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Mr. Andersen details the history of modern atomic theory.
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Transcript Provided by YouTube:
00:04
Hi. This is Mr. Andersen and today I’m going to be talking about atoms
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and elements. This here is a picture of helium. And if we look at helium right here, you know
00:13
that this is the nucleus here in the center. To give you an idea of scale the nucleus,
00:18
which is going to be the tiny bit down here is actually measured in fentometers. So that
00:23
is 10 to the negative 15th meters. In other words this is really small. The angstrom unit
00:33
here. But atoms are incredibly small. And so in this podcast what I’m going to talk
00:37
about is the history of atoms, how it came to be known that there exists an atom and
00:42
then finally we’ll talk about what are protons, neutrons, electrons. And then how are they
00:47
organized. Okay. So let’s start a little bit with history. And so when we get into the
00:50
modern day science, I love this guy. This right here is Mendeleev. And so what Mendeleev,
00:56
a Russian scientist, was doing is he took playing cards and we would put information
01:02
about each of the known atoms that we have. So maybe the atomic mass, and properties of
01:10
it. He’d write that on the playing cards. And then he just laid them out on a table.
01:14
And what he finally figured out is that you put them in a certain areas or certain order
01:18
according to their atomic number. And kind of what we had was the modern day periodic
01:23
table came out of that. If you really want to waste some time, this is a wonderful website
01:28
to go to. This is periodictable.com. This was developed by a person by the name of Theodore
01:34
Gray. I think he also started mathematica. But the cool thing about it is that you can
01:38
click on certain things. So this carbon for example and what you can get are these wonderful,
01:45
he’s collected all these different atoms, excuse me all these different elements. And
01:49
he has real world examples of it and then a bunch of properties of that as well. And
01:54
so you could waste a lot of time looking at that. And I have an app for this on my iPad
01:58
that is just really really cool. And so I love this picture here, because it shows you
02:03
all the different things. And what you find on here is that there’s all these relationships.
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So if we look for example right here, copper, silver and gold are all organized at the same
02:12
point on here. And so as you start to play around with the periodic table you find all
02:16
this cool stuff. So let’s go back in time a little bit. And so if we go through the
02:20
important people in the history of the atom, in my class I don’t think it’s super important
02:25
that you memorize all of these, but it is important to give us a little bit of context.
02:28
And so let’s first start with this guy. His name is Democritus. And he was greek and so
02:33
what he essentially did is said, imagine if we take everything and then we divide everything
02:38
in half. So let’s say a block of wood. And then we divide it in half again. And we divide
02:42
it in half again, divide it in half again, divide it in half again, divide it in half
02:46
again, divide it in half again, divide it in half again. If we just kept doing that
02:48
over and over and over again, we eventually get something that’s really, really small.
02:53
And that really small thing he called atomos. And that just means, atomos I think that’s
03:00
right, undivisible. In other words we can’t break it down anymore. And so he’s the first
03:04
guy to come up with this idea of atoms. But he wasn’t a scientist. So then we fast forward
03:10
a little bit and this here is, quite a bit actually, this right here is John Dalton.
03:14
And John Dalton was doing studies. So he was measuring the mass of things. And he came
03:19
up with our modern atomic theory. And so the modern atomic theory is this. He knew that
03:24
hydrogen for example, there would be two hydrogens for every one, let’s see, one oxygen. And
03:33
so he thought they were hooked together. They were like the little hooks in them that would
03:38
hook those together. And he knew that you were going to have two hydrogen for every
03:42
one oxygen that you’re going to have. There was a specific ratio. But that first theory
03:47
is that elements are made up of specific atoms and they come together in a certain ratio
03:52
to make a compound. So that sounds a lot like what we think of today. How’s that different
03:56
from our modern theory? Dalton thought they were just these round balls. So they weren’t
04:00
made up of a different things. And so let’s go forward a little bit. So who’s up next?
04:05
This guy right here. His name is JJ Thomson. And JJ Thomson discovered the electron. Let
04:10
me show you how he did that. He used something called a cathode ray tube. So a cathode ray
04:15
tube looks like this. You essentially have gas and then you run electricity through it.
