Lecture 8 part I Chem 102 IR(2)

Lecture 8 part I Chem 102 IR(2) - Chapter 8 Part I Bonding...

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Chapter 8 Part I Bonding
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Questions to Consider • What is a “ chemical bond ”? • Why do atoms bond with each other to form compounds? • How do atoms bond with each other to form compounds? The way atoms combine influences both the physical and chemical properties of the compounds formed.
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A chemical bond • Forces that hold groups of atoms together and make them function as a unit. • A bond will form if the energy of the aggregate is lower than that of the separated atoms. System always wants to achieve the lowest possible energy state.
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Types of bonding • Ionic Bonding – electrons are transferred ; Electrostatic interaction between cation and anion Ionic Solids (NaCl) • Covalent Bonding – electrons are shared equally; Electrons are attracted to both positively charged nuclei Diatomics H 2 • Polar covalent bonding- electrons unequally shared; HF
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Chemical Bonds Covalent Ionic
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Covalent Bonding Molecules want to achieve the lowest possible energy configuration Interaction of two hydrogen atoms Lowest E state is where a bond occurs
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Covalent Bonding Molecules want to achieve the lowest possible energy configuration H H The length of the bond will be defined by this energy state When very far apart the H atoms experience no interaction As they approach one another their electron density begins to spread out as they start to interact H-H Continues until they form a molecule (optimal distance)
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Covalent Bonding H-H H--------F Polar covalent molecules exhibit a DIPOLE MOMENT δ + δ - Polar Covalent shared electrons are not equally distributed, leading to negative and positive ends of a molecule Pure Covalent shared electrons are equally distributed, leading to an electronically neutral molecule H 2 , N 2, CH 4 CO, HF The more electronegative atom will pull the electron density towards itself
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Polar covalent bonding Molecules with a dipole moment orient themselves in an electric field H--------F δ + δ -
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Electronegativity The ability of an atom in a molecule to attract shared electrons to itself. For a molecule HX , the relative electronegativities of the H and X atoms are determined by comparing the measured H–X bond energy with the “expected” H–X bond energy.
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Electronegativity • On the periodic table, electronegativity generally increases across a period and decreases down a group. • The range of electronegativity values is from 4.0 for fluorine (the most electronegative) to 0.7 for cesium and francium (the least electronegative).
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The Pauling electronegativity values Determined electronegativity values
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Electronegativity Arrange the following bonds in order of increasing polarity Cl-H, O-H, H-H, S-H, F-H H-H < S-H < Cl-H < O-H < F-H The larger the difference in electronegativities between the atoms in a compound, the more ionic character the compound will exhibit
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Example 1.
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This note was uploaded on 04/21/2011 for the course CHEM 102 taught by Professor Peterpastos during the Spring '08 term at CUNY Hunter.

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Lecture 8 part I Chem 102 IR(2) - Chapter 8 Part I Bonding...

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