GSCI 163

1. GSCI 163 Lecture 11

2. Crystals How does the physical properties of the materials depend on how the atoms and molecules are organized? Why is a paper clip malleable and a glass rod is not? Why do gemstones have the face they do? Why had things are usually brittle?

3. Peaking at the microscopic level… • Atoms of a solid substance will arrange themselves according to electromagnetic forces. Example of NaCl – The sodium ions are positively charged and repel each other while attracted to the negatively charged chlorine ions. Thus they form an ordered arrangement that repeats itself in space. This is called a crystalline structure.

4. Different crystals The way each compound arranges itself into a crystalline structure depends on the electrostatic attractions between them. Here are examples of different ways to arrange atoms in a crystalline structure.

5. Visualize the crystals Log in in your computer and reach this page http://www.uwsp.edu/chemistry/tzamis/solidcrystalexamps.html Log in information: User: physics

6. Cleaving a crystal How do you break a ionic crystal? Imagine trying to force a top layer of crystals to move past each other. Because of the repulsion between similar elements, you will most likely break the ionic bonds between them rather then make them adjust to the new configuration. The crystal break and leave a smooth surface. The same thing happens for covalent bond crystals. Diamond, for example is cut in such a way to preserve the smooth faces to make them sparkle and show their brilliance.

7. Structure of metals As you have already experience by bending silverware, clips, soda cans and the like, metals are typically ductile and malleable. Why is that? Metallic ions give up electrons which are free to move in metallic compounds. These free electrons can screen the charge of the ions causing them to easily slide pass each other when stressedor compressed. When many rows pass each other they change shape macroscopically.

8. Polycrystalline and amorphous materials Ions line up to each other quite well, but is difficult for them to do it for vast regions of space making a single crystal. What you have then are regions where you find various crystals packed together. They are polycrystalline. Sometimes the solid does not have any crystalline structure at all. In this case we called them amorphous. Glass is the most common example of an amorphous material.

9. Next class • Carbon chemistry