Module #5

Chapter Overview

This chapter focuses on molecular structure and the theories of covalent bonding, particularly through the lens of VSEPR (Valence Shell Electron Pair Repulsion) Theory and Valence Bond (VB) Theory. It delves into the geometry of various molecular shapes and types of bonding, providing a foundational understanding necessary for diverse applications in chemistry.

Chapter Goals

1. A Preview of the Chapter: Understanding the core theories of covalent bonding and molecular geometries.

2. Valence Shell Electron Pair Repulsion (VSEPR) Theory: Introduction to predicting shapes of molecules.

3. Polar Molecules: Investigating how molecular geometry influences polarity.

4. Valence Bond (VB) Theory: Examines overlap of atomic orbitals in bond formation.

VSEPR Theory

Fundamental Concepts

• VSEPR theory is founded on the principle that regions of high electron density (electron groups, EG) will arrange themselves to minimize repulsion.

• Each bonded atom and lone pair on the central atom counts as one EG regardless of bond type (single, double, or triple).

Electronic Configurations

• Linear Geometry (AB2): Achieved when there are two electron groups, resulting in a 180° bond angle.

• Trigonal Planar Geometry (AB3): Three electron groups at 120° angles.

• Tetrahedral Geometry (AB4): Four electron groups around a central atom create a 109.5° angle.

Molecular Geometry and Stability

• The arrangement of atoms determines a molecule's stability. For instance, a molecule is considered most stable when the electron groups are maximally spaced.

• An example includes methane (CH4), where both electronic and molecular geometries are tetrahedral.

• Water (H2O), despite having a tetrahedral electronic geometry, exhibits bent molecular geometry due to lone pairs.

Influence of Molecular Geometry on Polarity

Conditions for Polarity

• A molecule becomes polar if it possesses a polar bond or a lone pair that cannot symmetrically cancel out. An example of a polar molecule is ammonia (NH3), featuring a trigonal pyramidal shape that leads to an overall dipole moment.

• Conversely, diatomic molecules like Cl2 are nonpolar as their symmetrical arrangements lead to cancelled dipole moments.

Valence Bond Theory

Orbital Overlap

• Valence Bond Theory describes covalent bonding as a sharing of electrons via the overlap of atomic orbitals, with hybridization playing a role in forming new orbitals to support molecular geometry as predicted by VSEPR.

• For various electronic geometries, specific hybridizations can be identified:

  • Linear (sp): one s and one p orbital hybridized.

  • Trigonal planar (sp²): one s and two p orbitals hybridized.

  • Tetrahedral (sp³): one s and three p orbitals hybridized.

Summary of Geometries

Molecular Shapes

• Trigonal Bipyramidal (AB5): All five substituents lead to a nonpolar configuration, with distinct geometries if lone pairs are involved (e.g., seesaw, T-shaped).

• Octahedral Geometry (AB6): Molecules may adopt a square pyramidal shape with one lone pair or a square planar shape with two, leading to polar or nonpolar outcomes depending on substituent symmetry.

• The chapter concludes with examples of double and triple bonds in organic compounds, reinforcing concepts of electron sharing and hybridization.

Final Thoughts

Understanding molecular structure is essential in comprehending chemical behavior and interactions. The insights gained from VSEPR and VB theories not only aid in predicting molecular shapes but also in grasping the essence of molecular polarity, which has significant implications in chemical reactions and bonding scenarios.