In this activity you will explore aspects of the makeup of crystals.

Crystals are much sought after by jewelers and industrialists alike. Their value is directly related to their perfection.

How and why does order exist in a crystal?

Crystals are common forms of solids. They are characterized by their order, compared with the disorder of liquids and the independence of atoms in gases. They are made of repeating units that can be as simple as a single kind of atom or as complex as a protein molecule.

Imagine we have created a slice through a crystal that is only one atom deep as shown in the model below. Run the model by clicking the PLAY arrow at the bottom of the model window.

1. Your model has a thermometer inside the model. Run the model and raise the temperature by clicking slowly on the red arrow on the thermometer.
2. Notice how the particles move and the ways in which they are constrained. Then, focus on one atom that is not on the edge, and look at the atoms around it. Repeat for other atoms.
3. Pause the model when you see “melting.”
4. Continue to run the model until you achieve complete melting.

4. Many manufacturers grow crystals for their industries. But in order to do this effectively, they have to know how to harness the forces that hold them together and allow them to grow. You have seen how atoms in crystals relate to their neighbors. Now you can discover how close atoms need to be in order to be added to a crystal. While the model is running, click on the Add One Atom (FAR) button and watch what happens to the atom added from a distance.

5. Now, while the model is still running, click on the Add One Atom (NEAR) button and watch the second atom.

6. Reset the model by clicking the BACK button of the player. Check the box to show forces under the model frame, and then run the model and follow steps 2 and 3 once again while the model is running.

1. What do you notice about how particles move in a crystal?
2. What is the first sign of a crystal melting?
3. What can you conclude about the attractions among atoms and how they contribute to crystal growth?

An industrial process wants to develop a high quality large crystal. What should they consider?

1. No crystal is perfect. There are several types of imperfections in crystals; the simplest type is called a “point defect.” Point defects are important to avoid in making the silicon wafers of semiconductors.

There are three important types of point defects:

• vacancies – a missing atom
• interstitials – atoms that occupy a site in the crystal structure where there is usually not an atom
• substitutionals – a different species of atom that is incorporated in the crystal structure

Defects are not just considered a problem; sometimes they are deliberately introduced to regulate certain processes, such as diffusion and oxidation. They are “one of the major ‘buttons’ to fiddle with when designing new materials” (Foll, University of Kiel). We still know very little about the effects of defects.

The model contains all three types of point defects. Can you identify them?

2. You want to cleave a crystal. But scientists do not really all agree about how a crystal cut propagates or travels along a crystal. Where might a break begin and how would it travel? Please provide a reasonable atomic-level hypothesis.