VESPR Theory
Related Examples and Practice Problems
Additional Worked Out Examples/ Practice
Identifying classification types: Differentiation between elements, compounds or mixtures and homogeneous and heterogenous mixtures
Separation techniques: Selected and explaining limitation of appropriate separation
Relating Properties to Composition: Predicting classification based on descriptive properties
Topic Summary & Highlights
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Core Concept
VSEPR (Valence Shell Electron Pair Repulsion) Theory is used to predict the geometry of molecules based on the repulsion between electron pairs in the valence shell of the central atom.
Purpose: VSEPR theory helps explain molecular shapes and bond angles by considering that electron pairs, both bonding and nonbonding (lone pairs), will arrange themselves as far apart as possible to minimize repulsion.
Practice Tips
Bonding vs. Lone Pairs: Lone pairs repel more strongly than bonding pairs, affecting bond angles.
Geometry Determination: Molecular geometry is determined by the number of bonding and lone pairs around the central atom.
Applications: VSEPR helps predict physical and chemical properties like polarity, reactivity, and intermolecular forces.
Basic Principles of VSEPR Theory
Electron Repulsion: Electron pairs around a central atom will repel each other, and they arrange themselves in a way that minimizes this repulsion.
Types of Electron Pairs:
Bonding Pairs: Electrons shared between atoms that form bonds.
Nonbonding (Lone) Pairs: Electrons not involved in bonding but located on the central atom.
Effect of Lone Pairs: Lone pairs occupy more space than bonding pairs because they are only attached to one atom, which affects molecular geometry and bond angles.
3. Predicting Molecular Shapes Using VSEPR
Step-by-Step Process:
Draw the Lewis structure of the molecule.
Count the number of bonding pairs and lone pairs on the central atom.
Use the total number of electron pairs (bonding + lone pairs) to determine the electron geometry.
Consider only the positions of the atoms to determine the molecular geometry.
Adjust bond angles for lone pair repulsion if necessary.
Electron Geometry: Considers both bonding and lone pairs and describes the spatial arrangement of all electron pairs around the central atom.
Molecular Geometry: Considers only the position of atoms (bonding pairs) and describes the shape of the molecule.
Examples:
Methane (CH₄): Tetrahedral electron geometry and tetrahedral molecular geometry.
Ammonia (NH₃): Tetrahedral electron geometry, but trigonal pyramidal molecular geometry.
Water (H₂O): Tetrahedral electron geometry, but bent molecular geometry.
Special Cases in VSEPR Theory
Expanded Octets: Atoms in the third period or beyond (like phosphorus in PCl₅ or sulfur in SF₆) can have more than 8 electrons in their valence shells, leading to trigonal bipyramidal or octahedral geometries.
Multiple Central Atoms: Larger molecules may have more than one central atom, each of which follows VSEPR theory (e.g., ethane, C₂H₆).
