Lecture 4 revised? Week 1

Lecture Overview

Topics Covered:

• Electron Geometry: The arrangement of electron groups around a central atom, determining the overall shape of the molecule.

• Molecular Geometry (Shape): The three-dimensional arrangement of atoms in a molecule, taking into account the positions of lone pairs of electrons.

• Effects of Lone Pairs of Electrons: Lone pairs influence the geometry and bond angles of molecules by repelling bonded electron pairs more than bonded atoms do.

• Bond and Molecular Polarity: Understanding how bond polarity translates into molecular polarity, affecting the molecule's physical properties.

Electron Geometry

Types and Electron Groups:

• 2 Electron Groups: Linear Geometry - A straight line with a bond angle of 180°, suitable for diatomic molecules.

• 3 Electron Groups: Trigonal Planar Geometry - Bond angles of 120°, typically seen in molecules like BF₃.

• 4 Electron Groups: Tetrahedral Geometry - Bond angles of 109.5°, commonly observed in methane (CH₄).

• 5 Electron Groups: Trigonal Bipyramidal Geometry - Features a combination of bond angles (90° and 120°), seen in PCl₅.

• 6 Electron Groups: Octahedral Geometry - All bond angles are 90°, exemplified by SF₆.

Lone Pairs Effect:

• Lone pairs occupy more spatial volume than bonded atoms, causing increased repulsion among electron pairs. This leads to skewed molecular geometries, as lone pairs push bonded pairs closer together, altering the ideal bond angles.

Molecular Geometry (Shapes)

Shapes Based on Electron Groups and Lone Pairs:

• 3 Electron Groups:

  • 0 Lone Pairs: Trigonal Planar (e.g. BF₃)

  • 1 Lone Pair: Bent (e.g. SO₂)

• 4 Electron Groups:

  • 0 Lone Pairs: Tetrahedral (e.g. CH₄)

  • 1 Lone Pair: Trigonal Pyramidal (e.g. NH₃)

  • 2 Lone Pairs: Bent (e.g. H₂O)

• 5 Electron Groups:

  • 0 Lone Pairs: Trigonal Bipyramidal (e.g. PCl₅)

  • 1 Lone Pair: Seesaw (e.g. SF₄)

  • 2 Lone Pairs: T-shaped (e.g. ClF₃)

  • 3 Lone Pairs: Linear (e.g. XeF₂)

• 6 Electron Groups:

  • 0 Lone Pairs: Octahedral (e.g. SF₆)

  • 1 Lone Pair: Square Pyramidal (e.g. BrF₅)

  • 2 Lone Pairs: Square Planar (e.g. XeF₄)

  • 3 Lone Pairs: T-shaped (e.g. ICl₃)

  • 4 Lone Pairs: Linear (e.g. RnCl₂)

Electron-Domain Geometries

Electron-Domain and Molecular Geometries with Lone Pairs:

• The arrangement of electron domains around the central atom helps predict the geometry:

  • 3 Electron Domains:

    • AB₂ with 1 Lone Pair: SO₂ - exhibits bent molecular structure.

  • 4 Electron Domains:

    • AB₃ with 1 Lone Pair: NH₃ - exhibits trigonal pyramidal shape.

    • AB₂ with 2 Lone Pairs: H₂O - again, shows a bent shape.

  • 5 Electron Domains:

    • AB₃ with 2 Lone Pairs: Trigonal Bipyramidal - T-shaped

    • AB₂ with 3 Lone Pairs: Linear

  • 6 Electron Domains:

    • AB₄ with 2 Lone Pairs: Square Planar

    • AB₂ with 4 Lone Pairs: Linear

Additional Effects of Lone Pairs

• Lone pairs can significantly alter molecular geometries and angles. The presence of lone pairs leads to smaller bond angles compared to ideal geometries because of the higher repulsion they exert on adjacent bonded atoms, resulting in distorted angles.

Geometries of Larger Molecules

• Analyzing larger molecules requires assessing each central atom in respect to its bonded atoms and lone pairs. This method helps predict the overall shape and chemical behavior of larger, complex molecules (e.g., CH₃CH₂COOH) where each central atom may have different electron configurations.

Bond and Molecular Polarity

Bond Polarity:

• The difference in electronegativity between atoms in a bond creates bond polarity, illustrated as a vector leading to the more electronegative atom. For example, in HCl, chlorine is more electronegative than hydrogen, resulting in a polar covalent bond.

Molecular Polarity:

• The molecular polarity derives from the vector sum of individual bond polarities. A molecule is polar if the vectors do not cancel out due to its asymmetrical shape (e.g., H₂O), while symmetrical molecules (e.g., CCl₄) can be non-polar as the vectors cancel each other out.

Chemical Formula Identification

• Utilizing an Electrostatic Potential Map helps identify possible chemical formulas by visualizing charge distribution within the molecule, relevant for molecules such as OCS, CO₂, SF₂, I₂, and HCN. Understanding these maps and their implications for molecular behavior is crucial for predicting reactivity and interactions between molecules.