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MOLECULAR GEOMETRYMOLECULAR GEOMETRY
Bonding Unit
VSEPR • Valence Shell Electron
Pair Repulsion theory.• Most important factor in
determining geometry is relative repulsion between electron pairs.
Molecule adopts the shape that minimizes the electron pair repulsions.
Molecule adopts the shape that minimizes the electron pair repulsions.
MOLECULAR GEOMETRYMOLECULAR GEOMETRY
Some Common GeometriesLinear
Trigonal Planar
Tetrahedral
VSEPR charts
• Use the Lewis structure to determine the geometry of the molecule
• Electron arrangement establishes the bond angles
• Molecule takes the shape of that portion of the electron arrangement
• Charts look at the CENTRAL atom for all data!
Rules for Predicting Molecular Structure using the VSEPR Model
1. Two pairs of electron on a central atom in a molecule are always placed 180o apart (linear).
2. Three pairs of electrons on a central atom in a molecule are always placed 120o apart on the same plane as the central atom (trigonal Planar).
3. Four pairs of electrons on a central atom in a molecule are always placed 109.5o apart (tetrahedral).
Rules for Predicting Molecular Structure using the VSEPR Model
4. When every pair of electrons on the central atom is shared with another atom, the molecular structure has the same name as the arrangement of the electron pairs.Number of Pairs Name of Arrangement
2 Linear
3 Trigonal Planar
4 tetrahedral
Rules for Predicting Molecular Structure using the VSEPR Model
5. When one or more of the electron pairs around a central atom are lone pairs, the name for the molecular structure is different from the arrangement of electron pairs.
Structure Determination by VSEPR
Structure Determination by VSEPR
Water, H2O
The electron pair geometry is TETRAHEDRAL
The electron pair geometry is TETRAHEDRAL
The molecular geometry is BENT.
The molecular geometry is BENT.
H O H••
••
H O H••
••
2 bond pairs
2 lone pairs
Methane, CH4
Ammonia, NH3
Water, H2O
Determining polarity of a molecule based on the symmetry of the molecule.
