University of Virginia
Physics Department

## Cloud Model of the Atom I

A Physical Science Activity

Adapted from: Schwaner, T.D., Petty, J. T., Schwaner, L.A. (1994). Modeling Atoms and Molecules: A New Lesson for Upper Elementary and Middle School Students, The American Biology Teacher, 56:8, pp. 488-491.

2003 Virginia SOLs

• PS.1
• PS.3
• PS.4

Objectives

Students will

• describe the arrangement of electrons in atoms according to their energy levels;
• describe the structure of a water molecule.

Background Information

This activity offers a three dimensional model of atoms based on the cloud or quantum model. Students use B.B.'s to represent electrons inside of balloon electron clouds. The activity can be conducted as a student-centered activity where students work in small groups, or the teacher can lead the class through the steps with discussion. There are shortcomings within the model in that no nucleus is represented and the student must imagine its presence. Of particular value in the model of the more complex atom, oxygen, however, is the arrangement of electrons into two distinct orbital levels as represented by one balloon inside the other.

### Student Activity

Materials

• 1 12-inch and 1 5- inch balloon of identical color (described as "red" in the procedure)
• 2 5-inch balloons of identical color but of a different color than the first set of balloons (described as "green" in the procedure)
• 10 BB's

Procedure

1. Place 1 BB in each of the small green balloons. Carefully (without inhaling the B.B.) blow up each green balloon so it is approximately the size of a hen's egg. Tie each off. Each now represents the simplest possible atom, hydrogen. Strictly speaking, the model of hydrogen is incomplete as no nucleus is present, but one can imagine the presence of a nucleus at the center of the balloon. Rotate one of the balloons so the BB moves around on the inner wall. Then, add energy to the atom model by spinning the balloon faster. As in a real atom, the electron reflects the added energy. In a real atom, the electron will "jump" to a new energy level. In the model, the electron simply moves faster.
2. For a more complex atom, place 2 BB's in the small red balloon. Place 6 BB's in the large red balloon. Insert the uninflated small red balloon into the large red balloon. Hold the ends of the balloons together and carefully inflate the small red balloon while it remains inside the large balloon. Only inflate it enough so the walls stand out away from the B.B.'s. Tie it off. Next, inflate the large red balloon while continuing to push the small red balloon farther inside. Continue to inflate the large red balloon until it is significantly larger than the small balloon now inside. Tie off the large balloon. This atom now contains 8 electrons and represents oxygen. Point out to the students that the 8 electrons are grouped as 2 in the inner orbital (balloon) and 6 in the outer. Though this model is still deficient in that atoms are not distributed into orbitals, it still demonstrates that they occupy varying distances from the nucleus.
3. The oxygen atomic model and hydrogen atomic models can further be combined to represent a water molecule that would demonstrate the stability of an octet of electrons in the outer valence levels of atoms involved in chemical bonds.

Extensions

1. Students research historical development of atomic models.
2. Students research chemical properties of an element.

Students with Special Needs

This activity requires fine motor skills if it is to be conducted as a student-centered activity. It will work well as a teacher-centered demonstration if students are not able to manipulate the materials comfortably.

Click here for further information on laboratories with students with special needs.

Assessment

1. Students describe how to construct a model of atoms different than those in the activity using the materials included in the activity.
2. Students build atomic models using materials they choose.