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Estrutura Atômica II Modelo Atômico de Thomson - Descoberta do elétron — Transcript

by Aprendendo Química - Prof. Alexandre Oliveira · 2,084 words · 321 segments · language en · Watch on YouTube

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0:10So, we saw that Dalton's atomic model
0:11was in 1803. The second atomic model
0:15would take quite a while to appear. It
0:19would only emerge from experiments
0:21carried out in Crookes tubes by Thomson
0:24. Let's talk a little bit about that
0:28before we start discussing the second
0:30atomic model, which will be the plum
0:33pudding model, Thomson's model. But
0:37first, let's see what happened
0:39previously so that Thomson could arrive
0:42at his atomic model. So we have a
0:45physicist here, Crookes, who performed
0:48some experiments in this tube that
0:51became known as the Crookes tube. What
0:54is a Crookes tube? It is a glass tube,
0:56as you can see there. And he created a
1:00vacuum, a gas under low pressure. The
1:03pressure here is on the order of 0.01
1:06atm. So, he placed a vacuum pump on
1:10this glass tube, so that only a
1:12residual gas remained inside, that is,
1:15a gas under low pressure. And he
1:19applied a very high potential
1:22difference here, on the order of 20,000
1:25V. When he applied this 20,000 V
1:27potential difference, he observed a
1:30light beam going from the cathode to
1:32the anode. And be careful, we will
1:37study electrochemistry, batteries, and
1:40electrolysis in physical chemistry in
1:43the second semester. And we will learn
1:45that the signs of the cathode and anode
1:47in batteries and electrolysis are
1:49opposite. The cathode in a battery is
1:52the positive pole. The anode in a
1:54battery is the negative pole. However,
1:57in electrolysis, it is the opposite.
1:59The cathode in electrolysis is the
2:01negative pole. And, excuse me, the
2:04cathode in electrolysis is the negative
2:06pole and the anode in electrolysis is
2:08the positive pole. The Crookes tube is
2:11not a spontaneous phenomenon; it is not
2:13a battery. We have a potential
2:16difference, a battery providing a
2:19potential difference. So here in the
2:22Crookes tube, you use the notation of
2:25electrolysis. Therefore, the cathode is
2:28the negative pole of the field and the
2:31anode is the positive pole of the field
2:34. I say this because many of my
2:35students who have already studied
2:37batteries and electrolysis say: "
2:39Alexandre, but isn't the cathode the
2:41positive pole?" I say, wait, the
2:43cathode is the positive pole of a
2:45battery. Here you use the notation of
2:47electrolysis, as this is a
2:49non-spontaneous phenomenon. So, the
2:51cathode is the negative pole in the
2:54Crookes tube and the anode is positive.
2:58Hence the name cathode rays, because
3:01these rays traveled from the cathode
3:04toward the anode. In other words, they
3:08left the negative pole and headed
3:11toward the positive pole. You will see
3:16that Thomson discovers that these
3:18cathode rays are, in fact, electrons.
3:22That is why we say that the one who
3:24discovered the first subatomic particle
3:27, which was the electron, was Thomson.
3:30But look, Crookes, when working with
3:33these tubes, was already working with
3:35cathode rays. And what are cathode rays
3:38? They are electrons. But it wasn't
3:41Crookes who discovered electrons,
3:43because he didn't say: "Look, cathode
3:46rays are a fundamental particle of
3:48matter.""They are present in any type
3:51of matter and, therefore, the atom is
3:53divisible.""This is a subatomic
3:54particle," and he could have given it
3:56any name, which Thomson will call
3:58electrons. So, Crookes worked with
4:01cathode rays, he worked with electrons,
4:04but he did not discover electrons. The
4:07one who will do that is Thomson, as I
4:09am about to narrate to you now. So, do
4:11you understand what a Crookes tube is?
