Valence shell electron pair repulsion (VSEPR) model: Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs. The idea here is that the bonding and nonbonding pairs around a given atom will be positioned as far apart as possible. This chart is NOT provided on the AP exam!

Arrangement of electron pairs VSEPR A = central atom B = atoms bonded to central atom E = lone pairs (non-bonding) of electrons on central atom Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB2 2 linear linear B Bond angle

0 lone pairs on central atom 2 atoms bonded to central atom Cl Be 0 lone pairs on central atom 2 atoms bonded to central atom

Arrangement of electron pairs VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB2 2 linear linear trigonal planar trigonal planar AB3 3

VSEPR – Trigonal Planar arrangement Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry trigonal planar trigonal planar AB3 3 trigonal planar AB2E 2 1 bent

Arrangement of electron pairs VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB2 2 linear linear AB3 3 trigonal planar AB4 4 tetrahedral tetrahedral

VSEPR – Tetrahedral Arrangement Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB4 4 tetrahedral tetrahedral trigonal pyramidal AB3E 3 1 tetrahedral

Arrangement of electron pairs Molecular Geometry VSEPR – Tetrahedral Arrangement Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB4 4 tetrahedral tetrahedral AB3E 3 1 tetrahedral trigonal pyramidal AB2E2 2 2 tetrahedral bent H O

CH4 NH3 H2O Number of lone pairs 1 2 Bond angle 109.5o 107o 104.5o In these molecules, the arrangement of the electron pairs predict tetrahedral geometry. Why is the geometry different from predicted? Lone pairs require more room than bonding pairs and tend to compress the angles between the bonding pairs.

Arrangement of electron pairs VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB2 2 linear linear AB3 3 trigonal planar AB4 4 tetrahedral tetrahedral trigonal bipyramidal trigonal bipyramidal AB5 5

VSEPR – Trigonal Bipyramidal Arrangement Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry trigonal bipyramidal trigonal bipyramidal AB5 5 trigonal bipyramidal See-Saw (distorted tetrahedron) AB4E 4 1

VSEPR – Trigonal Bipyramidal Arrangement Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry trigonal bipyramidal trigonal bipyramidal AB5 5 AB4E 4 1 trigonal bipyramidal See-Saw trigonal bipyramidal AB3E2 3 2 T-shaped Cl F

VSEPR – Trigonal Bipyramidal Arrangement Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry trigonal bipyramidal trigonal bipyramidal AB5 5 AB4E 4 1 trigonal bipyramidal See-Saw AB3E2 3 2 trigonal bipyramidal T-shaped trigonal bipyramidal AB2E3 2 3 linear I

Arrangement of electron pairs VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB2 2 linear linear AB3 3 trigonal planar AB4 4 tetrahedral tetrahedral AB5 5 trigonal bipyramidal AB6 6 octahedral octahedral

VSEPR – Octahedral Arrangement Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB6 6 octahedral square pyramidal Br F AB5E 5 1 octahedral

VSEPR – Octahedral Arrangement Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB6 6 octahedral AB5E 5 1 octahedral square pyramidal AB4E2 4 2 octahedral square planar Xe F

Predicting Molecular Geometry Draw Lewis structure for molecule. Count number of lone pairs on the central atom and number of atoms bonded to the central atom. Use VSEPR to predict the geometry of the molecule. What are the molecular geometries of SO2 and SF4? (In SF4, S uses an expanded octet of 10.) S F AB4E S O S O See-Saw (distorted Tetrahedron) AB2E bent

Dipole Moments and Polar Molecules H F electron rich region electron poor region d+ d- A molecule such as HF that has a center of positive charge and a center of negative charge is said to have a dipole moment. The dipole moment is often represented by an arrow pointing to the negative charge center with the tail of the arrow indicating the positive center of charge.

Which of the following molecules have a dipole moment? H2O, CO2, SO2, and CH4 O H S O dipole moment polar molecule dipole moment polar molecule C H C O no dipole moment nonpolar molecule no dipole moment nonpolar molecule