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Unformatted text preview: C HAPTER 12 M OLECULAR S TRUCTURE 447 ts Monday morning, and youd like a cup of coffee, but when you try cranking up the stove to reheat yesterdays brew, nothing happens. Apparently, the city gas line has sprung a leak and been shut down for repairs. The coffee cravings are strong, so you rummage in the garage until you find that can of Sterno left over from your last camping trip. Youre saved. Both Sterno and natural gas contain compounds that burn and release heat, but the compounds in each of these substances are different. Natural gas is mostly methane, CH 4 , while Sterno contains several substances, including methanol, CH 3 OH. H H H H C C H H H O H methane methanol The oxygen atom in methanol molecules makes methanols properties very different than methanes. Methane is a colorless, odorless, and tasteless gas. Methanol, or wood alcohol, is a liquid with a distinct odor, and is poisonous in very small quantities. Chemists have discovered that part of the reason the small difference in structure leads to large differences in properties lies in the nature of covalent bonds and the arrangement of those bonds in space. This chapter provides a model for explaining how covalent bonds form, teaches you how to describe the resulting molecules with Lewis structures, and shows how Lewis structures can be used to predict the three-dimensional geometric arrangement of atoms in molecules. Methane or methanol? We use both to heat food. How do their moleculesand propertiesdiffer? 12.1 A New Look at Molecules and the Formation of Covalent Bonds 12.2 Drawing Lewis Structures 12.3 Resonance 12.4 Molecular Geometry from Lewis Structures Given a periodic table, identify the number of the group to which each element belongs. (Section 2.3) Given a chemical formula, draw a Lewis structure for it that has the most common number of covalent bonds and lone pairs for each atom. (Section 3.3) Write or identify the definitions of valence electrons, electron-dot symbol, lone pairs, Lewis structure, double bond, and triple bond. (Chapter 3 Glossary) Write or identify the definition of atomic orbital. (Section 11.1) Write electron configurations and orbital diagrams for the nonmetallic elements. (Section 11.2) Review Skills The presentation of information in this chapter assumes that you can already perform the tasks listed below. You can test your readiness to proceed by answering the Review Questions at the end of the chapter. This might also be a good time to read the Chapter Objectives, which precede the Review Questions. 12.1 A New Look at Molecules and the Formation of Covalent Bonds In Chapter 3, you were told that carbon atoms usually have four bonds, oxygen atoms usually have two bonds and two lone pairs, and hydrogen atoms form one bond. Using guidelines such as these, we can predict that there are two possible arrangements of the atoms of C 2 H 6 O....
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- Fall '06