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.