Post on 20-Jan-2021
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Chapter 10Molecular
Geometry and Chemical
Bonding Theory
10 | 1
Molecular geometry• General shape of a molecule
• Determined by relative positions of the atomic
nuclei
A good theory of chemical bonding must describe the molecules correctly.• Molecular Geometry
• Chemical Bond Distance and Strength
• Other Properties
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in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.
EXPLAINING MOLECULAR GEOMETRY
Valence Shell Electron Pair Repulsion TheoryPhysical Theory of Molecular Geometry
Valence Bond TheoryQuantum Mechanical Theory of Chemical Bonding
Molecular Orbital TheoryQuantum Mechanical Theory of Chemical Bonding
Molecular Geometry and Polarity10 | 3©2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted
in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.
EXPLAINING MOLECULAR GEOMETRY
Valence Shell Electron Pair Repulsion TheoryPhysical Theory of Molecular Geometry
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Valence Shell Electron Pair Repulsion Model(VSEPR)
• Predicts the shapes of molecules and ions• Electrons repel each other• Valence electron pairs keep maximum distance
from one another as possible• Minimizing electron pair repulsion lowers energy• The number of electron pairs around a central
atom determines the shape
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Electron pairs include• electrons in bonds• lone pairs
• 2 to 6 pairs of electrons can be positioned around a central atom
• Each arrangement has a unique set of angles based on geometry
• The name matches the number of electron pairs
Electron Pairs around a Central Atom
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2 Electron Pairs
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3 Electron Pairs
• Electron Pair Geometry: count all electron pairs• Molecular Geometry: ignore electron pairs
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4 Electron Pairs
• Electron Pair Geometry: count all electron pairs• Molecular Geometry: ignore electron pairs
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5 Electron Pairs
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6 Electron Pairs
10 | 12
Examples of Balloon Models and Ball-and-Stick Models of Molecules.
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For these examples,Electron Pair Geometry = Molecular Geometry
10 | 13
• Long bonds give a large atom more space.• sp3 bond angles with large atom increase slightly to > 109.5o
• Other sp3 bonds angles decrease to < 109.5o.
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Large atoms need more space than small ones.
10 | 14
• Bonds get pushed away from electron pairs.• sp3 bond angles between shrink slightly to < 109.5o.
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Electron pairs need more space than bonds.
10 | 15
• Single, double, triple bonds treated as one electron pair unit• Bonds get pushed away from multiple electron pairs• sp2 bond angles around multiple bonds increase to > 120o
• sp2 bond angles between other atoms shrink slightly to < 120o
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Multiple bonds need more space than single bonds.
Predicting Molecular Geometry Using VSEPR
1. Draw the Lewis (electron-dot) structure.
2. Count the total number of bonding and nonbonding electron pairs around the central atom
3. Assign the electron pair geometry based on the total number
4. Assign the molecular geometry based on the number of bonded atoms
5. Single, double, triple bonds treated as one electron pair unit
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10 | 17
Methane, Ammonia, and Water
4 electron pairstetrahedral electron pair geometry
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4 bonds - tetrahedral molecular geometry3 bonds – trigonal pyramidal geometry2 bonds – bent molecular geometry
10 | 18
Sulfur Hexafluoride, SF4
5 electron pairs 4 bondstrigonal bipyramid electron pair geometry seesaw molecular geometry
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10 | 19
Xenon Difluoride, XeF2
5 electron pairs 2 bondstrigonal bipyramid electron pair geometry linear molecular geometry
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10 | 20
Chlorine Trifluoride, ClF3
5 electron pairs 3 bondstrigonal bipyramid electron pair geometry T-shaped molecular geometry
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10 | 21
Iodine Pentafluoride, IF5
6 electron pairs 5 bondsoctahedral electron pair geometry square pyramidal molecular geometry
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10 | 22
Tellurium Tetrachloride, TeCl4
5 electron pairs 4 bondstrigonal bipyramidal electron pair geometry seesaw molecular geometry
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10 | 23
Organic Molecules
4 electron pairs around carbondifferent electron pair geometries
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4 bonds – tetrahedral molecular geometry3 bond groups – trigonal planar geometry2 bond groups – linear molecular geometry