Video Transcript
In this video, we will learn about
the very specific signatures of light produced by atoms or ions of different metals
when they’re excited. We’ll look at the specific colors
of visible light produced, examine how they’re produced, and identify elements based
on their specific patterns.
Generally, the colors we see arise
from electromagnetic waves hitting special sensing cells in our eyes. This triggers an electrical
response that’s transferred to our brain. The brain turns that information
into color, like red, green, or blue. Electromagnetic waves, otherwise
known as light waves, can be visible or invisible. Our eyes only detect certain types
of electromagnetic wave. At one end of the visible light
spectrum is light we call red. At the other end is light we call
blue. In between, we have many of the
other colors we’re familiar with like orange, green, and yellow. But some colors don’t actually show
up in this spectrum because our brain makes new colors for some combinations of
light. For instance, a mixture of red and
blue light is experienced as purple.
In this video, we’re going to be
looking at individual packets of light called photons. Photons of red light have less
energy each than photons of blue light. A photon of red light has about
half the energy necessary to break a single carbon-carbon bond, while a photon of
blue light has the energy necessary to break about 1.2 carbon-carbon single
bonds. However, these figures are here
just to provide some background. In this video, we’re not actually
going to be doing any number crunching it all. In fact, we’ll look at the origins
of atomic emission spectroscopy in flame tests.
We can identify some of the
elements in the sample based on the color they will turn a Bunsen flame. For instance, sodium ions in a
flame produce a bright orange color. Lithium or strontium will turn it
red. And samples containing barium,
boron, or copper can produce green flames. Even with practice, it’s sometimes
difficult to tell which color corresponds to which element. And it’s almost impossible if
you’ve got a mixture of different colors.
But if we look in more detail at
the pure signals from each element, we actually see many different colors all
overlapping. We can see the individual colors if
we use a prism to separate them out. For the case of sodium, we end up
with bright lines of yellow orange and dimmer lines elsewhere in the visible
spectrum. The important thing is that each
pattern of colors and their intensities is unique to each element. This is where atomic emission
spectroscopy comes in.
Let’s break down what this name
means. The atomic means we’re on the
atomic scale, dealing with phenomena from atoms and ions. Emission means we’re dealing with
light being emitted, in this case, from atoms or ions. And spectroscopy relates to the
viewing of an image or a spectrum. Therefore, altogether, atomic
emission spectroscopy is the examination of light emitted by atoms or ions across a
spectrum. Let’s break down how it works. There’s nothing wrong with a basic
flame test, so we can start with that. We put our sample in a flame. The flame provides the energy that
excites atoms and ions to emit light, which we can separate using a prism or a
diffraction grating.
What we get out of atomic emission
spectroscopy is an emission spectrum. This might be a print or it could
be on the computer. This is made up of lines, some
bright, some dim, with dark areas in between because we didn’t detect any light at
these frequencies. Emission spectra represent the
light and their respective intensities across a range of the electromagnetic
spectrum, visible or invisible. When an emission spectrum consists
of sharp lines with dark areas, it’s called a line emission spectrum. So, what does all this mean? Well, firstly, we’re going to look
at the signatures of a few different elements, and we’ll learn about how we can use
those signatures to identify the elements in a sample. Then, we’ll look at why these
sharply defined emissions occur in the first place and what these lines tell us
about the atoms or ions that produced them.
We know that when excited at a
flame, atoms or ions of elements will emit light. This light will be a mixture of
different colors that sharp on a spectrum as very sharply defined lines which, like
a fingerprint, give away the element that produced them. This is the atomic emission
spectrum for a sample containing sodium. This bright yellow orange here is
responsible for the orange-yellow color of a sodium flame. But, while in flame test we can
only see the overall color, in atomic emission spectroscopy we can see the
individual lines exhibited by that element. No matter how you prepare your
sample, the signature, the fingerprint, will be pretty much identical. The sample can be solid, liquid, or
even a gas. Or it can be pure or in some form
of compound. The signature will be the same.
But there is one important point to
make. When we do flame tests or atomic
emission spectroscopy, we usually examine ionic compounds and not pure metals. This is because a lot of metals
will burn in air, reacting with oxygen if heated. This generates far more heat than
necessary to produce the emission we’re interested in. And the heat can be so great that
particles will glow red, yellow, or white hot. This makes it very hard to pick out
the specific colors we’re looking for to identify an element using atomic emission
spectroscopy. However, ionic compounds are not
pure elements.
So what about the nonmetal in our
ionic compound? Well, for reasons that are beyond
the scope of this video, metals show up an atomic emission spectroscopy, but
nonmetals are much harder to detect, so we generally don’t see them. Boron is an unusual exception. Otherwise, the elements we’ll see
through atomic emission spectroscopy will all be very clearly metals. Let’s say we’ve run a sample
through our spectrometer, focusing on the visible spectrum. The machine comes back with a
pretty graph. There were dark areas indicating
that none of this type of light was emitted. And we see a pattern of lines. These lines may be colored or
simply black and white. And we may have labels along the
bottom for wavelength or frequency of light.
