Atoms and Elements, Part 2 of 6: What makes an atom itself

Atoms and Elements, Part 2 of 6: What makes an atom itself


All right, so what we talked about in the
last one is what is an atom made up of right? And we decided that an atom up was
made up of protons, neutrons, and electrons. But what we didn’t talk about
is which one of those defines an atom? Is it all of them? I mean is it just the
mixture of protons, neutrons, electrons that says this one’s calcium, this one’s
carbon? Or is there another way to it? We talked about how they’re organized, a
small nucleus containing … what is it? Yeah, the protons and the neutrons and
the electrons really far out, like really really, really, really, really far out
there. Okay so we’re gonna talk about in this one is what makes an atom itself?
How do we know an atom from one element versus an atom from another element? And
it turns out again long story short, what we figured out is that it’s the number
of protons that determines an atom. So if we want to know whether we have
carbon, nitrogen, hydrogen, or whatever like that, the only thing we have to look
at is the number of protons. Okay how do we know? Well if we start from the
beginning of the periodic table, so let’s go to the top left here. And we look at
hydrogen up there it turns out hydrogen’s the first element on the
periodic table for a good reason because it has a single proton. So hydrogen has a
single proton. So hydrogen is one proton. Okay and turns out any element with only
one proton is going to be hydrogen. Doesn’t matter what else you put on
there neutrons and electrons, you’re still going to have hydrogen. Now those
neutrons and electrons those are gonna do things, they’re gonna be important coming
up but they’re not what defines the atom. If we go to the next element on the
periodic table, if you look at your periodic table, by the way in the class
it’s always a good idea to have a periodic table on hand. Print one out,
have it available for yourself. It’s got two protons. And anything that has two
protons is always going to be helium. If you’ve got one proton you’re hydrogen or
two protons you’re helium. So let’s go back to our periodic table. And if you
look at the periodic table, might be a little hard to see on this one. If you
look at hydrogen the top left of the periodic table there’s a little one in
the top left here. We get helium there’s, which is on the right-hand side of the
periodic, table there’s a little two in the top left. What is that telling us? Well it turns out those numbers in the top left are
telling us how many protons are in that element. Or, conversely, if we know how
many protons we have we can look up our element. So again have a periodic table
on hand. Try to find which element has six protons. So you go along the periodic
table. Hydrogen, helium, lithium, beryllium, right. Lithium has three, beryllium has
four, boron has five, and carbon has six. So anytime you’ve got six protons you
know you’ve got yourself a carbon atom. Again we’ll find out we can have more
neutrons, less neutrons, more electrons less electrons, it’s still carbon. And
that’s what distinguishes one type of atom from another type of atom. So let’s
just do one more. Let’s look down in that yellow section. The first thing in the yellow section is an Sc which stands for scandium. How many
protons does it have? Hopefully you came up with the number 21. It’s got 21
protons. So that’s what we use the periodic table for. So it turns out
it’s really, really important to know how many protons there are, in whatever
you’re doing, because it tells you how many atoms. you have. Again if you have
two protons you’ve got a helium, if you’ve got six protons you’ve got your
calcium (should have said Carbon)so the counting is very important we need to keep track of the
number of protons. And if you look at my words here it says you keep track of the
number of neutrons because that’s important as well. Well, if the neutrons
don’t matter for identity why does it matter? Cuz it matters for how big
and heavy the atom is. So if you have something, right, the neutrons weigh
about the same as the protons we said they were about the same size. So let’s
say we have carbon, six protons, let’s say we had one with six neutrons it’s gonna
weigh a certain amount but if we start adding neutrons… seven neutrons, eight
neutrons, it’s going to get heavier. And how heavy something is does make a lot
of difference for how it works. Okay so it doesn’t change it from carbon but it
does change its mass. So it’s important to keep track of that. Turns out it’s
also really important to keep track of electrons and we’ll get to why later on
in the class. But we’re also gonna need to keep track of electrons. So how do we
know? Hey well we saw that protons were given on the periodic table, and we call
that atomic number. So the number of protons is the atomic number which is
sometimes abbreviated Z. So I’m gonna use carbon as an example here … we’re gonna
write down its elemental symbol C. Remember there’s no letters after that,
some things just have one letter. So carbon right there is C. Now, in order to
keep track of things, we have a kind of standard notation that we use. And the
standard notation is to always put the number of protons in the lower left
corner. And so we’re going to write a 6 down there in the lower left corner to say
that carbon has six protons. Now sometimes people will leave that off
because you can always get it from the periodic table to know that carbon has 6
protons. But sometimes people will put it there. Now how many electrons do we have?
