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.