University of Virginia
Physics Department

Demonstrating Quanta

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.3

Objectives

Students will

• define "quantum";
• describe atomic structure as quantized.

Motivation for Learning

Discrepant Event: Light Emission

Use a lightbulb, flashlight, or match to demonstrate the emission of light. Light the source of your choice, and ask students why the source is emitting light. If they respond, "Because it is burning," ask them what they think happens at the atomic level when something burns. Explain to them that when you "light" the source, you are adding an amount of energy to it. For an example, consider a neon bulb. When voltage is applied across the bulb, current flows through the gas within it. This flow of electrons impacts the electrons of the neon atoms within the bulb. In these collisions, kinetic energy is given to electrons with the atom, and they move to a higher energy level or "quantum" state. They may only remain there for an instant. Then they are pulled back down to their original quantum state. Since their energy must be conserved, they emit energy in the form of light energy. It is this light energy that we see when a light bulb, match, or flashlight "burns."

To aid in this difficult concept, follow the link to http://www.colorado.edu/physics/2000/quantumzone/bohr2.html. The applet halfway down the page allows teachers and students to interact with an atomic model. Explain to students that each dotted circle surrounding the nucleus represents a different energy or quantum level. The farther away from the nucleus, the higher the quantum state. By clicking on a higher energy level, a photon of light energy is shot into the orbiting electron. The electron then has gained enough energy to attain orbit at a higher quantum level. Eventually, the electron will emit a photon of light energy and drop to a lower quantum energy level, where it is more stable.

Background Information

Students often find the idea that electrons in atoms exist in quantized energy levels very difficult to grasp since most of our everyday experiences are with phenomena that are continuous. However, once students are introduced to a macroscopic example of quantization, they will become more comfortable with the idea. The activity described is one that a teacher could integrate into a class discussion of the quantum model of the atom.

Student Activity

Materials

• Stairs, chorus risers, or bleachers and/or
• Bookshelf and/or
• Chairs

Procedure

1. Introduce the definition of quantum to students. Many may have heard the term "quantum leap" or may have seen the old television show by that title. In this context, quantum is referring to a very sudden, discrete change or leap. This is very much how electrons behave within an atom when they absorb or emit energy. Electrons cannot be halfway between two energy levels.
2. Since we do not usually think of nature in a quantum sense, students will benefit from some common, quantized examples. One example is a staircase. When we ascend stairs, we can only be on a step-we cannot rest in between steps. (Students may argue that they can be "between" steps by having their feet on different steps, but still they are not truly between steps. Before introducing this example, explain to the students that this example is only a model of the idea. Also, establish a definition for what "being on a step" means.) Once the students accept the model, the arrangement of electrons within an atom can be accomplished by lining two students on one stair then several on a second stair (up to eight) and so on. Additionally, the students will readily see that in order to move from their assigned quantum state (stair) to a higher one, energy must be put into the system; that is, they must climb to the next available step.
3. Another example of quantization is a bookshelf where books can only exist on one shelf at a time and never in between.
4. Chairs can also be used to demonstrate the concept. Line up several chairs or desks (perhaps in groups of two, eight, and eighteen to replicate the electron arrangement for the first three energy levels of an atom). Place students in the chairs playing the role of electrons. Emphasize the impossibility of sitting between chairs to parallel the idea that electrons will not exist between energy levels.
5. An important short-fall in this demonstration is that within an atom, the energy levels are not evenly spaced as in the examples cited. The bookshelf example could include a bookshelf with moveable shelves that could be variably spaced.

Questions

1. What are the 3 main particles that make up atoms?
2. Can these particles be broken down into smaller particles?
3. Do we ever know exactly where an electron is as it orbits a nucleus?
4. What experiment did Ernest Rutherford perform and what did it tell us about the nature of atoms?
5. Can a chemical change alter the identity of an atom (change its atomic number)?
6. Can electrons exist between quantized energy levels?
7. How many electrons can each of the first 3 energy levels hold?

Extensions

1. Students conduct historical research on the development of the quantum model of the atom.
2. Students construct a time-line of the major developments of the atomic theory from ancient to modern times.

Students with Special Needs

Some students may not be able to participate in the stairs portion of the activity. Click here for further information on laboratories with students with special needs.

Assessment