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Unformatted text preview: Chapter 9: Chemical Bonds Chemical bonds are described as a result of attractive and repulsive forces. Ionic bonding involves transfer of electrons and attraction of the resulting ions, while covalent bonding involves sharing of electrons. Electronegativity is used to predict bond polarity and as an aid in drawing Lewis structures, for which other strategies are presented. Resonance and exceptions to the octet rule are examined. Bond lengths and bond energies are related to types of bonds, and bond energies are used to determine enthalpy changes. The chapter concludes with a discussion of bonding in alkenes and alkynes, and an introduction to polymers. Summary of Sections 9.1 Chemical Bonds: A Preview Chemical bonds are presented as the balance between the attractions between nuclei and electrons, and the repulsions between nuclei and between electrons. 9.2 The Lewis Theory of Chemical Bonding: An Overview The section opens with statements of the fundamental ideas of Lewis theory including the noble gas rule (or the octet rule). A Lewis symbol consists of the symbol for the element surrounded by dots, one dot for each valence electron. Usually the dots occupy each side (right, left, top, bottom) singly until there are more than four, when they pair up. Further note that the number of dots equals the family number for representative elements. The rest of the chapter builds on this section. O BJECTIVES : Write Lewis symbols for elements and compounds. Know what the octet rule is and how to use it. 9.3 Ionic Bonds and Ionic Crystals The reaction of sodium atoms with chlorine atoms to form sodium chloride occurs because each sodium atom gives up an electron to form a sodium ion with an electron configuration the same as the noble gas neon. In addition, each chlorine atom gains an electron to form a chloride ion with the same electron configuration as the noble gas argon. These oppositely charged ions are attracted by an ionic bond. Eventually, many ions join together to form a crystal of NaCl. This all is shown with spdf electron configurations. 9.4 Using Lewis Symbols to Represent Ionic Bonding Lewis symbols are presented as a way to represent ionic bonding in a simpler, more compact way than spdf notation. The symbols express the role of valence electrons in ion formation. O BJECTIVE : Describe ionic bonds. 9.5 Energy Changes in Ionic Compound Formation Now there is sufficient information to speak of the energetics of forming ionic compounds. Thus, the Born-Haber cycle for several simple ionic compounds is sketched out. The cycle is presented as an application of Hesss law, without an indication of how lattice energies are calculated from crystal structures. These concepts need to be briefly explained as they are introduced in lecture....
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