Experiment 2

Atomic Bb's

A Particle Model of Metals

Objective:

The objective of this lab is to learn more about the basic particle model for metals.

Review of Scientific Principles:

Metallic crystals are not perfect. Sometimes there are empty spaces, vacancies, where an atom should be. There are also small mismatches, dislocations, in the rows of atoms, and these are found in all metals.

Applications:

Defects in the crystal structure of metals control many of their properties including hardness and ductility.

Time: 40 minutes

Materials and Supplies:

Plastic Petri Dish

Bb's

Procedure:

Your team will be given a covered Petri dish containing Bb's. Keep the dish flat on the table so no Bb's spill out! Answer the Questions which correspond to a given procedure. Try to write complete idea answers; that increases understanding.

1. The Bb's represent the atoms in a metal.

2. If you move the Petri dish back and forth the atoms move. This simulates the movement of atoms in a metal when it is heated.

3. Move the dish back and forth and try to get the atoms arranged as neatly as possible. Slowing the motion of the dish and gradually stopping it simulates the formation of the crystal.

4. Make a sketch below showing how the Bb's are arranged. You don't have to draw all the Bb's!

5. Make a sketch below showing the arrangement of Bb's around an empty space. When this happens in the metal it is called a vacancy. In a real crystal when atoms are out of line it is called a dislocation.

6. Move the dish back and forth somewhat rapidly. (Don't spill the Bb's). This simulates heating the metal. (Heating the metal gives the atoms more kinetic energy.)

Questions:

1. Describe the bonding between the atoms in a metal.

2. What type of energy do "moving" atoms possess?

3. Do the atoms in a crystal move?

4. Describe the arrangement of the Bb's. Are there any empty spaces, i.e., places where a Bb is missing?

5. Are the Bb's arranged perfectly? Would you expect atoms to be perfectly arranged?

6. Do more or less defects exist in the metal when it is heated?

Answers to Questions

1. The valence electrons of a metal atom are loosely held and considered to be shared by all the atoms in the crystal. This is called the electron sea model.

2. Kinetic energy.

3. Yes, they vibrate about an equilibrium position.

4. Student answer.

5. It is unlikely that the atoms are perfectly arranged. Some disorder is expected.

6. More defects exist at higher temperatures.