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.