4:14It is a glass tube with residual gas,
4:17that is, gas under very low pressure,
4:200.01 atm, where he applies an electric
4:23field, a very large potential
4:26difference, on the order of 20,000 V,
4:29and a ray emerges from the cathode to
4:32the anode, called cathode rays. That
4:37was the only thing Crookes did, right,
4:40that he managed to deduce. But Thomson,
4:45taking advantage of what Crookes
4:47discovered, performed some more
4:49sophisticated experiments to study the
4:52nature of these cathode rays. What was
4:56that? What was this luminous beam that
4:59came out of the negative pole of the
5:01cathode toward the positive pole of the
5:03anode, which we were calling cathode
5:06rays? So, in 1887, remember that
5:09Dalton's atomic model was in 1803. And
5:13here we have 1887, 84 years later,
5:16Thomson took Crookes 'tubes and
5:18performed some very interesting
5:21experiments to understand what these
5:24cathode rays were. Okay? First, Thomson
5:28discovered that the rays, the cathode
5:30rays had mass. Why? Because when he
5:34took a Crookes tube and made some
5:37modifications, for example, placing a
5:39small pinwheel here, let's say, in the
5:42direction of the cathode, between the
5:44cathode and the anode, he observed that
5:46this pinwheel began to spin. So, why
5:50did it start to spin? Because these
5:54cathode rays that were coming out of
5:56the cathode, upon colliding with this
5:59small pinwheel, made this pinwheel spin
6:01on its path from the cathode, which is
6:04the negative pole, to the anode, which
6:06is the positive pole. So, the first
6:11postulate or first important deduction
6:14by Thomson is that cathode rays have
6:17mass, so they are particles; they have
6:20a particle nature. Second ingenious
6:24modification that Thomson made, right?
6:29He took the Crookes tube and placed a
6:32screen in front of the cathode. And he
6:36observed that the cathode rays traveled
6:39in a straight line. Why did they travel
6:42in a straight line? Because they caused
6:46, they generated a shadow of that
6:48screen placed in front of the cathode,
6:50on the opposite wall of the tube. Look
6:54at the important deductions Thomson was
6:56reaching. The first is that cathode
6:59rays have mass because they were
7:01spinning that little pinwheel. The
7:05second is that cathode rays travel in a
7:08straight line, because by placing any
7:11screen in front of the cathode, which
7:14is the negative pole of this electric
7:16field, it cast a shadow of this screen
7:19on the opposite wall of the Crookes
7:22tube, okay? And Thomson continued
7:26making modifications to the Crookes
7:29tubes here, better understanding what
7:31these cathode rays were. After all, he
7:35had already deduced two pieces of
7:37information he had: cathode rays had
7:38mass and traveled in a straight line.
7:42Now, Thomson was spectacular. He placed
7:45a combination of an electric field and
7:49a magnetic field here in the Crookes
7:52tube. This he would also do. And then,
7:56by altering the intensities of the
7:58electric and magnetic fields, he would
8:00manage to discover the ratio between
8:01the mass and the charge of the electron
8:03. But first, he placed just an electric
8:07field here. When he placed an electric
8:09field, what did he observe? Here is a
8:11Crookes tube, always with the cathode
8:13and anode, cathode rays always leaving
8:14the cathode. He placed a screen here to
8:18let out a beam of cathode rays. This
8:22beam was deflected toward the positive
8:24pole of the field. Conclusion. If the
8:29cathode ray beam is deflected toward
8:32the positive pole of the field. Third,
8:37very important and ingenious conclusion
8:40by Thomson: cathode rays are negative
8:43because they are attracted by the
8:46positive pole of the field. Okay? Right
8:52. Three postulates. Cathode rays have
8:56mass, cathode rays travel in a straight
8:59line, and cathode rays are negative,
9:02they possess a negative charge. And
9:06then, very ingeniously, Thomson started
9:09changing the type of residual gas in
9:12the tube here. He used hydrogen gas,
9:16helium gas. Whatever gas he put in, the
9:19results were the same. The cathode rays
9:22traveled in a straight line. The
9:25cathode rays had mass, the cathode rays
9:27were negative. What was his conclusion?
9:30These cathode rays are present in any
9:32type of matter. Because regardless of
9:36the matter he used for the gas, to fill
9:38that gas, to compose the residual gas
9:41in the bulb, the experiments had the
9:43same results. And here showing how he
9:47was even more ingenious, spectacular,
9:49right? In 1897, he managed to calculate
9:55the ratio between the charge and mass
9:57of the electron, of the cathode ray.
10:00Here is what he did, look. He placed a
10:03combined electric field and magnetic
10:06field here. And then, according to the
10:11intensity of the electric and magnetic
10:13fields, he could deflect this beam.
