Review of Lewis Structures and Molecular Geometry

Review of Lewis Structures and Molecular Geometry

Introduction

  • Setting: Chemistry review session in a classroom, focusing on Shakes Molecule and Organic Chemistry.

  • Reminder: Reference to a website (loopstructure.net) for additional practice on Lewis structures.

Schedule Overview

  • Current Focus: Completing the unit on Shakes Molecules before transitioning to Organic Chemistry.

Key Concepts of Lewis Structures

Overview of Previous Lessons

  • Lewis Charges: Introduction to how to calculate Lewis structures of molecules.

  • Electron Groups: Understanding how to categorize based on the VSEPR (Valence Shell Electron Pair Repulsion) theory, specifically around the AXE notations.

    • Example AX2E: Two terminal atoms with one lone pair results in a specific molecular geometry.

  • Electron Group Geometry: Categorizing electron arrangements.

    • Trivial Cases: Raw geometry for three electron groups gives a trigonal planar arrangement.

Examples: Geometry with Different Electron Groups

  1. Five Electron Groups

    • Geometry: Trigonal bipyramidal.

    • Bond Angles: 120 degrees (equatorial) and 90 degrees (axial).

    • Visualization: Description of a five-sided figure called a trigonal bipyramid, highlighting the separation of electron groups.

    • Variations with Lone Pairs: Lone pairs change the geometry to shapes like seesaw, T-shaped, and linear.

    • AXE Cases:

      • A5 (no lone pairs)

      • A4E (one lone pair)

      • A3E2 (two lone pairs)

      • A2E3 (three lone pairs)

    • Homework Instructions: Use provided charts to find information based on the number of electron groups.

  2. Six Electron Groups

    • Geometry: Octahedral.

    • Description: Discussion about arranging bonds to achieve equitable 90-degree angles in three-dimensional space.

    • Visualization: Analogy with a Rubik's Cube; six electron groups positioned at the faces of the cube, emphasizing the three-dimensional arrangement.

    • Noteworthy Molecule: Sulfur hexafluoride (SF6) as an example of an octahedral structure; it exhibits the exceeding of the octet rule due to sulfur's larger size and lower electronegativity.

Exceeding the Octet Rule

  • Definition: Atoms like sulfur can exceed the octet rule when surrounded by highly electronegative atoms (e.g., fluorine).

  • Context: Various AX cases that allow for exceeding the octet rule.

Molecular Shape Examples

Acetone (C3H6O)

  • Reference: Acetone as a common organic molecule, essential in both chemistry and everyday products like fingernail polish remover.

  • Lewis Structure: Provided by the textbook; involves multiple central atoms.

    • Carbon Atoms: Each end carbon (AX4) is tetrahedral while the central carbon (AX3) is trigonal planar.

  • Angles and Polarity: Discussion of bond angles being slightly less than the theoretical values due to double bonds and lone pairs.

  • Determining Polarity: Application of positive and negative dipole moments across the molecule leads to determining overall molecular polarity.

Geometric Isomers of CCl2H2

  • Illustration of Lewis Structures: Correct and incorrect Lewis structures create different isomers:

    • Cis and Trans Isomers:

      • Trans Isomer: Chlorine atoms on opposite sides, resulting in nonpolar characteristics.

      • Cis Isomer: Chlorine atoms on the same side, leading to polar characteristics.

  • Measuring Polarity: Defined using the dipole moment (BC) in units of Debye (D).

    • Example Values: Nonpolar has a dipole moment of 0 D, while the polar version may measure around 4.9 D.

  • Impact on Physical Properties: Connection made with boiling points in polar molecules like water (100°C) compared to nonpolar counterparts, highlighting the relationship between polarity and boiling points due to intermolecular forces.

Introduction to Formal Charge and Lewis Structure Assessments

Formal Charge Calculations

  • Definition: Formula used for calculating formal charge on individual atoms in a molecule.

  • General Understanding: Concept that formal charge helps determine the best Lewis structure among alternatives, favoring arrangements where higher electronegativity atoms carry negative charges.

  • Example with BF3: Analyzing two Lewis structures, considering boron’s lack of an octet versus fluorine exceeding its expectations leads to discussion regarding the 'better' structure based on formal charge distribution.

Summary and Conclusion

  • Critical Takeaways: Reiteration of the importance of Lewis structures, molecular geometry, polarity, and the formal charge method as vital tools in understanding chemical bonding and molecular interactions.

  • Transition to Organic Chemistry: Following these foundational principles, the course will now move into the domain of organic chemistry, where these concepts find specialized applications.