Molecule shape & effect

Molecule Shape (VSEPR) & Molecular Polarity

  • Learning Checklist:

    • Explain the difference between a bonding electron pair and a lone electron pair.

    • Discuss the differences in repulsion between:

      • Two lone pairs

      • A lone pair and a bonding pair

      • Two bonding pairs

    • Use VSEPR theory to determine molecular geometry and shape.

    • Analyze molecule polarity based on bond polarity.

    • Develop competency in representations of electrostatic potential.

Assigning Formal Charge

  • Formal Charge Formula:

    • Formal charge = (number of valence e-s in neutral atom) - (number of e-s in lone pairs) - (number of e-s in covalent bonds/2)

Key Principles

  • Pauli's Principle: No two electrons can have the same quantum state within an atom.

  • Aufbau Principle: Electrons occupy the lowest energy orbital available.

  • Hund's Rule: Every orbital in a subshell is singly occupied before any orbital is doubly occupied.

  • Electron Configuration: Explains how electrons are distributed in atomic orbitals.

  • Schrödinger Equation: Fundamental equation of quantum mechanics.

  • Atomic Orbitals: Regions in space where there is a high probability of finding electrons.

  • Electromagnetic Radiation: Light behaves both as a particle and a wave.

  • Molecular Orbital Theory: Describes the behavior of electrons in molecules.

Molecular Geometry

  • Shapes dictated by:

    • Non-bonded atoms avoiding mutual repulsion of bonding electrons.

    • Bonding electrons being repelled by lone pair electrons.

  • VSEPR (Valence Shell Electron Pair Repulsion) Theory explains these shapes:

    • Lone pairs hold more volume and cause distortion in angles.

Representations of Molecular Shapes

  • 2D bond representation of 3D structures:

    • Each bond involves two electrons.

  • Example: Methane (CH4) Lewis structure:

    • Predict molecular geometry as tetrahedral, with bond angles of 109.5°.

Lone Pairs vs Bonding Pairs

  • Lone Pairs:

    • Located close to a single nucleus, e.g., ammonia (NH3).

    • Take up more space than bonding pairs, affecting molecular shape.

  • Bonding Pairs:

    • Shared between two nuclei, e.g., in NH3.

Analyzing Molecular Shapes

  • Methane (CH4):

    • Draw Lewis structure and predict 3D geometry.

    • Use molecular shapes simulation to visualize and draw representations.

VSEPR Geometry Patterns

  • 4 bonded electron pairs: Tetrahedral (109.5°)

  • 3 bonded + 1 lone pair: Trigonal pyramidal (107.5°)

  • 2 bonded + 2 lone pairs: Bent (104.5°)

    • Lone pairs distort the bonding angles.

Magnitude of Repulsion

  • Bond angles trend: H-C-H > H-N-H > H-O-H

  • Relative magnitude of repulsion:

    • Lone Pair/Lone Pair > Bonding Pair/Lone Pair > Bonding Pair/Bonding Pair

Determining Molecular Geometry

  • Examples:

    • CHCl3: Tetrahedral

    • NCl3: Trigonal pyramidal

    • PCl5: Draw Lewis structure and determine geometry (trigonal bipyramidal).

Octahedral Geometry

  • Example: XeF4:

    • Central atom with 6 electron pairs leading to square planar geometry.

    • Lone pairs influence bond positioning based on repulsion.

Assessing Molecular Polarity

  • A polar molecule exhibits a dipole with charge separation.

  • Steps to determine polarity:

    1. Draw Lewis structure.

    2. Analyze molecular shape and lone pairs using VSEPR.

    3. Identify polar bonds by comparing electronegativities.

    4. Determine if there is a net molecular dipole.

    5. Study intermolecular interactions based on polarity.

  • Examples to assess: CO2, H2O, C2H6, C2H5OH.

Resonance Structures & Hybridization

  • Learning Checklist:

    • Identify resonance structures and preferred forms using formal charge.

    • Understand hybridization as a bonding theory explaining molecular shape.

    • sp3 hybridization utilized for tetrahedral geometry.

Resonance and Delocalization

  • Molecules can be represented by multiple Lewis structures.

  • Average representation reflects actual structure (e.g., carbonate ion).

  • Steps in drawing resonance structures involve valence counting and octet fulfilment.

Hybridisation Abstracts

  • Hybridization = mixing of atomic orbitals to create equivalent orbitals.

  • sp3 hybridization creates tetrahedral shapes (methane example).

  • Sigma bonds formed by overlaps of hybrid orbitals.

  • C-H bonds in methane demonstrating equivalent bond lengths and angles.