Chemistry CH 11

Chemistry CH 11 - 11 Chemical Bonding I: Basic Concepts...

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11 Chemical Bonding I: Basic Concepts 11-1 Lewis Theory : An Overview 11-2 Covalent Bonding: An Introduction 11-3 Polar Covalent Bonds 11-4 Writing Lewis Structure 11-5 Resonance 11-6 Exception to the Octet Rule 11-7 The Shapes of Molecules 11-8 Bond Order and Bond Length 11-9 Bond Energies Focus On Polymer — Macromolecular Substances
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11-1 Lewis Theory: An overview Lewis theory was proposed by two Americans, G. N. Lewis and Irving Langmuir, and a German, Walther Kossel in the period 1916-1919. 1. Electrons, especially those of the outermost (valence) electronic shell, play a fundamental role in chemical bonding. 2. In some cases, electrons are transferred from one atom to another. Positive and negative ions are formed and attract each other through electrostatic forces called ionic bonds. Some fundamental ideas associated with Lewis's theory are
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3. In other cases, one or more pairs of electrons are shared between atoms. A bond formed by the sharing of electrons between atoms is called a covalent bond. 4. Electrons are transferred or shared in such a way that each atom acquires an especially stable electron configuration. Usually this is a noble gas configuration, one with eight outer-shell electrons, or an octet. Lewis Symbols and Lewis Structures A Lewis symbol consists of a chemical symbol to represent the nucleus and core (inner-shell) electrons of an atom, together with dots placed around the symbol to represent the valence (outer-shell) electrons.
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Si Si a chemical symbol to represent the nucleus and core (inner-shell) electrons the valence (outer-shell) electrons For Example, Notice Lewis symbols are commonly written for main- group elements but much less often for transition elements.
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Example 11-1 Writing Lewis Symbols. Write Lewis symbols for the following elements: (a) N, P, As, Sb, Bi; (b) Al, I, Se, Ar. Solution (a) These are the elements of group 15. Their atoms all have five valence electrons (ns 2 np 3 ). The Lewis symbols all have five dots. N P As Sb Bi
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(b) Al is in group l3; I, in group l7 ; Se, in group l6; Ar, in group 18. I Al Se Ar Note that for main-group elements, the number of valence electrons, and hence the number of dots appearing in a Lewis symbol, is equal to the group number for the s-block elements and to the "group number minus 10" for the p-block elements.
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A Lewis structure is a combination of Lewis symbols that represents either the transfer or the sharing of electrons in a chemical bond. Na ¡Á Cl [ Na ] [ Cl ] ¡Á H ¡Á Cl H Cl ¡Á Ionic bonding (transfer of electrons) Lewis symbols Lewis structure Lewis structure Lewis symbols Covalent bonding (sharing of electrons)
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In these two examples, we designated the electrons involved in bond formation differently—(×) from one atom and (·) from the other. This helps to emphasize that an electron is transferred in ionic bonding and that a pair of electrons is shared in covalent bonding. Of course, it is impossible to distinguish between electrons, and henceforth we will use only dots (·) to represent electrons in Lewis structure.
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This note was uploaded on 05/07/2010 for the course CHEM 401 taught by Professor Chemistry during the Spring '10 term at Uni Potsdam.

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Chemistry CH 11 - 11 Chemical Bonding I: Basic Concepts...

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