Well it turns out the electrons in what we call a neutral atom, and we’ll
discuss what that means later on class, same number as the protons. If you
remember, protons were positive, electrons were negative. Nature loves balance and
so the number of protons and the number of electrons are going to be equal. We’re
not going to put that anywhere in these symbols. Okay, if we have an equal number
of electrons nothing appears we’ll see what happens when we have an unequal
number later on. Now it turns out even though we need to know the number of
neutrons nobody tells you that directly. Oh, what a pain! They only give you that
number indirectly. And they give you that by putting a number in the top left here
which is called the mass number. Now what is the mass number? The mass number is
the number of protons plus the number of neutrons. And so I have a mass number of
12. That’s there, we know that there’s six protons because we already said that all
carbon atoms have six protons. And so we’re just trying to do is figure out
here how many neutrons do we have? Simple right? 12 equals six plus what? And we get
that the number of neutrons is just 12 minus 6 or six. So in this particular
case for the symbol I wrote down, I say this is carbon and it’s got a 12 up in
the top left it’s got a six in the lower left and that means it has six protons
and six neutrons. Now, that’s kind of hard to say that symbol out loud with the 12
in the top left and the six on the bottom left, so we have a way of saying
this. And if you were to try to say this out loud you just say this is carbon-12.
We put a dash there when we write it but we don’t say the dash. So carbon-12 that is
a dash not an underscore. Okay, so this is carbon-12. Now how come we don’t tell you in this that there’s six protons? Because like I said we can figure that
information out in other ways, so we don’t need that. We can look at a
periodic table. All righty, so what if I had a different element on the periodic
table? So let’s go down and say pick calcium. Find calcium on the periodic
table. It’s an alkaline earth metal. Remember
those ones in the second group? And it’s number 20. So we’re gonna take calcium
and write that down there, and it’s number 20. Where do I put the 20 on the
top or on the bottom? Right. Number of protons goes on the
bottom. And then I might want to know how many neutrons are in here. But again
nobody tells me directly, so I’m gonna write 42 here. And I’m just making that
number up just for an example. We’ll figure out ways of how do you know that
somebody has to tell you that information. Okay, so how many protons… sorry… how many neutrons are in this example? Well we use the same math up there. I’m
not going to do all the steps, but we end up with basically our mass number minus
our atomic number, and we figure out that that’s 22 neutrons in this guy. Now I’ve
got to turn myself off. Here 22 neutrons in this guy because 42- 20=22.
Now another similar type of problem is what would be the elemental symbol for,
let’s say, nitrogen with eight neutrons? Okay, so how would we do that? Our
periodic table is always our friend so let’s go over to the periodic table and
we’re going to find nitrogen on the periodic table. Now you can’t read this one very well but hopefully you have a paper one in front
of you. If you look along the second row of the periodic table you see that N in
the middle there, and that is nitrogen. So we’re gonna go back and now we’ve got
nitrogen there. And we want the elemental symbol for nitrogen with eight neutrons.
Well we need to know how many protons it has. We go back to our periodic table.
Nitrogen up in the middle there has a seven in its top left and by the way
that top left some periodic tables put it in different places. It’s whatever the
whole number is. Okay, it’s not the decimal number that you’re looking for.
