Instructor’s Notes
Finger Printing the Elements
Materials
Colored pencils
Gas Discharge Lamps
Hand-held Spectrometer
Learning Objectives
1. A hand-held spectrometer can be used in obtaining atomic line spectra.
2. Atomic line spectra are the result of electronic energy transitions.
3. Atomic line spectra can be used to identify elements.
Question to Investigate
1. How does a spectrometer record line spectra from a gaseous elemental substance or compound?
2. What is the origin of line spectra?
3. How can line spectra be used to identify an element?
Background
Any pure elemental substance, when vaporized and excited with some energy source like heat or
voltage, will emit light with components at very specific discrete wavelengths. This is known as a
line spectra, and work on this started in 1752 with Thomas Melvill who first observed that the
spectra of flames into which metals or salts were introduced showed these characteristic lines. This
phenomena was used in the late 1800s to identify elements in a sample, but couldn’t be explained
using existing atomic theories. The hydrogen line spectra was mathematically described by
Johannes Rydberg, a Swedish physicist, in 1890. He used the following equation to describe the
line spectra:
 1 1 
1
= 1.097 x 10 7 m-1  2  2 

n1 n 2 
This equation is a more generalized version of roughly the same equation proposed by Johann
Balmer, a schoolteacher, in 1885, to describe specifically the visible hydrogen spectra:


 1 = 1.097 x 107 m-1  12  12 , n = 3, 4, 5, 6
2 n 



In 1913 Niels Bohr proposed his atomic theory and this helped explain the origin of line spectra.
He theorized that electrons in the hydrogen atoms existed in specific energy levels – were quantized
– and that the lines observed in these emission spectra corresponded to the energy emitted by
electrons as 
they make transitions between levels, or jump from higher to lower levels (see figure
below). The Rydberg equation above gives you the wavelength of the emitted line as a function of
the energy levels n1 and n2 involved in the transition. These energy levels and possible transitions
between levels, associated with both absorption and emission of light by hydrogen, are shown
schematically below.
Electronic Transitions
Hydrogen Emission Line Spectrum
Spectroscopy uses light to study matter. This activity uses emission spectroscopy to measure line
spectra of several gaseous substances. In basic emission spectroscopy, energy is given to the
sample you’re studying, in the form of heat (possibly from a flame, as in flame tests) or an electric
arc or spark (as in gas discharge tubes). The elements present in the sample will absorb only
specific energies that will cause transitions of the electrons in each element. This elevates the
electrons from the ground state to excited states. The electrons, being unstable in excited states due
to the higher energy, return to the ground state and in doing so, re-emit the energies they absorbed.
This results in a bright line spectrum, which can be recorded after the light is dispersed, or broken
into its components, by a grating or prism. The wavelengths of the emitted light can be used to
identify the element as they correspond to the particular electronic transitions occurring within it.
Thus, elements can be characterized by their spectra, as each element has its own unique electron
configuration, so its own characteristic line spectra, sort of like a fingerprint.
Emission Spectrum
Absorption Spectrum
Procedural notes
Set up the lamps before class and make sure they’re all operating properly. Photocopy enough
sheets of the spectra strips for each student to have enough for each lamp they need to observe.
Instruct the students ahead of time on how to use the spectrometers and how to safely observe the
lamps. Students should not look directly at the lamps but rather observe the line spectra only
through the spectrometers.
1. This exercise could be used concurrently with a discussion on atomic structure or the discovery
of the elements.
2. You could expand this exercise to include graphing, by having students make a calibration
curve. To do this, have them observe the mercury lamp and plot wavelength as the dependant
variable and scale reading as the independent variable. They should fit this data with a linear fit
and generate an equation for this fit (using a graphing program like Excel). They should then
record both the scale reading and wavelength for lines observed from the other lamps, and use
the calibration curve to calculate the wavelengths and compare to their readings.
3. You could make this exercise part of a history of science lesson by talking about how
spectroscopy was used to identify several of the elements.
4. You could also extend this lesson to astronomy by discussing how physicists identify which
elements are present in stars.
Finger Printing the Elements
Purpose
For this experiment, you will work in groups of two or three. Your goals are to:
o Use a spectrometer to observe atomic emission spectra.
o Identify an unknown element based on its emission spectrum.
Safety
Some of the discharge tubes emit a significant amount of ultraviolet radiation, which can be harmful
to your eyes, so PLEASE do not STARE at the lamps directly.
Procedure
1. Obtain a spectrometer and take a few minutes becoming familiar with it.
o Locate the slit (where the light enters the device), the eyepiece (which is covered with the
grating piece and is what you look through) and the scale inside.
2. Look at the room lights with this spectrometer.
o What do you observe? Record your qualitative observations.
o Make a sketch of the spectrum below. Do your best to match the scale you observe.
3. Use the spectrometer to observe the mercury and hydrogen gas discharge tubes.
o What do you observe? Record your qualitative observations.
o Make a sketch of each spectra.
Mercury (Hg)
Hydrogen (H)
o Are either of these elements present in the room lights?
4. Use the spectrometer to observe the gas discharge tubes available.
o Which element has the most complicated line spectrum? Which element has the simplest
line spectrum?