The first thing we need to ask is,
does this spectrum correspond to just one element’s signature? If the sample is a pure element,
our job is relatively easy; all we need to do is compare the pattern of lines to the
patterns in a database of spectra for pure elements. The samples used to make these
spectra in the first place would’ve been verified as pure elements in other
ways. We know we found a match when all
the lines in our spectra match up with a sample in the database. If every single line is accounted
for by comparing to just one element, then we know that the sample contains that one
element and no other metals. But what if some of the lines don’t
match? There must be another element.
This is a spectrum from a sample,
and there are lots and lots of lines. If we look in our database, we find
that there’s a lot of lines that match up with the spectrum for pure barium. But we can still see there are lots
of lines that can’t be from barium because they don’t show up in the spectrum for
barium. So we have to keep looking. We keep looking through our
database, and we find a particular match for this cluster of blue lines and lots of
other lines that match up as well in the spectrum for boron. That’s most of the lines accounted
for, but a few more to go. So back to the database we go. This time we focus on this little
cluster of blue lines, and we find a match in copper. It took a lot of searching, but we
finally ticked off every single line in our sample. So we know that our sample is a
mixture of barium, boron, and copper.
Sometimes is enough to focus on a
smaller area of the spectrum. What’s quite cool about atomic
emission spectroscopy is that if we did flame tests, we wouldn’t be able to tell
these elements apart, and we wouldn’t be able to tell them apart from the
mixture. But using our spectrometer, we’re
able to separate out the individual frequencies of light and see the unique
signature for each element. Atomic emission spectroscopy is
simple in some ways and complex in others. The patterns we get from each
element are unique. That’s what allows us to simply
compare them and be sure we know which elements are involved. But the reasons these patterns
happen in the first place are much harder to understand, so we’ll look at a
simplified version.
This is an electron-shell diagram
for lithium atom, two electrons in the inner shell and one electron in the outer
shell. And here are a couple of extra
shells, which are empty. When a lithium atom is excited, for
instance, by a flame, electrons absorb some of that energy and move away from the
nucleus into higher energy shells and subshells. These high-energy electrons are
likely to lose their energy in one form or another. One thing that can happen is that
when an electron moves from a higher-energy level to a lower-energy level, it emits
a photon.
The bigger the difference in energy
between the high-energy energy level and the low-energy energy level, the greater
the energy of the emitted photon. A photon of blue light has more
energy than a photon of green light. And a green proton has more energy
than a photon of red light. But atoms or ions can also emit
invisible infrared or ultraviolet light. The pattern of lines from each
element we see in atomic emission spectroscopy arises from the unique energy levels
that atoms or ions of that element exhibit. A lithium ion and a lithium atom
produce the same spectra because their energy levels are almost identical. A single spectrum arises from many
different transitions.
Unfortunately, there’s no easy way
to predict the color we get from the flame test of a specific element. In the case of lithium, the red
color we see when we put lithium into a flame is caused by this red line. Now it’s about time we had some
practice.
The image below shows the flame
emission spectra of four metals and an unknown mixture of metals. By using the spectra, what metals
does the unknown mixture contain?
Flame emission spectra is another
name for atomic emission spectra. Our sample has been heated in a
flame. The light’s been collected and
passed through a prism or scattered off a diffraction grating, and the separated
frequencies of light have been collected and put together into a spectrum. We’ve been given the spectra of
samples that contain only magnesium, aluminum, copper, or lithium. The lines we see in each spectrum
are unique to that element. So, if those lines appear in our
mixture, we know our mixture contains that element.
The easiest way to approach this
question is to identify the most distinctive line, and that’s this green one over
here. The only spectrum with an identical
line that we’ve been given is that of magnesium. So we’ve accounted for that
line. But we should also see all the
other lines from the magnesium spectrum in our mixture spectrum. So what we need to do is match up
all these lines with lines in our mixture spectrum. Every single one of the lines in
the magnesium spectrum can be found in the spectrum from our unknown mixture, so we
know for certain our mixture contains magnesium.
However, there are still three
lines for which we don’t have a match. There appear to be two different
elements, lithium and aluminum, that are a match for this red line. However, the other two lines on the
aluminum spectrum match up perfectly with the other two unaccounted-for lines, while
in the spectrum of lithium there are no extra lines in the right places. If there was lithium in our sample,
we’d expect this bright orange line to show up here. But it doesn’t. Since all the lines in the aluminum
spectrum show up in our mixture spectrum, we know we also have aluminum in our
mixture. And since we’ve accounted for all
the lines in our unknown mixture spectrum, we know that our sample contains only
magnesium and aluminum.
Let’s finish up with our key
points. Atomic emission spectroscopy is the
examination of light emitted by atoms or ions across a spectrum. And this is what an atomic emission
spectrum looks like: sharp lines of color that are bright or dim, with dark areas in
between. We call this style of spectrum a
line spectrum, although there are other varieties of line spectrum. Atoms or ions of each element give
out a unique signature characteristic of the element. It’s this we use to identify the
element from the spectrum. Photons of light are emitted when
excited electrons move from higher-energy levels to lower-energy levels. And blue light has higher-energy
photons than green light, which has higher-energy photons than red light.
