Overview
This lecture introduces how to use VSEPR theory to predict molecular shapes and explains how molecular structure affects polarity and dipole moments.
Molecular Shapes: Basics
- Molecular structures are three-dimensional, described by bond distances and bond angles.
- VSEPR theory (Valence Shell Electron-Pair Repulsion) predicts molecular structure by minimizing repulsion between regions of electron density.
- Electron regions include single, double, triple bonds, and lone pairs.
Electron-Pair Geometry & Molecular Structure
- Electron-pair geometry considers all electron regions (bonds + lone pairs); molecular structure considers only the atoms' arrangement.
- When there are no lone pairs on the central atom, electron-pair geometry and molecular structure are identical.
- Lone pairs occupy more space than bonding pairs, causing bond angles to deviate from ideal values.
Common Geometries & Angles
- Linear: 2 regions, 180°
- Trigonal planar: 3 regions, 120°
- Tetrahedral: 4 regions, 109.5°
- Trigonal bipyramidal: 5 regions, 90° & 120°
- Octahedral: 6 regions, 90°
Lone Pairs and Molecular Shape
- Lone pairs reduce bond angles and change the molecular structure name (e.g., bent, trigonal pyramidal, seesaw, T-shaped, square planar).
- In trigonal bipyramidal, lone pairs prefer equatorial positions; in octahedral, they are placed opposite each other.
VSEPR Prediction Steps
- Write the Lewis structure.
- Count regions of electron density around the central atom (including lone pairs and all bonds).
- Assign electron-pair geometry based on the number of regions.
- Determine molecular structure by considering the number and arrangement of lone pairs.
Example Structures
- CO₂: linear geometry, linear structure (both 180°).
- BCl₃: trigonal planar geometry & structure (120°).
- NH₄⁺: tetrahedral geometry & structure (109.5°).
- H₂O: tetrahedral geometry, bent structure (104.5°).
- SF₄: trigonal bipyramidal geometry, seesaw structure.
- XeF₄: octahedral geometry, square planar structure.
Polarity and Dipole Moments
- Polar covalent bonds arise from a difference in electronegativity between atoms, resulting in partial charges and bond dipoles (measured in Debye units; μ = Q × r).
- The overall molecular dipole is the vector sum of individual bond dipoles.
- For a molecule to be polar, it must have polar bonds and a shape where dipole moments do not cancel.
- Nonpolar molecules: bond dipoles cancel (e.g., CO₂, CH₄, BF₃).
- Polar molecules: bond moments do not cancel (e.g., H₂O, NH₃, CH₃Cl).
Properties of Polar Molecules
- Polar molecules align in an electric field.
- Polar solvents dissolve polar substances; nonpolar solvents dissolve nonpolar substances.
Key Terms & Definitions
- VSEPR theory — predicts molecular shapes based on electron pair repulsions.
- Electron-pair geometry — arrangement of all electron regions around a central atom.
- Molecular structure — arrangement of atoms (not lone pairs) in a molecule.
- Bond angle — angle between two bonds from the same atom.
- Bond dipole moment — measure of bond polarity due to differences in electronegativity.
- Dipole moment (μ) — overall measure of molecular polarity.
Action Items / Next Steps
- Practice predicting molecular structures using VSEPR for assigned molecules.
- Use the molecular shape simulator online for additional practice.
- Review homework problems on molecular polarity and dipole moments.