Recording-2025-03-25 chem. ch. 7

Shape of Molecules and Bonding Theory

• Electron Domains: Discuss the concept of electron domains and how they help predict both electron domain geometry (EDG) and molecular geometry of compounds.

• Types of Intermolecular Forces: Be familiar with various intermolecular forces:

  • Dipole-Dipole Interactions: Occur between polar molecules.

  • Hydrogen Bonding: Strong type of dipole-dipole interaction, particularly with H bonded to F, O, or N.

  • Dispersion Forces: Weak forces due to temporary dipoles in nonpolar molecules.

  • Ion-Dipole Interactions: Forces between an ion and a polar molecule.

• Valence Bond Theory (VBT):

  • Bonds are formed by the overlap of atomic orbitals.

  • For bonding to occur, two conditions must be met:

    • Atomic orbitals must be singly occupied.

    • Electrons must have opposite spins (spin up and spin down).

• Hybridization:

  • Definition: Mixing of atomic orbitals to explain molecular bonding/geometry that cannot be explained by VBT alone.

  • Concept of Promotion: Involves moving an electron from a lower energy orbital to a higher one (for instance, in carbon, promoting from 2s to 2p). This creates excited states (e.g., Beryllium star, Carbon star).

  • Hybridization allows the forming of equal-energy hybrid orbitals (e.g., sp, sp², sp³, sp³d, sp³d²).

Examples of Hybridization

• Beryllium Chloride (BeCl₂):

  • Beryllium has 4 electrons but all occupied; requires hybridization to explain bonding.

  • One 2s and one 2p orbital mix to form two sp orbitals, allowing overlap with Chlorine.

• Boron Trifluoride (BF₃):

  • Boron has 5 electrons needing to share 3 for BF₃; utilizes hybridization to form three sp² orbitals after promoting an electron.

• Methane (CH₄):

  • Carbon has 4 electrons; undergoes promotion to create excited state carbon and then hybridizes to form four sp³ orbitals.

• Phosphorus Pentachloride (PCl₅):

  • Phosphorus has 5 total electrons; promotion allows access to 3d orbitals. Hybridizes to form five sp³d orbitals.

• Sulfur Hexafluoride (SF₆):

  • Hybridizes using sp³d² to bond with six fluoride atoms, allowing for six bonds.

Electron Domain Geometry

• Important Rule: The number of electron domains (single, double bonds, or lone pairs) determines hybridization:

  • 2 Electron Domains → sp

  • 3 Electron Domains → sp²

  • 4 Electron Domains → sp³

  • 5 Electron Domains → sp³d

  • 6 Electron Domains → sp³d²

Molecular Orbital Theory

• Purpose of Molecular Orbital Theory:

  • Explains phenomena like paramagnetism and diamagnetism.

• Bond Order:

  • The formula for bond order:[ \text{Bond order} = \frac{(\text{Number of electrons in bonding orbitals}) - (\text{Number of electrons in antibonding orbitals})}{2} ]

  • Hydrogen (H₂) Example: Bond order = 2/2 = 1 (exists)

  • Helium (He₂) Example: Bond order = 0/2 = 0 (does not exist)

• Paramagnetic vs Diamagnetic:

  • Paramagnetic = at least one unpaired electron.

  • Diamagnetic = all electrons paired.

Summary of Learning Objectives

• Understand hybridization and promote theory in bonding phenomena for various compounds.

• Be able to predict shapes and bond types in molecular structures.

• Prepare for quizzes and exams based on detailed examples and bond theories covered in class.

Preparation Tips

• Review the hybridization process for various compounds thoroughly, as hybridization is used frequently to explain the geometry and bonding.

• Practice predicting molecular shapes using VSEPR and understanding the intermolecular forces.

• Make sure concepts such as electronegativity, electron affinity, and other ionic interactions are well understood in correlation with bonding theory.

Make sure to ask questions during lab or office hours if unsure of concepts.