VSEPR and Molecular Geometry

V.S.E.P.R. & Molecular Geometry

Overview of Valence Shell Electron Pair Repulsion (VSEPR)

  • Definition: VSEPR is a modeling system that provides a framework to visualize how molecules orient themselves in three-dimensional space.

  • Importance: The three-dimensional shape of a molecule is crucial because:

    • Structure is connected to function.

    • Structure is connected to properties:

    • Boiling Point (BP)

    • Melting Point (MP)

    • Reactivity

Structure and Function of Molecules

  • Active Site: Location on an enzyme where substrate binds.

  • Enzyme: Biological catalyst that accelerates chemical reactions.

  • Substrate: The reactant molecule upon which an enzyme acts.

Guiding Principles of VSEPR

  • Minimization of Repulsion: The fundamental concept is to minimize repulsion between like charges by:

    • Moving electrons farther apart to increase their distance.

    • Maximizing attraction between opposite charges.

Analyzing Molecular Shape Using VSEPR

  • Key Factors to Consider:

    1. The number of atoms present in the molecule.

    2. The presence of lone pairs on the central atom, which are significant as they exert force on other electrons.

  • Electron Domains:

    • Definition: An electron domain is either a lone pair of electrons or a bond (single, double, or triple).

    • Count: A single bond, double bond, or triple bond counts as one domain.

    • A lone pair also counts as one domain.

Molecular Geometry Based on Electron Domains

Molecules with 2 Atoms

  • Domains: 1

  • Shape: Linear

  • Bond Angle: 180°

  • Example: Oxygen (O2)

Molecules with 3 Atoms (No Lone Pairs)

  • Domains: 2

  • Shape: Linear

  • Bond Angle: 180°

  • Example: Carbon Dioxide (CO2)

Molecules with 3 Atoms (With Lone Pairs)

  • Domains: 3 or 4

  • Shape: Bent

  • Bond Angle: 104.5°

  • Example: Water (H2O)

Molecules with 4 Atoms (No Lone Pairs)

  • Domains: 3

  • Shape: Trigonal Planar

  • Bond Angle: 120°

  • Example: Boron Trifluoride (BH3)

Molecules with 4 Atoms (With Lone Pairs)

  • Domains: 4

  • Shape: Trigonal Pyramidal

  • Bond Angle: < 109.5°

  • Example: Ammonia (NH3)

Molecules with 5 Atoms (No Lone Pairs)

  • Domains: 4

  • Shape: Tetrahedral

  • Bond Angle: 109.5°

  • Example: Methane (CH4)

Molecules with 5 Atoms (With Lone Pairs)

  • Domains: 5

  • Shape: See Saw

  • Bond Angles: Not Applicable (NA)

  • Example: Sulfur Tetrafluoride (SF4)

Molecules with 6 Atoms (No Lone Pairs)

  • Domains: 5

  • Shape: Trigonal Bipyramidal

  • Bond Angles: Not Applicable (NA)

  • Example: Niobium Pentabromide (NbBr5)

Molecules with 7 Atoms (No Lone Pairs)

  • Domains: 6

  • Shape: Octahedral

  • Bond Angle: 90°

  • Example: Sulfur Hexafluoride (SF6)

Practice Molecular Geometry Determination

  • Exercises: Determine molecular shape for the following:

    • Xenon Trioxide (XeO3)

    • Methane (CH4)

    • Phosphorus Pentachloride (PCl5)

    • Sulfur Dioxide (SO2)

    • Ammonia (NH3)

Cyanic Acid Lewis Diagrams Analysis

  • Two Possible Lewis Diagrams:

    • Diagram 1: N=C-O-H

    • Diagram 2: N-C=O-H

  • Experimental Data for Cyanic Acid:

    • Nitrogen bond lengths: 115 pm, 143 pm

    • Bond angles: 176°, 97°, 106°

  • Task: Choose the diagram that best reflects the experimental data based on VSEPR theory and justify the choice.