Learning Goals / Objectives:

• A hand-held spectrometer can be used in obtaining atomic line spectra.

• Atomic line spectra are the result of electronic energy transitions.

• Atomic line spectra can be used to identify elements.

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 spectrum, and work on this started in 1752 with Thomas Melvill who first observed the spectra of flames into which metals or salts were introduced showed these characteristic lines. This phenomenon was used in the late 1800s to identify elements in a sample, but couldn’t be explained using existing atomic theories. The hydrogen line spectrum was mathematically described by Johannes Rydberg, a Swedish physicist, in 1890. He used the following equation to describe the line spectrum:

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:

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.

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.

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.

• This exercise could be used concurrently with a discussion on atomic structure or the discovery of the elements.

• 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 dependent 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.

• 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.

• You could also extend this lesson to astronomy by discussing how physicists identify which elements are present in stars.

Anticipatory Set:

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?

Instructions / Activities:

1. Obtain a spectrometer and take a few minutes becoming familiar with it.

• 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.

• What do you observe? Record your qualitative observations.

• 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.

• What do you observe? Record your qualitative observations.

• Make a sketch of each spectra.                                                                                                                                                                            Mercury (Hg)                                                                                  Hydrogen (H)    

• Are either of these elements present in the room lights?

• Use the spectrometer to observe the gas discharge tubes available

• Which element has the most complicated line spectrum? Which element has the simplest line spectrum?

Assessment:

Teacher’s Discretion

Wrap-up / Closure:

Teacher’s Discretion

Attachments:

These materials are contained in a kit in our Library!