Chapter 9 Molecular Geometry

# Chapter 9 Molecular Geometry - Chapter 9 Chapter Molecular...

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Chapter 9 Chapter 9 Molecular Geometry and Bonding Molecular Geometry and Bonding Theories Theories

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Lewis structures give atomic connectivity: they tell us which atoms are physically connected to which. However, do not show their overall shape) A molecule’s shape is determined by its bond angles. • Consider CCl 4 : experimentally we find all Cl-C-Cl bond angles are 109.5 ° . Therefore, the molecule cannot be planar. All Cl atoms are located at the vertices of a tetrahedron with the C at its center. Molecular Shapes Molecular Shapes
Molecular Shape of CCl Molecular Shape of CCl 4 4

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In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts which ever 3D geometry minimized this repulsion. We call this process V alence S hell E lectron P air R epulsion ( VSEPR ) theory. • There are simple shapes for AB 2 and AB 3 molecules (see page 347). VSEPR Theory VSEPR Theory
Molecular Shape: Five Fundamental Geometries

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When considering the geometry about the central atom, we consider all electrons (lone pairs and bonding pairs). When naming the molecular geometry, we focus only on the positions of the atoms. Naming Molecular Geometry
To determine the shape of a molecule, we distinguish between lone pairs (or non-bonding pairs, those not in a bond) of electrons and bonding pairs (those found between two atoms). 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. VSEPR Model VSEPR Model

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To determine the electron pair geometry: draw the Lewis structure, count the total number of electron pairs around the central atom, 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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We determine the electron pair geometry only looking at electrons. We name the molecular geometry by the positions of atoms. We ignore lone pairs in the molecular geometry. All the atoms that obey the octet rule have tetrahedral electron pair geometries. The Effect of Nonbonding Electrons
By experiment, the H-X-H bond angle decreases on moving from C to N to O: Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs. Therefore, the bond angle decreases as the number of lone pairs increase. 104.5 O 107 O N H H H C H H H H 109.5 O O H H The Effect of Nonbonding Electrons on Bond Angles

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Similarly, electrons in multiple bonds repel more than electrons in single bonds. C O Cl Cl 111.4 o 124.3 o The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles
• Atoms that have expanded octets have AB 5 (trigonal bipyramidal) or AB 6 (octahedral) electron pair geometries. For trigonal bipyramidal structures there is a plane

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Chapter 9 Molecular Geometry - Chapter 9 Chapter Molecular...

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