ABSTRACT
The atoms that make up all matter in the universe arrange themselves in different ways to form rocks, water, galaxies, trees, or people. Different atoms interact in different ways to form a substance. When the interacting atoms of a substance are not moving about too much, they occupy more or less xed positions and arrange themselves in a geometric pattern which minimizes the interaction energy. This pattern is repeated throughout the substance. When this happens the substance is called a crystal (Figure 9.1).
