Molecular geometry

Molecular Geometry

Course Information

• Course Code: CHEM 1066 Introduction to Chemistry 1
• Instructor: Michael M Forde PhD MRSC

Topics and Outcomes

• Shapes of Molecules
• VSEPR Theory: Used to predict shapes of molecules
• Symmetry
• Symmetry Elements: Identify symmetry elements in molecules and apply to orbitals
• Point Groups: Assign point groups based on symmetry elements
• Polarity, Dipole Moments, and Chirality: Use symmetry as applied to polarity and chirality of molecules

Lewis Structures Recap

Definition of Basic Concepts

• Covalent Bond: Sharing an electron pair between two atoms constitutes a single bond.
• Lone Pair: Unshared pairs of valence electrons contribute to shape and chemical properties.
• Octet Rule: In general, each atom shares electrons with neighbors to achieve a total of 8 valence electrons.
• Hypervalent Species: Atoms can bond to achieve more than an octet of electrons.

Advanced Concepts

• Resonance: The blending of Lewis structures that accounts for the averaging of bond lengths and lowering of bond energy over a single contributing structure.
• Oxidation Number: Refer to the rules table on the course shell.
• Formal Charge Formula: \text{Formal Charge} = V - L - \frac{1}{2}P where:
  • V = Number of valence electrons on parent atom
  • L = Number of lone pairs in molecule
  • P = Number of shared electrons

Classroom Activities

• Collaborative educational activities related to Lewis Structures.

Evidence of Molecular Shapes

Examples from Crystal Structures

• Visual representation of the beryl hexagonal crystal (Crystal structure of Be3Al2SiO_{18}).
• X-ray Diffraction Pattern: Consideration of molecular size and arrangement of atoms.

Techniques for Studying Shapes

1. Constant-height STM image of graphite.
2. Simultaneously Recorded AFM Image: Shows repulsive interactions within the material.
3. Charge Density Estimations: Charge density variations evaluated through different parameters such as k = 1800 ext{ N/m}, A = 0.3 ext{ nm}, f_0 = 18076.5 ext{ Hz}, and Q = 20000.

VSEPR Theory from Lewis Structures

VSEPR Theory Overview

• Provides a good approximation of the shape of molecules based on valence electrons rather than identity of the atoms involved.
• VSEPR Model Principles: Regions of shared (enhanced) electron density occupy positions as far apart as possible:
  • Lone pair/lone pair > lone pair/bonding pair > bonding pair/bonding pair.
  • If a lone pair does not affect the geometry, it is stereochemically inert.

Steps for Using VSEPR Theory

1. Draw the Lewis Structure.
2. Count Electron Pairs: Include both bond pairs and lone pairs but count multiple bonds as one pair.
3. Arrange Electron Pairs: To minimize repulsion amongst them.
4. Position Atoms: To minimize lone pair-lone pair repulsion if there is more than one lone pair.
5. Name Molecular Geometry: Based on atomic positions.

Applying VSEPR Theory

Example Structures

• CH4 (Methane), NH3 (Ammonia), H2O (Water): Drawing and identifying molecular shapes and angles.

Overview of Molecular Geometry

Electron Groups Table

• Types and Corresponding Molecular Geometries:

  • 2 Electron Groups: Linear [$AX_2$]
  • 3 Electron Groups: Trigonal planar [$AX_3$]
  • 4 Electron Groups: Tetrahedral [$AX_4$]
  • 5 Electron Groups: Trigonal bipyramidal [$AX_5$]
  • 6 Electron Groups: Octahedral [$AX_6$]

• Lone Pair Scenarios:

  • Linear [$AX_2E$]
  • Bent (V-shaped) [$AX2E2$]
  • Trigonal Pyramidal [$AX_3E$]
  • Square Planar [$AX4E2$]

AXmEn Notation

Limitations of VSEPR Theory

• Energy Value Variability: Pure atomic orbital energy values vary significantly by electron configuration, especially in heavy atoms due to screening effects.
• Bond Angles in Lone Pairs: It is challenging to predict bond angle distortions in molecules with lone pairs.
• Model Limitations: Only sigma bonded electron pairs and lone pairs are considered; many molecules with sideways overlap cannot be accurately modeled using VSEPR.

Applications of Molecular Geometry

Dipole Moment Consideration

• If the bonds in a molecule are polar (i.e., formed between atoms of different electronegativities) and the molecule is not symmetrical, a dipole moment exists.
• Dipole Moment: Measures the vector sum of distances between the charges in a molecule.

Theoretical Approaches to Bond Theory

• Lewis Structures and VSEPR Theory: Initial starting points for understanding molecular shapes.
• Valence Bond Theory: Provides more detailed insights compared to previous models.
• Molecular Orbital Theory: Represents a further improvement in understanding molecular interactions.

Symmetry in Molecules

Introduction to Molecular Symmetry

• Molecular Symmetry: Affects physical properties and reactivity of molecules.
• Symmetry Definition: An object is symmetric if it remains invariant under a specific transformation.

Symmetry Operations and Elements

• Symmetry Operation: Movements that convert a molecule into a configuration that is indistinguishable from its original form.
• Symmetry Element: Geometric constructs (line, plane, point) about which symmetry operations are performed.

Classification of Symmetry Operations

• Identified operations include:
  • E (Identity)
  • Cn (Rotation)
  • σ (Reflection)
  • Sn (Improper Rotation)

Specific Examples of Symmetry Operations

1. Inversion Operation: Consider moving an atom to the center and then returning it to the original position on the opposite side.
2. Reflections in Molecules: Different types of reflections, such as vertical or horizontal planes, must be discerned in complex molecular geometries.
3. Composite Operations: Involves combining multiple symmetry operations for comprehensive analysis.

Point Groups

Definition and Importance

• Point Group: A collection of symmetry operations that all intersect at a single point.
• Examples include the C3v point group for H2O including symmetry elements E, 2C3, and 3σv.

Tracer Chart for Point Group Identification

• A method to systematically identify symmetry elements in molecules.

Applications of Symmetry in Chemistry

Polarity of Molecules

• Polar Molecules: Possess a permanent electric dipole moment. A molecule cannot be polar if:
  • It has a center of inversion.
  • Its charge distribution is symmetric about its center.
  • It belongs to a D point group or cubic point groups T or O.

Chirality Considerations

• Chirality: A molecule is chiral if it cannot be superimposed on its mirror image.
  • Any molecule with an Sn symmetry element cannot be chiral.
  • Example: CH4 is not chiral, while CHBrClF is chiral despite having a tetrahedral structure due to different point groups (Td vs C1).