University of Virginia Physics Department Cloud Model of the Atom I A Physical Science Activity

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. 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.