Okay, so seven protons where does that go top or bottom? You’re right it goes on
the bottom. Now I’ve got eight neutrons so it’s tempting just to put an eight up
there. But that is not correct because remember, what goes up there is not the
number of neutrons, but the number of protons plus the number of neutrons. And
we know we have seven protons and we know we have eight neutrons because
someone told us. For a total of 15. So I’m gonna put a 15 up
there. And how would we say that out loud? We’d call it nitrogen-15. All right, so
that’s what we do when we’re writing elemental symbols. We use protons and
neutrons as our mass number up on top. And we use our atomic number on the
bottom. Now why do we keep track of the neutrons can’t we just say well you know
it’s carbon it’s got this many neutrons? Well the answer is no, because it turns
out that there’s most elements on the periodic table don’t just have one type
of atom they have several. They all have the same number of protons right because
we wouldn’t have bromine if we didn’t have 35 protons. But it turns out there’s
different ways of making bromine with different numbers of neutrons. So let’s
just do a quick example here just to refresh ourselves. In this top one how
many neutrons would I have? 35? 79? You know right 35 is the number of protons. This
one is the number of protons plus the number of neutrons and so we’ve got 79.
Number of protons is 35 plus the number of neutrons so the number of
neutrons is 79 – 35, which is just equal to 44 neutrons. All right and we
call that element bromine-79. All right I’ve got two of them down here why do
I have two of them? Cuz like I said, for most elements on the periodic table
there’s many different ways to form it. How do you know which one to form?Someone will always tell you. Okay ,this is not something you need to know off
the top of your head. We will always be given information that will allow you to
know which one you’re talking about. So let’s look at the one on the left here.
We’ve got 127, 53, and I. So go to your handy-dandy periodic table look at that,
that’s iodine. Okay, and how many protons in iodine? All right, 53. All right, number
of protons is 53. How about the number of neutrons? Well, number of neutrons,
I’m gonna skip a few steps here because we’ve done this a few times now, is gonna
be 127 – 53 which is going to be 74 neutrons. But it turns out you can also
make iodine-126 which is going to have, what, 73 neutrons. And again you’ll always
be given that information. So we have iodine-127 then left we have iodine-126
on the right. Two different forms, both of iodine, and we call these isotopes. Then what are isotopes? They are the same element with different numbers … numbers of neutrons. Okay, so on the Left
we notice we had 74 neutrons. On the right we had 73 neutrons. So we ended up
with an isotope. Same element different number of neutrons. All righty, so a
couple examples for you to try. First one. How many protons are in potassium-39? So take a moment trying to answer that one. All righty, what did you
come up with? Well we need to go over to our handy-dandy periodic table and find
potassium. Potassium is one of those odd ones it’s K. It’s got 19 protons. All
right, all right, oh wait! That’s what the question asked for. So right, we just get
that straight off the periodic table that K has a 19 on it. And so we get 19
protons and that is our answer to that question. So might have tricked you because we
were doing neutron, neutron, neutron then we did proton. That was kind of the point.
You just kind of pay attention to the question and we’re finding number of
protons. There, all right, what do we do next? We do how many neutrons are in zinc-65? All right, go over, find your handy dandy periodic table and see if you can come
up with the answer to that. All righty, Zinc. if you look on your periodic
table the Zn. Did you find Zn on the periodic table?
Let’s go looking. Okay, we see Zn over there. It’s the top right of the yellow
ones, and it’s got a 30 on it. So we’re gonna put a 30 down here. We always put
the atomic number on the lower part. We’re going to put a 65 up here. And the
nice thing is if you look at this, it’s kind of set up for you like a
subtraction problem. already. We take 65 minus 30 we get 35 as our number of
neutrons. And our answer right there is 35. All right, so that’s how we count
protons, neutrons, and electrons. The periodic table tells us protons. Neutrons,
we have to be given that information somehow, either directly or by giving the
mass number. And electrons for now is equal to the number of protons. So
the zinc one here that we have an example of since it has 30 protons has
30 electrons. Thank you so much.