Lect_chap_9_for_lecture_use

Lect_chap_9_for_lecture_use - Chapter 9 Molecular Geometry...

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Chapter 9 Chapter 9 Molecular Geometry and Bonding Theories Molecular Geometry and Bonding Theories CHEMISTRY The Central Science Professor Li Niu
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Lewis structures give atomic connectivity –which atoms are physically connected to which in a molecule, but not the shape and size of the molecule The shape of a molecule is determined by its bond angles and bond length --- consider CCl 4 molecule below experimentally we find all Cl-C-Cl bond angles are 109.5 ° . Thus, the molecule cannot be planar – cannot be predicted from Lewis structure drawing 9.1 Molecular Shapes 9.1 Molecular Shapes Tetrahedron object Ball-and-stick model Space-filling model
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Angles, Angles, and and Shapes Shapes the key words for this chapter the key words for this chapter to think in to think in three dimensions three dimensions is a must
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To predict molecular shape, we assume the valence electrons repel each other . Therefore, the molecule adopts whichever 3D geometry to minimize this repulsion We call this model V alence S hell E lectron P air R epulsion ( VSEPR ) theory ( pronounced as pronounced as “vesper vesper ) Remember: like charges repel and unlike charges attract 9.1 Molecular Shapes 9.1 Molecular Shapes
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Examples for AB 2 and AB 3 molecules Fig. 9.2
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In general, there are five fundamental geometries for molecular shapes based on AB n composition – AB n molecule Fig. 9.3
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When considering geometry, we consider the central atom the corner atoms can be removed which creates more geometries. We consider all electrons, i.e., lone pairs (non-bonding pair) and bonding pairs . When naming the molecular geometry, we focus only on the positions of the existing atoms. 9.1 Molecular Shapes 9.1 Molecular Shapes
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To determine the shape of a molecule, we define the electron domain geometry by the positions in 3D space of all electron pairs (bonding or non-bonding) The electrons adopt an arrangement in space to minimize e - -e - repulsion This implies that whenever possible, a symmetry is adopted 9.2 The VSEPR Model 9.2 The VSEPR Model
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To determine the electron pair geometry 1. draw the Lewis structure, 2. count the total number of electron pairs around the central atom, 3. arrange the electron pairs in one of the above geometries to minimize e - -e - repulsion, and count multiple bonds as one bonding pair. VSEPR Model VSEPR Model
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The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles We determine the electron pair geometry by only looking at electrons We name the molecular geometry molecular geometry by the positions of atoms ( ignore lone pairs in the molecular geometry) All the atoms that obey the octet rule have tetrahedral electron pair geometries 9.2 VSEPR Model 9.2 VSEPR Model
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The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles • By experiment, the H-X-H bond angle decreases on moving from C to N to O (CH 4 , NH 3 and H 2 O ): • Therefore, the bond angle decreases as the number of lone pairs increases.
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This note was uploaded on 01/18/2009 for the course CHM 120 taught by Professor Pande during the Spring '08 term at SUNY Albany.

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Lect_chap_9_for_lecture_use - Chapter 9 Molecular Geometry...

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