Laboratory Atomic Emission Spectrum

Transcription

1 Laboratory Atomic Emission Spectrum Pre-Lab Questions: Answer the following questions in complete sentences by reading through the Overview and Background sections below. 1. What is the purpose of the lab? 2. How is spectroscopy used by scientists? 3. What is the charge of an electron and where is it located? 4. What is the ground state? What is the excited state? How can an electron move from one energy level to the next? 5. At what point is a photon emitted? 6. What is a spectrum? Objective In this lab students will learn about atomic energy levels, atomic emission spectroscopy, and the spectral fingerprints of elements. Overview Students will draw their own continuous spectrum. Then they will analyze the emission lines from a number of different atomic emission light sources. These light sources are gas discharge tubes filled with gaseous samples of various elements. They will record the spectra they observe in such a way as to relate them to the continuous spectrum they drew. They will then use the spectra they drew to identify several unlabeled atomic emission lamps. This is how scientists identify elements found in distant stars. Background The electrons in an atom occupy different energy levels, as you know. When all of the electrons are at the lowest possible energy level they are said to be in the ground state. Electrons do not always stay in the ground state. Sometimes they can be promoted to a higher-energy electron shell. This can happen in two ways. First, the electron can absorb a photon of just the right amount of energy to move it from one quantum shell to another. Second, when atoms are heated or energized with electricity their electrons can gain energy. This promotes them to the higher-energy shell. When an electron is in a higher-energy shell it is said to be in an excited state. Electrons in excited states do not usually stay in them for very long. When electrons lose their energy they do so by emitting a photon of light. A spectrum is the scientific name for a rainbow: light broken into the different wavelengths that make it up. You can see spectra using a spectroscope, a prism or a diffraction grating. A spectroscope is a device which uses a diffraction grating to create a visual spectrum in a way that places the spectrum on a scale. This enables the user to measure the wavelengths of light being observed. The back of an ordinary CD is a reflective diffraction grating. Atoms produce very sharp lines in a spectrum when they are heated. You will look at these lines in this lab. These lines show the energy differences between the excited states and the ground state. The atomic spectrum of hydrogen is shown below: Hydrogen Emission Spectrum When you look at the hydrogen gas discharge tube you will see a mixture of these four colors. To see the lines you have to use a diffraction grating or a prism. Even so, the mixed color alone can be enough to identify an element. Put simply, each element produces a unique color spectrum when energized sufficiently. Because every element has a unique spectrum the spectrum of an element can be used to identify it. Distant stars are too far away for us to take a sample to analyze in a lab. Even so, we can gather information about what they are made of by looking at the spectrum of light they produce. By collecting data here on Earth for every element we can record their spectral fingerprints. These can be used to identify them in far off stars and galaxies.

2 Procedure: Observing Atomic Emission Spectra 1. Go to Using this site, view and record the line spectra for the elements Ar, H, He, Hg, N, and Ne on Spectra Data Sheet 1 (the sheet that has these elements already labeled by name). Be as careful as you can to place the lines of the spectrum as close as possible to the correct numerical value for the wavelength. This will be critical for identifying the spectrum later. Use the color spectrum at the top of the page to line up the lines as best you as can so that you can estimate the wavelength of the lines you draw. The lines need not be in color, unless you wish them to be. 2. Observe the spectra of all elements (labeled A-F) and record the atomic emission spectrum of each of the different elements on Spectra Data Sheet 2 (the sheet that has the elements labeled A through F). Again, be as careful as you can to place the lines of the spectrum as close as possible to the correct numerical value for the wavelength. 3. Compare the Spectra on Data Sheet 2 (the ones you observed) to those on Spectra Data Sheet 2 (the ones you copied from the website). Do your best to identify elements A through F as being Ar, H, He, Hg, N, or Ne by seeing which spectra most closely match up to one another. Write the identities of elements A through F on next to the appropriate spectra on Data Sheet 2. Questions: Answer in complete sentences by referring back to the Background as well as what you observed in this lab. 1. What is a spectroscope and what is it for? Remember, you used a spectroscope in this lab. 2. You observed the spectral lines for a variety of different elements. What is happening within an atom that causes it to emit light in specific lines in a spectrum? 3. Why do the elements in the tubes have to have high voltage electricity run through them before the colored light is emitted? In other words, why don't the cold, un-energized tubes glow? 4. How can you use the emission spectrum of an element to identify it? 5. What were the identities of the lamps your teacher set out for you to identify? Give the letter and element for each lamp you identified.

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4 Re red orange yellow green blue indigo violet

5 ANSWERS 18 pts (3 pts each) 1. The purposes are to learn about atomic energy levels, atomic emission spectroscopy, and the spectral finger prints of elements. 2. Can be used to identify elements and/ or analyze light from distant stars. 3. The charge is -1 and it is located outside of the nucleus. 4. The ground state is the lowest possible energy level for an electron. The excited state is when an electron is in a higher energy shell. Electrons move between states when they gain energy from heat or electricity. 5. When electrons lose their energy (and drop down from an excited state). 6. A spectrum is a rainbow. SPECTRAL DATA SHEETS 10 pts (5 pts each) 12 pts (3 pts each) 1. A spectroscope is used to see a spectrum. It is a device that uses a diffraction grating to create a visual spectrum. 2. The electrons jump to excited states, but don t stay there long. When they return to the ground states, they emit energy in the form of light. 3. The electrons need to be excited by energy in the form of heat or electricity. 4. Each element produces a unique spectrum, like a fingerprint. 10 pts (2 pts each) 5. A: Helium B: Hydrogen C: Neon D: Mercury E: Nitrogen F: Argon