Geometry and Shape of Molecules

Introduction to Molecular Geometry

The three-dimensional structure of a molecule is determined primarily by minimizing repulsions between electron domains (clouds) around a central atom. This arrangement influences the physical and chemical properties of substances.

• Electron domains include both bonding pairs (single, double, triple bonds) and non-bonding pairs (lone pairs).

• Lewis structures show connectivity and electron arrangement but do not represent the actual 3D shape.

• Ball-and-stick models and space-filling models are used to visualize molecular shapes and bond angles.

Example: Carbon Dioxide (CO2)

• Molecular formula: CO2

• Lewis structure: O=C=O

• Ball-and-stick model: Linear shape, bond angle of 180°

Valence Shell Electron-Pair Repulsion Theory (VSEPR)

Basic Principles of VSEPR

VSEPR theory predicts the arrangement of electron pairs around a central atom to minimize repulsion and achieve the lowest energy configuration.

• Electron domains (bonding and non-bonding) repel each other.

• The number of electron domains (Steric Number, SN) determines the geometry.

• Each single bond, double bond, triple bond, and lone pair counts as one electron domain (SN = 1 for each).

Common Molecular Geometries and Examples

Trigonal Planar Electronic Geometry

SN = 3: Three Electron Domains

When three electron domains are present around a central atom, the electronic geometry is trigonal planar, with ideal bond angles of 120°.

• All bonding domains: Molecular geometry is trigonal planar (e.g., BF3).

• Two bonding domains, one lone pair: Molecular geometry is bent, with bond angles slightly less than 120° (e.g., SO2).

Examples and Applications

• CH2O (formaldehyde): SN = 3, trigonal planar geometry, bond angles ≈ 120°.

• SO2: SN = 3, bent geometry, bond angles < 120° due to lone pair repulsion.

Tetrahedral Electronic Geometry

SN = 4: Four Electron Domains

Four electron domains around a central atom result in a tetrahedral electronic geometry, with ideal bond angles of 109°.

• All bonding domains (AX4): Molecular geometry is tetrahedral (e.g., CH4).

• Three bonding domains, one lone pair: Molecular geometry is trigonal pyramidal, bond angles ≈ 107° (e.g., NH3).

• Two bonding domains, two lone pairs: Molecular geometry is bent, bond angles ≈ 105° (e.g., H2O).

Octahedral Electronic Geometry

SN = 6: Six Electron Domains

Six electron domains around a central atom result in an octahedral electronic geometry, with bond angles of 90° and 180°.

• All bonding domains: Molecular geometry is octahedral (e.g., SF6).

• Five bonding domains, one lone pair: Molecular geometry is square pyramidal (e.g., BrF5).

Key Definitions and Concepts

• Steric Number (SN): The total number of electron domains (bonding and non-bonding) around a central atom.

• Bonding domain: A region where electrons are shared between atoms (single, double, or triple bonds).

• Non-bonding domain: A region where electrons are localized on a single atom (lone pairs).

• Electronic geometry: The spatial arrangement of all electron domains around a central atom.

• Molecular geometry: The spatial arrangement of only the atoms (ignoring lone pairs).

Important Equations

• Steric Number (SN):

• Formal Charge (FC):

Summary Table: Electronic vs. Molecular Geometry

Example: In water (H2O), the central oxygen atom has two bonding domains and two lone pairs, resulting in a bent molecular geometry with a bond angle of approximately 105°.

Additional info: These notes expand on the provided slides by including definitions, equations, and a summary table for quick reference.