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  • Hi. It's Mr. Andersen and this AP Physics essential video 4. It is on the atom. In the

  • last video we talked about how Ernest Rutherford and his gold foil experiment had helped scientists

  • discover this positive small nucleus in the center of an atom. But that did not tell us

  • what the electrons were doing. And he just speculated that they were moving around the

  • nucleus almost like planets in orbit around the sun. But one of the researchers working

  • underneath Rutherford, Niels Bohr spotted a problem in this. He knew that any charged

  • particle that is moving is going to be giving off electromagnetic radiation. As it does

  • that it is losing some of that energy. And so it is quickly going to spiral into the

  • middle and annihilate essentially the whole atom. So he knew that was not right. He also

  • knew that as it gives off radiation, the wavelength of that radiation is going to vary. And as

  • it varies we are going to get this nice smooth spectrum, spectrum of electromagnetic radiation

  • given off by high energy atoms. But when we started to look into space what we found is

  • that light was not smooth. It had these discrete units within it. And so that spectra had to

  • be described. And Bohr's model helped to do that. And so if you think of it like this,

  • and this works for hydrogen, is what the Bohr model is built on, you have these energy levels.

  • And so an electron can be in energy level one, energy level two, energy level three.

  • But it can never be found in the middle. It is quantized. It has to be in one of those

  • levels or another level. And so how does it move between levels? Well, if it absorbs energy

  • from a photon, electromagnetic radiation for example, it will jump to a higher level. And

  • as it moves down it is going to emit those photons. And that helped to describe what

  • we were seeing in the spectra. And so that improved our model. So we now had the cloud

  • that had the electrons in it. And then the nucleus. And so we found these negative electrons

  • in the cloud and then protons and neutrons were found in the nucleus. And in a neutral

  • atom the number of protons and electrons are going to be equal. And the electrons tell

  • us a lot about the properties of that atom. In fact the whole periodic table is built

  • on the electrons, electrons especially we have in these outer levels. Now the Bohr model

  • helps us explain what those electrons are doing and how they are moving. They move into

  • these discrete energy states and that helps us to explain the spectra. And so if you look

  • at any kind of an atom on the periodic table the atomic number 2 tells us the number of

  • protons we have. And so we are going to have these positive protons that are going to be

  • found in the nucleus. We can kind of figure out how many neutrons roughly we are going

  • to have in an average atom by taking the mass number, subtracting the atomic number. And

  • so we would know in helium for example that we are going to have two neutrons. Now since

  • the number of protons and electrons are the same in a neutral atom we can figure out that

  • we have got these electrons moving around

  • the outside. But there were problems with this planetary model. Electrons were not orbiting

  • like planets. They were actually jumping between orbits according to Niels Bohr. And so they

  • did not just move back and forth on all these infinite number of orbits around the nucleus,

  • giving off a smooth amount of spectrum. It is almost like a ladder, that an electron

  • can be here, but it could also be here. And it can never be found in the middle. We call

  • that being quantized. It has to be in a specific unit to exist. Now how do you move an electron

  • to a farther level? Well you have to put a little bit of energy into it. So if we had

  • a lot of energy we could jump it up to this energy level. And as it falls back down it

  • is going to release a certain amount of energy. And so this is a visual or a model of what

  • the Bohr model might look like. And so as it orbits around the center, if it receives

  • a photon it jumps to a higher level. If it gives off an equal photon it will drop down

  • to a lower level. And so it is only existing in these quantized orbits. And this helped

  • to explain spectra. Because before the model was discovered or was put forth, people had

  • started discovering spectra. They were looking into space, not with just a prism, but a spectroscope.

  • So they were splitting the light into all of its different wavelengths. And they were

  • starting to see these lines. So when you are looking at the sun for example, which is mostly

  • hydrogen, we saw these different series. So the Lyman series was developed by one scientist

  • who was using spectroscopy. And he came up with an equation that explained what was going

  • on. But you could not see this spectra because it was into ultraviolet. We all saw the Paschen

  • series that was showing the similar relationship. But this was in the infrared. And the Balmer

  • series was seeing the same thing. And so what really he was explaining, let's throw the

  • Balmer series up here, is that they were seeing these discrete units of light. And so where

  • was that light coming from? If you look at hydrogen, well you can see here as we move

  • from this energy level 2 up to energy level 3, it requires a certain amount of energy.

  • And as the electron falls back down it is going to give off that energy. It is going

  • to give off that light. And so the Bohr model predicted what these numbers were and they

  • fit perfectly with the numbers that we were seeing in the spectra. And so again this only

  • works for hydrogen. And so it is a good step model, or a good model to get you started

  • on understanding how the atom is really put together. But did you learn the energy level

  • structure of an electron in an atom at the appropriate scale being investigated? In this

  • case it is at these energy levels in a hydrogen atom. I hope so. That is the Bohr model. And

  • I hope that was helpful.

Hi. It's Mr. Andersen and this AP Physics essential video 4. It is on the atom. In the

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玻爾原子 (The Bohr Atom)

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    Bravo001 發佈於 2021 年 01 月 14 日
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