A CAROLINA ESSENTIALS INVESTIGATION

Bright-Line Spectroscopy and Atomic Structure

Total Time

Physical Science

Grade Level

9-12 High School

Introduction

This activity employs a simple procedure in which students use a spectroscope to observe the excited state of hydrogen and several additional small, nonmetal gases. They use wavelength data to calculate the energy associated with electron level transitions. The activity can be used to introduce Bohr’s model of the atom or to compare metal flame test data to nonmetal gas data. In both cases, the wave properties of wavelength and frequency are central to energy calculations that are useful in describing electron behavior. Prior to the activity, have students explain the phenomenon of ground state and excited hydrogen.

Phenomenon

Observe the non-energized (ground state) hydrogen spectral tube.
Observe the energized (excited state) hydrogen spectral tube.
How can you explain what’s happening? What needs to be investigated?

Essential Question

How does the color of light emitted from excited gas samples provide evidence of atomic structure?

Activity Objectives

Determine the wavelength, frequency, and energy of the bright lines for the spectrum of several nonmetal, gaseous elements.
Use the activity data and observations to justify Bohr’s atomic model.

Next Generation Science Standards* (NGSS)

PE HS-PS4-1. Use mathematical representations to support a claim regarding relationships among frequency, wavelength, and speed of waves traveling in various media.

SCIENCE & ENGINEERING PRACTICES

Using Mathematics and Computational Thinking

DISCIPLINARY CORE IDEA

PS4.A: Wave Properties

CROSSCUTTING CONCEPTS

Energy and Matter: Flows, Cycles, and Conservation

Materials Needed

Recommended Safety

Spectral tubes get very hot. Have a hot glove or towel available for the removal of tubes from the power source.

Teacher Prep and Disposal

Prior to the activity, make sure the electrodes of the spectral tubes are not dusty or corroded in any way. Also check the connectors in the power source before plugging it in. They should be clean. After the activity, store the spectral tubes and power source in a clean, dry area.

Determine prior to the activity if you will introduce the terms ground state and excited state, or if you will have students generate them during a discussion of the phenomenon. Review the terms if they were introduced earlier. Make sure students notice that the excited state (additional electrical energy for this activity or additional heat energy for flame tests) is always associated with a unique color for every element.

Student Procedures

Look at the broad end of the spectroscope. Locate the small, vertical slit. This slit should take in the light you want to analyze. Hold the narrow end of the spectroscope to your eye. Position the spectroscope so that the slit is filled with the light for analysis. Without moving the spectroscope, adjust your gaze so you can see the spectrum and wavelength guide. Do not shake or drop the spectroscope. This may dislodge the diffraction grating that separates the light into bands.
Record the element and color of the excited gas in the spectral tube.
Using the spectroscope, record the spectrum colors with the colored pencils or markers and the wavelength of each bright line in the element’s spectrum. You will need to estimate the wavelength as closely as possible.
Repeat for all elements.

Teacher Preparation and Tips

1. Demonstrate to students how to use the spectroscope. The slit points directly at the light source. Students should see the colored light through the slit. You may want to have them practice using an overhead light first.
2. Remind students of Bohr’s experiments with the hydrogen spectrum and how the evidence led to his model of the atom.
3. If students are having a hard time seeing the spectrum, turn off the classroom lights.
4. Not all students will see the same colors. If you have a color-blind student, pair him/her with another student for data collection.

Data and Observations

Analysis and Discussion

1. Use the wave and energy equations to derive one equation that relates wavelength to energy.

1. Explain which spectral color bands are the most energetic and least energetic. Justify your answer with sample calculations.
  Oranges and reds have larger wavelengths, so the associated energy is less because wavelength is in the denominator of the proportion (formula). Violets and blues have shorter wavelengths, so the energy is greater. Wavelength and energy are indirectly proportional. Examples:

Using the spectral data, determine if a relationship exists between the number of electrons an atom has and the number of spectral lines. Provide evidence for your answer.
Generally, the more electrons an atom has, the more spectral lines the element’s spectrum has.
Explain how the data from this activity can be used to support or refute Bohr’s model of the atom.
Data from this activity can be used to support Bohr’s model of the atom. As energy was added to the atoms in the spectral tube, the electrons went into the excited state. The energy gained was emitted as light energy as electrons returned to the ground state. Every atom has a unique number of electrons, with a unique pattern of bright-line light emission, and that supports Bohr’s model.
Electrons are located in discrete energy levels. As the electrons gain energy, they move to a higher energy level. Conversely, as electrons lose energy, they emit light energy and return to a lower energy level.

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*Next Generation Science Standards® is a registered trademark of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.