Bright-Line Spectra
INTRODUCTION
The current model of the atom assumes that electrons move around the nucleus only at certain fixed distances. These fixed distances are called energy levels. The closer the electron is to the nucleus, the less the energy it has. The farther the electron from the nucleus, the more energy it has.
An electron goes to a higher energy level (farther away from the nucleus) when energy is added to it. An electron that has energy added to it is in the excited state. The electron can return to its original (lowest energy) ground state in steps. On its way back to the ground state, the electron gives off the energy it gained when it became excited. The light energy (in the form of photons) can be classified according to wavelength in the electromagnetic spectrum. Only light energy with a wavelength between 3.8 x 10-7 meters and 7.5 x 10-7 meters is visible to the human eye. Because these numbers are so small, wavelength is often reported in nanometers (109 nm = 1 m). The energy in the visible region can be observed using the simple spectroscopy equipment provided.
Emission spectra or bright-line spectra are extremely important tools for scientists in identifying elements. Because the electron patterns around every kind of atom are unique, and because these electrons interact with light in different ways because of their different positions, you can determine what kinds of atoms are present in a substance by the kind of light absorbed or emitted by the substance. Every atom has a kind of "fingerprint" in the normal light spectrum that is measured with a device called a Spectrometer. This instrument uses a prism to split the incoming light into its composite colors.
As an example, a marine ecologist may suspect that the reason many bottom-dwelling organisms are dying in a local harbor is because of a chemical pollutant. She samples the mud and chemically extracts a type of metal ion, but she’s not sure what kind of metal it is. She injects the metal ions into the hot flame of an atomic emission spectrometer and observes two bright lines. The lines correspond to the wavelengths of 563 nanometers (nm) and 615 nm. This combination is the "fingerprint" for tin. The ecologist may then trace the tin to a particular type of ship's paint or a nearby industrial source.
In this lab, you have a little bit easier task than separating metal ions from sea mud. You are observing various light sources including tubes that have been filled with various types of gases. As electrical energy passes through these tubes, the electrons in the atoms of the gas absorb some of the energy and become excited, jumping to higher energy levels. Eventually, the electrons release the energy and fall back to a more stable configuration closer to the nucleus. When the electrons release this energy, we see it in the form of light. You will compare the spectra of these gas tubes with incandescent (regular light bulb) sources and a fluorescent light bulb. Specifically, you will be asked to identify the gas that is used to fill fluorescent light tubes.
Objectives:
1. Determine the emission spectra of various known gases from gas discharge tubes (“neon” lights)
2. Determine what element is in a fluorescent light bulb based on emission spectraDirections:
For each emission spectra you observe:
· mark the limits of your visual range
· measure and draw the wavelength of any line spectra you see using colored pencils
· note the regions where the colors are most bright
1. Color of sodium vapor discharge tube: ______
Emission Spectra
Where have you seen sodium vapor lamps before?
2. Color of mercury vapor discharge tube: ______
Emission Spectra
3. Color of hydrogen gas discharge tube: ______
Emission Spectra
4. Incandescent light bulb (white light)
This is a "regular" light bulb.
Incandescent light bulbs work by passing an electric current through a filament (thin wire) of tungsten. The tungsten heats up and its electrons become excited and then fall back to the ground state, emitting light energy. What’s the symbol for tungsten? ______
Emission Spectra
5. Fluorescent light bulb
This type of lighting is very common in schools and offices because it uses much less electricity than traditional incandescent light bulbs.
Emission Spectra
Analysis Questions
1. Helium was discovered in the Sun's corona during the eclipse of 1868. In 1888, traces of helium were isolated here on Earth. How could scientists determine that this was the same gas that had been identified on the Sun?
2. Compare the results of the various gas tube spectra with the spectrum observed using the standard fluorescent light tube. Based on your results, what gas do you think is used in fluorescent light tubes?
3. Observe the temperatures of the incandescent light and the fluorescent light bulb. Why do you think fluorescent lights are considered more "energy efficient" than incandescent light bulbs?
4. Now that you know more about what gases are contained in fluorescent lights, why do you think there is a concern about the breakage and improper disposal of these tubes?
5. In this activity, you have observed examples of fluorescence and phosphorescence (i.e. “glow in the dark” materials). “Glow sticks” are plastic tubes that contain chemical dyes that emit light once activated by mixing with other reactants. This type of light emission is called chemiluminescence because it is generated during a chemical reaction. (The reaction taking place in a glow stick is actually very similar to reactions that take place in some living things, such as fireflies, many deep-sea creatures, and some algae. When living things “glow” – this is called bioluminescence.) Explain the differences between fluorescence, phosphorescence, and chemiluminescence.