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And what you get is a ray that goes right through the middle of it like that. Now we
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have things in our house, like a CRT, like your television, most of the televisions that
04:31
we have are a cathode ray tube. It’s a little bit different than the new tvs that we have
04:35
today. But essentially what JJ Thomson found is if you held a magnet up here, so if we
04:40
put a magnet right here, what it did is it bent the cathode ray. So it bent that ray
04:45
of electrons. And so he used magnets and he also used electric fields. And what he found
04:51
is that since we’re pushing that, when there is a like charge, he discovered, I think he
04:57
called them corpuscles or something like that. But we call this today the electron. So he
05:02
discovered the electron. What else did he come up with? Well he came up with what we
05:06
call the plum pudding. In other words he thought that atoms, instead of just being that ball
05:11
that Dalton believed is that you had a positive charge inside it. But then you had all these
05:16
negative charges inside here as well. So you had negative electrons that were kind of embedded
05:21
in this positive ball. Okay. So let’s go forward a little bit. And tweak that. So this is Ernest
05:27
Rutherford. Ernest Rutherford is famous for his gold foil experiment. And essentially
05:31
what he did is took a bit of gold, so this is gold foil. And then he shot alpha particles
05:38
out of it. And so alpha particles were shooting out through here and they would travel through
05:43
the gold. He had a little sensor on this side that would sense where the alpha particles
05:48
go. Now what is an alpha particle? Alpha particle is essentially like the, it’s two protons
05:53
and two neutrons. And so it’s the nucleus of a helium. And so what happened is as these
06:00
alpha particles came shooting through, some of them would kind of be turned to the side.
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But occasionally one of these alpha particles would come shooting right back. And when it
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came shooting right back it kind of freaked out Rutherford. And Rutherford said it was
06:14
like shooting a giant shell. Or a bullet at a piece of tissue paper and occasionally one
06:20
of those bullets comes flying back at you. So what did he discovered was the nucleus.
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And he knew that it had a positive charge because those positive alpha particles were
06:31
being shot back. So he discovered the nucleus. And so let’s go to the Rutherford model. Rutherford
06:37
model said that we had a positive charge in the middle and then we had these negative
06:41
electrons going around the outside. And so they went kind of zipping around the outside
06:45
like that. And he had realized that most of that space on the inside was that. It was
06:49
mostly just space with a tiny little nucleus. Because just a few of those alpha particles
06:54
actually came shooting back out. Alright. Let’s go next. Next, this is Niels Bohr. Niels
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Bohr, let’s circle him down here, Niels Bohr is Danish. He works in Rutherford’s lab. And
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the problem with Rutherford’s lab is that they discovered this positive charge on the
07:10
inside and this idea that you have electrons going around the outside. But physics predicts
07:15
that if you have a negative charge here and a positive charge out here, that quickly that
07:20
negative charge is going to be attracted to it and so the whole Rutherford model didn’t
07:23
really work. And so what did Bohr figure out? Bohr figured out that we’ve got this positive
07:29
charge on the inside. We’ve got electrons going around the outside. But they go in specific
07:34
orbits around the outside. So those electrons were not traveling here, but they had to travel
07:38
in this specific orbit. And so that’s the Bohr model. That you got a positive nucleus
07:43
and then electrons going around the outside in specific obits. Okay. Let’s go next. Next
07:48
we get into the weird world of quantum physics. And so this right here is a man by the name
07:53
of Schrodinger. Schrodinger is famous for Schrodinger’s cat, if you’ve ever heard of
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that. And so what Schrodinger said is that electrons aren’t particles. That they’re actually
08:03
waves. And so there’s this duality. And so now we had this kind of competition between
08:08
those people like Bohr who said electrons are particles. Schrodinger said it was more
08:12
like a wave. And so we had to sort out this. And so we eventually get to Heisenberg. But
08:18
first we’ve got this guy. Chadwick. And what Chadwick discovers is the neutron. It’s weird
08:24
but it took us way longer to discover the neutron. That’s because it has no charge.
08:28
But it accounted for that mass. And then finally we have his guy. This is Werner Heisenberg.
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And why Heisenberg figured out is that both Rutherford, excuse me. Both Bohr and Schrodinger
08:43
were right. In other words what does an atom look like? It’s got a positive charge which
08:48
we now know neutrons on the inside. But there’s kind of this wave of probability out here.
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And so is an electron a particle? Yeah. But it’s described by Schrodinger’s idea of a
09:00
wave. So we got pretty deep. But what is this? This is this quantum theory that we have today.
09:06
It describes how electrons move.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video.