Electrons and Light—Using Indirect Evidence
In this activity, which includes two demonstrations and a lab, students will learn how light energy emitted from chemical compounds is used to provide evidence for the structure of atoms that make up the compounds. The first demonstration is a standard cathode-ray-tube demonstration. The second demonstration will show students the emission spectra of several elements. And the lab activity teaches students that the color of the flame produced by an element is characteristic of that element. The flame-test lab is a less rigorous version of emission spectroscopic analysis.
Student Ability Level and Grouping
This activity is appropriate for students in a general high-school chemistry class. Students can work in pairs, with two pairs at each lab table.
Expected Student Background and Skills
Students should know basic atomic structure and some of the history of the development of the current atomic model. It would be helpful for students to understand basic characteristics of light, especially frequency and wavelength. Students should also be familiar with basic lab techniques like lighting and adjusting a lab burner.
Time and Materials Required
Both demonstrations can be done in a single class period. Materials required are
- Demonstration cathode-ray tube
- Power supply
- Strong magnet
- Spectrum tubes
- Power supply
- Diffraction gratings or spectroscopes (one for each student)
The lab activity will also require a full class period, including discussion time. Materials required (per lab group) are
- Cation solutions (20 to 25 mL)
- Wood splints
- Eye protection
Students will learn how indirect evidence can be used to describe the structure of atoms. Students will also learn about the structure of the atom itself, as well as the behavior of electrons that produces the line spectra.
National Science Education Standards
Science as Inquiry
- Abilities necessary to do scientific inquiry
- Understandings about scientific inquiry
- Structure and properties of matter
- Chemical reactions
History and Nature of Science
- Science as a human endeavor
- Nature of scientific knowledge
- Historical perspective