2010-01-28 - Intermolecular forces

2010-01-28 - Intermolecular forces - LIQUIDS AND SOLIDS...

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LIQUIDS AND SOLIDS What Are the Key Ideas? The condensed phases of matter result from attrac- tive intermolecular forces. Atoms, ions, and molecules form solids with their atoms in characteristic arrangements when they have insufficient energy to escape from their neighbors. They form liquids when they can move past their neighbors but cannot escape from them entirely. Why Do We Need to Know This Material? The materials of tomorrow will be formulated by understanding how the arrangements of their atoms and mole- cules determine their properties. Scientists understand and design new materials by considering how the properties and interactions of individual particles trans- late into bulk properties. This chapter continues the important task begun in Chapter 4, that of relating atomic and molecular properties to the structure and properties of bulk matter. What Do We Need to Know Already? This chapter uses the concepts of potential energy (Section A), coulombic interactions (Section 2.2), polar molecules and dipoles (Section 3.3), and intermolecular forces in gases (Section 4.15). M olecules attract one another. From that simple fact spring fundamentally important consequences. Rivers, lakes, and oceans exist because water molecules attract one another and form a liquid. Without that liquid, there would be no life. Without forces between molecules, our flesh would drip off our bones and the oceans would be gas. Less dramatically, the forces between molecules govern the physical properties of bulk matter and help to account for the differences in the substances around us. They explain why carbon dioxide is a gas that we exhale, why wood is a solid that we can stand on, and why ice floats on water. At very close range, molecules also repel one another. When pressed together, molecules resist further compression. That is why substances have a def- inite bulk and cannot be squashed down to a point. In Chapter 4, we considered gases, in which intermolecular forces play only a minor role. Here, we deal with liquids and solids, in which the forces that hold molecules together are of crucial importance for determining the physical proper- ties of bulk samples. Individual water molecules, for instance, are not wet, but bulk water is wet because water molecules are attracted to one another and to other substances. Individual water molecules neither freeze nor boil, but bulk water does, because in the process of freezing molecules stick together and in boil- ing they separate. We have to refine our atomic and molecular model of matter to see how bulk properties can be interpreted in terms of the properties of individual molecules, such as their size, shape, and polarity. We begin this chapter by exploring intermolecular forces, the forces between
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2010-01-28 - Intermolecular forces - LIQUIDS AND SOLIDS...

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