Ch 10 Chem

Chapter Overview

  • Focus on Chemical Bonding II: Molecular Geometry and Hybridization of Atomic Orbitals.

  • Introduced by Jason Overby, Copyright © 2022 McGraw Hill LLC.

Valence Shell Electron Pair Repulsion (VSEPR) Model

  • The VSEPR model predicts the geometry of a molecule based on the electrostatic repulsions between electron pairs (bonding and nonbonding).

Key VSEPR Classes

  • Notation:

    • AB extit{n} where:

    • A = central atom

    • B = atoms bonded to central atom

    • n = number of bonded atoms or lone pairs.

Basic Classifications

  1. AB2

    • Number of atoms bonded to central atom: 2

    • Number of lone pairs: 0

    • Arrangement of electron pairs: Linear

    • Molecular Geometry: Linear

    • Example: Beryllium Chloride (1l)

  2. AB3

    • Number of atoms bonded to central atom: 3

    • Number of lone pairs: 0

    • Arrangement of electron pairs: Trigonal Planar

    • Molecular Geometry: Trigonal Planar

    • Example: Boron Trifluoride ()

  3. AB4

    • Number of atoms bonded to central atom: 4

    • Number of lone pairs: 0

    • Arrangement of electron pairs: Tetrahedral

    • Molecular Geometry: Tetrahedral

    • Example: Methane ()

  4. AB5

    • Number of atoms bonded to central atom: 5

    • Number of lone pairs: 0

    • Arrangement of electron pairs: Trigonal Bipyramidal

    • Molecular Geometry: Trigonal Bipyramidal

    • Example: Phosphorus Pentachloride (  )

  5. AB6

    • Number of atoms bonded to central atom: 6

    • Number of lone pairs: 0

    • Arrangement of electron pairs: Octahedral

    • Molecular Geometry: Octahedral

    • Example: Sulfur Hexafluoride ()

Summary of VSEPR Table (10.1)

  • Shows electron-pair geometry and molecular geometry for varying configurations.

Tetrahedral Bond Angles

  • Bond Angles:

    • Regular tetrahedral: 109.5°

    • Lone pair vs. bonding pair repulsions lead to varying angles.

    • Molecules with lone pair adjustments:

    • NH3: 107° (Trigonal pyramidal)

    • H2O: 104.5° (Bent)

Advanced VSEPR Configurations

VSEPR Class 3 Electron Groups

  • AB3: Trigonal Planar - Trigonal Planar

  • AB2E: 2 Bonds, 1 Lone Pair - Bent

VSEPR Class 4 Electron Groups

  • AB4: Tetrahedral - Tetrahedral

  • AB3E: 3 Bonds, 1 Lone Pair - Trigonal Pyramidal

  • AB2E2: 2 Bonds, 2 Lone Pairs - Bent

VSEPR Class 5 Electron Groups

  • AB5: Trigonal Bipyramidal - Trigonal Bipyramidal

  • AB4E: 4 Bonds, 1 Lone Pair - Distorted Tetrahedron

  • AB3E2: 3 Bonds, 2 Lone Pairs - T-shaped

  • AB2E3: 2 Bonds, 3 Lone Pairs - Linear

VSEPR Class 6 Electron Groups

  • AB6: Octahedral - Octahedral

  • AB5E: 5 Bonds, 1 Lone Pair - Square Pyramidal

  • AB4E2: 4 Bonds, 2 Lone Pairs - Square Planar

Predicting Molecular Geometry

  1. Draw Lewis Structure: Understand the arrangement of atoms.

  2. Count Lone Pairs and Bonded Atoms: Essential for determining geometry.

  3. Use VSEPR Theory at this stage for accurate predictions.

Example 10.1: Geometry Predictions

  • Molecules Analyzed:

    • (a) AsH3: Trigonal Pyramidal

    • (b) OF2: Bent

    • (c) C2H4: Trigonal planar (120°)

  • Molecule Shapes: Determined primarily by arrangement of bonded pairs versus lone pairs.

Dipole Moments and Polar Molecules

  • Definitions:

    • Dipole Moment (Q): Q is the charge multiplied by distance r between charges.

    • Polar molecules demonstrate electron density displacement, resulting in partial charges (+ and -).

Behavior of Polar Molecules

  • Polar molecules align their ends with an electric field, resulting in effects observed in microwave ovens and electric fields.

Summary of Results for Dipole Moments (Table 10.3)

  • Examples of dipole moments of specific molecules:

    • HF: Linear, Dipole Moment = 1.92 D

    • H2O: Bent, Dipole Moment = 1.87 D

Hybridization

  • Definition: Mixing of two or more atomic orbitals to form hybrid orbitals.

    • Hybrid orbital count equals the number of atomic orbitals mixed.

    • Types of Hybridization:

    1. sp: Linear (e.g., BeCl2)

    2. sp²: Trigonal planar (e.g., BF3)

    3. sp³: Tetrahedral (e.g., CH4, NH3, H2O)

    4. sp³d: Trigonal bipyramidal (e.g., PCl5)

    5. sp³d²: Octahedral (e.g., SF6)

Important Hybrid Orbitals (Table 10.4)

  • Overview of hybrid orbitals and their shapes corresponding to electron groups.

Example 10.3: Determining Hybridization State

  • Strategy: Count the lone pairs and bonded atoms to find hybridization.

    • Examples include BeH2, AlI3, and PF3.

Example 10.5: Formaldehyde Bonding

  • Explains bonding processes and hybridization in accordance with Lewis structures.

Molecular Orbital Theory

  • Interactions create bonding or antibonding molecular orbitals.

  • Key Points:

    • Number of molecular orbitals = number of atomic orbitals combined.

    • Stable bonding orbitals lower energy, while antibonding raise energy.

Bond Order Calculation

  • Equation: bond order = $(Nb - Na) / 2$

    • $N_b$ = number of electrons in bonding orbitals

    • $N_a$ = number of electrons in antibonding orbitals.

Delocalized Molecular Orbitals

  • Not confined between two adjacent atoms; example includes benzene (C6H6).

  • Allows for greater stability through electron delocalization.

Key Examples and Applications

  • Chemistry in Action: Mesh of chemical theory and practical applications such as microwave ovens and material structures like Buckyballs.

Conclusion

  • The breadth of molecular geometry and hybridization principles underscore the fundamentals of chemical bonding. Each model offers insights essential for understanding molecular structure, reactivity, and physical properties. Understanding these concepts equips students to predict the behavior and shape of molecules effectively.