10:17Notice that he made a little hole in
10:19the anode here, right? Here is the
10:21cathode, here is the anode, see. He
10:23made a hole in the anode for the beam
10:25of cathode rays to pass through the
10:26anode. Too ingenious, right? And then
10:31this beam was subjected to a
10:33combination of electric and magnetic
10:35fields. And when this beam passed
10:39through this electric and magnetic
10:41field, it underwent deflections. And
10:45then, by calculating the angle of these
10:47deflections and according to the
10:49intensity of the electric and magnetic
10:52fields he was using, he didn't discover
10:54the charge or the mass of an electron,
10:57but he discovered the ratio between the
10:59charge and the mass of an electron,
11:02which would be fundamental for another
11:04scientist, after him, to be able to
11:06discover the charge and the mass. He
11:10didn't discover either of the two; he
11:12discovered the ratio between the two,
11:14the ratio between the charge and the
11:16mass of the electron. So, as I said,
11:20according to the intensity of the
11:22magnetic and electric fields, the
11:25cathode rays underwent a greater or a
11:28lesser deflection. This means there was
11:32a proportion between the charge and the
11:35mass. The charge and the mass of the
11:38electron were altered according to the
11:40change in the intensity of the electric
11:43and magnetic fields, right? In other
11:46words, speaking better, the electron
11:49has a charge, has a mass, and as it
11:51propagates, according to this charge
11:53and this mass it has, this deflection
11:56was greater or lesser according to the
11:58intensity of the electric and magnetic
12:01field used. So he discovered the ratio
12:05between the charge and the mass of an
12:07electron to be 1.76 x 10 ^ 11 C/kg or
12:13converting to 1.76 x 10 ^ 8 C/g. Okay?
12:21What was Thomson's goal? It was to find
12:25the electron's charge to determine its
12:27mass. He didn't do either, but he
12:30already took a big step because he
12:32discovered the ratio between the charge
12:34and the mass of an electron. Whoever
12:37discovered the charge would notably be
12:40able to discover its mass as well. So
12:43Thomson discovered the first subatomic
12:46particle of matter. Which one was it?
12:50The electron. So the question is, who
12:52says which was the first subatomic
12:54particle? The electron. Who discovered
12:57the electron? Thomson. He was the first
13:00to work with electrons. No. The first
13:03to work with electrons, which are
13:04cathode rays, was Crookes. But he
13:07didn't have the capacity, right? He
13:11didn't perform the necessary
13:12experiments to say that electrons were
13:15present in any type of matter and that,
13:17therefore, the atom would now be
13:19divisible. Which would ruin the atomic
13:22model in vogue at the time, which was
13:24Dalton's atomic model, the experimental
13:27atomic model. The first experimental
13:29atomic model was Dalton's. And now the
13:32second atomic model emerges, because
13:34with this atomic model, matter now had
13:37a particle smaller than the atom, which
13:39were the electrons. And then Thomson
13:43announces his atomic model. This was in
13:471897, a model that became known as the
13:52plum pudding, right? Why? Because the
13:57atom would be a solid, homogeneous, and
14:00positive sphere. This sphere would be
14:05positive. And to neutralize this
14:10positive sphere, there were electrons
14:14embedded on the surface of this atom.
14:19Here is the famous plum pudding. Here
14:22we would have a positive sphere, a
14:25positive charge spread throughout the
14:28entire sphere. And to neutralize it,
14:31since the atom is a neutral species, we
14:34would have electrons embedded on the
14:36surface of this sphere. Okay? So, look
14:40at how we learn very important things
14:42from the Crookes tube and the
14:43experiments. What are cathode rays?
14:46They are electrons. What are the
14:48characteristics of cathode rays? They
14:51travel in a straight line, they have
14:53mass, they are negative. Actually,
14:57cathode rays are electrons and Thomson
14:59was the first to discover these
15:01electrons, the first subatomic particle
15:03. And he even discovered the
15:07charge-to-mass ratio of an electron as
15:11being 1.76 x 10 ^ 8 coulombs per gram.
15:17Here is the plum pudding atomic model.
15:19What was the first atomic model?
15:21Dalton's, the billiard ball model. The
15:23atom is a solid, indivisible,
15:25indestructible sphere. There are no
15:27subatomic particles. In Thomson's model
15:31, there are already subatomic particles
15:33, which in this case is one subatomic
15:35particle, the electrons. Okay?

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