This is an introductory course for students with limited background in chemistry; basic concepts such as atomic and molecular structure, solutions, phases of matter, and quantitative problem solving will be emphasized with the goal of preparing students for further study in chemistry.
Atom Emission Spectra
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來自 Duke University 的課程
Introduction to Chemistry: Structures and Solutions
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We will cover introduction to light, Bohr model of the hydrogen atom, atomic orbitals, electron configurations, valence versus core electrons. Don’t forget to use this week’s discussion forum for any questions and discussions.
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Prof. Dorian A. CanelasAssistant Professor of the Practice
Chemistry
In this video, we will be viewing a demonstration of
the atomic emission spectra of three elements, hydrogen, helium, and neon.
We will be observing the different colors of light come out of tubes filled with the
different gases, when they are excited by applying an external electric charge.
We'll be able to separate each spectrum into its component
colors using a very simple diffraction grating.
Recall that a continuous spectrum of white light can
be separated into its component colors by a glass prism.
The prism bends the light and some colors of the light
are bent more than others due to differences in their wavelengths.
We are observing the same process when droplets of
water in the atmosphere separate sunlight into its component colors.
The sun
produces white light because it is an example of an incredibly hot object.
But what about the light produced here on Earth by the chemical elements?
For example, most of you are familiar with the term neon sign.
What color of light does neon produce? Do other elements produce
different colors of light?
Are all of these lights filled with neon? What about these?
Are these light tubes filled with the gas neon?
First, let's look at what we call white light using a simple light bulb.
When the tungsten filament light bulb is brightly lit,
we can see a continuous spectrum of visible colors.
Before we jump straight to neon lights,
let's start with the simplest element, hydrogen.
When hydrogen is given sufficient energy, it also produces visible light.
The light from hydrogen is especially important in astronomy, because
much of the matter in the universe is made from hydrogen.
Here is what a giant lit up hydrogen
gas discharge tube looks like.
As you can see, the lamp itself glows with a bright pinkish purple color.
This is an example of the smaller discharge
tube that we'll be using in this experiment.
At the end of each experiment, we will scan back so that you can see the
tube itself in the apparatus that we're using
to apply the electric charge to the tube.
In this case, they have little clamps, which are going to
electric source.
Here is some video footage we took of our small hydrogen lamp.
First is the lamp itself over on the right.
If we pass that light through a grading, we can separate that
pinkish purple composite light we observe with our eyes into its component colors.
You can see
some bright vertical lines of different colors separated by dark black spaces.
Other than the pinkish purple lamp itself, what colored lines are you observing?
It makes sense that we can see red and blue when we split colors apart.
Because red and blue mix together to make
purple, the composite color of the lamp light.
As you've reported, we don't see all colors of the rainbow.
Hydrogen spectral discharge tube does not
yield a continuous spectrum of visible light.
The lines are some of the visible spectral lines for hydrogen.
We see duplicated spectra here.
Because the diffraction grating gives us first order,
second order, and third order, et cetera, diffraction.
If we pan back with the camera, then we can see the apparatus itself, which is a
not very expensive cardboard box, was a piece
of grating film tapped over a hole in it.
As Dr. Lyle removes the box, you can see the light itself and its power supply.
Next, let's look at some video footage we took of our helium lamp.
This time we are starting with the lamp itself in the center.
To the naked eye, this helium light is
a different color than the hydrogen light was.
It's bright pink, and you can see the bottom of the lamp if you look carefully.
Again, there are duplicate spectra observable through the grating
on either side of the lamp. Please tell me what you are observing.
Is the helium lamp giving off more or less lines of color than the hydrogen lamp?
Yes, hopefully you realize that more lines can be seen
in the helium atomic emission spectrum than we saw for hydrogen.
Finally is the neon light, again, the lamp itself is in the center.
To the naked eye, the neon light is very bright red.
The spectrum that is showing up has lots of red and some magenta and yellow.
This is one that everyone is familiar with, vintage red neon signs.
In the United States, these signs are often used to advertise for the
beverage beer.
The camera has been panned back now, so you can see
the window we made in our cardboard box covered with the grating.
And finally, the block with the grating taped to it is
being pulled away, so that you can see the light apparatus again.
What we just watched was a demonstration of spectral tubes.
The study of atomic and molecular
spectra is a scientific specialty called spectroscopy.
The electrons of each element were excited in a discharge tube using electricity.
The energy from the electricity was converted to kinetic
energy that allowed the electron to move further away
from its atom's nucleus.
But this excited state is a temporary state.
As the excited electrons fell from this higher energy level back to another
lower energy level closer to the nucleus, they emitted photons of light.
That gave us the atomic emissions spectrum, and we could
see some of that spectrum in the visible light range.
Each element has its own set of allowed energy levels for the
electrons, and therefore, each element will have its own set of colored lines.
Tune in to the next lecture to learn more
about the energetics of this process for the hydrogen atom.
I can review what happened to the hydrogen atom using simple diagrams here.
If I draw a very curved picture of the hydrogen atom, with a dot in the center
for the positively charged nucleus, and a circle
to show the sphere where the electron is orbiting.
Here's the electron.
Initially, we put some energy into the
system using the electricity from the light apparatus.
When we did that, the electron had to respond by moving to higher energy.
So what it did was it moved further away from its nucleus.
So now the electron is in an excited state.
It's further away from the nucleus than it needs to be.
It then relaxed over time.
And when it relaxes, it emits light.
And it gets closer to its nucleus again, so that it's backward started from.
We will be reviewing this process in more detail
in the next lecture, so please check it out.
