any charge okay this is where
4:09 we have one more proton than electron
4:11 what about this case here what if we had
4:14 five protons and three electrons in this
4:16 case the charges don't Balan out either
4:19 but now I have two more protons than
4:21 electrons I've got two more protons that
4:22 aren't getting balanced out by these
4:24 electrons so that means that I'm going
4:27 to have a net charge of two plus for
4:30 these two protons and again this is
4:32 going to be an ion because it's got a
4:34 charge now the opposite can happen as
4:37 well here is an example where an atom
4:41 has four electrons and three protons so
4:43 now it's got more electrons and protons
4:46 these three protons can only balance out
4:48 three of the electrons so I have one
4:50 more electron that's not getting
4:54 balanced out that means that this total
4:57 charge is going to be minus one that's a
4:59 charge of this one more electron that I
5:01 have in the the proton and if I had a
5:05 case in which I had five electrons oh
5:09 and I should have said we call this an
5:11 ion because it has a charge just like
5:14 this negative charge positive charge it
5:16 doesn't matter if it's got a charge we
5:19 call it an ion okay so anyway we have a
5:21 case where we have five electrons and
5:24 three protons now there are two more
5:28 electrons and protons so I have two
5:30 negative charges that aren't getting
5:32 balanced out by the proton which means
5:36 that I have a net charge of minus 2 and
5:39 this also is an ion so that's how you
5:41 can figure out net charge so what's the
5:44 net charge for this atom well we have 1
5:56 protons since they're different numbers
5:58 the protons and electrons aren't going
6:00 to be balancing each other out we have
6:03 one more electron than proton so that
6:05 means that we have a net negative charge
6:08 and since there is one more electron
6:11 than proton this atom here is going to
6:14 have a net charge of
6:16 minus1 so now that we've talked about
6:18 these three characteristics you can use
6:20 to describe an atom let's look at the
6:22 practice Problem video so that you can
6:24 go over this and a little bit more depth
