Valence shell electron pair repulsion VSEPR model Predict

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!

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 AB 2 2 0 Arrangement of electron pairs Molecular Geometry linear B Bond angle B

Cl Be Cl lone pairs on to central atom 20 atoms bonded central atom

VSEPR Class # of atoms bonded to central atom # lone pairs on central atom AB 2 2 0 linear AB 3 3 0 trigonal planar Arrangement of electron pairs Molecular Geometry

VSEPR – Trigonal Planar arrangement Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry trigonal planar bent AB 3 3 0 trigonal planar AB 2 E 2 1 trigonal planar

VSEPR Class # of atoms bonded to central atom # lone pairs on central atom AB 2 2 0 linear AB 3 3 0 trigonal planar AB 4 4 0 Arrangement of electron pairs tetrahedral Molecular Geometry tetrahedral

VSEPR – Tetrahedral Arrangement Class AB 4 AB 3 E # of atoms bonded to central atom # lone pairs on central atom 4 0 3 1 Arrangement of electron pairs Molecular Geometry tetrahedral trigonal pyramidal

VSEPR – Tetrahedral Arrangement # of atoms bonded to central atom # lone pairs on central atom AB 4 4 0 tetrahedral AB 3 E 3 1 tetrahedral trigonal pyramidal AB 2 E 2 2 2 tetrahedral bent Class Arrangement of electron pairs Molecular Geometry O H H

CH 4 NH 3 H 2 O Number of lone pairs 0 1 2 Bond angle 109. 5 o 107 o 104. 5 o 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.

VSEPR Class # of atoms bonded to central atom # lone pairs on central atom AB 2 2 0 linear AB 3 3 0 trigonal planar AB 4 4 0 tetrahedral 0 trigonal bipyramidal AB 5 5 Arrangement of electron pairs Molecular Geometry

VSEPR – Trigonal Bipyramidal Arrangement Class AB 5 AB 4 E # of atoms bonded to central atom # lone pairs on central atom 5 0 trigonal bipyramidal 1 trigonal bipyramidal See-Saw 4 Arrangement of electron pairs Molecular Geometry (distorted tetrahedron)

VSEPR – Trigonal Bipyramidal Arrangement # of atoms bonded to central atom # lone pairs on central atom AB 5 5 0 trigonal bipyramidal AB 4 E 4 1 trigonal bipyramidal See-Saw AB 3 E 2 3 2 trigonal bipyramidal Class Arrangement of electron pairs Molecular Geometry T-shaped F F Cl F

VSEPR – Trigonal Bipyramidal Arrangement # of atoms bonded to central atom # lone pairs on central atom AB 5 5 0 trigonal bipyramidal AB 4 E 4 1 trigonal bipyramidal See-Saw AB 3 E 2 3 2 trigonal bipyramidal T-shaped AB 2 E 3 2 3 trigonal bipyramidal linear Class Arrangement of electron pairs Molecular Geometry I I I

VSEPR Class # of atoms bonded to central atom # lone pairs on central atom AB 2 2 0 linear AB 3 3 0 trigonal planar AB 4 4 0 tetrahedral trigonal bipyramidal octahedral Arrangement of electron pairs AB 5 5 0 trigonal bipyramidal AB 6 6 0 octahedral Molecular Geometry

VSEPR – Octahedral Arrangement # of atoms bonded to central atom # lone pairs on central atom AB 6 6 0 octahedral AB 5 E 5 1 octahedral square pyramidal Class Arrangement of electron pairs Molecular Geometry F F F Br F F

VSEPR – Octahedral Arrangement # of atoms bonded to central atom # lone pairs on central atom AB 6 6 0 octahedral AB 5 E 5 1 octahedral square pyramidal AB 4 E 2 4 2 octahedral square planar Class Arrangement of electron pairs Molecular Geometry F F Xe F F

Predicting Molecular Geometry 1. Draw Lewis structure for molecule. 2. Count number of lone pairs on the central atom and number of atoms bonded to the central atom. 3. Use VSEPR to predict the geometry of the molecule. What are the molecular geometries of SO 2 and SF 4? (In SF 4, S uses an expanded octet of 10. ) F S O AB 2 E O bent S O O F S F AB 4 E F See-Saw (distorted Tetrahedron)

Dipole Moments and Polar Molecules electron poor region electron rich region H F 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? H 2 O, CO 2, SO 2, and CH 4 O H H dipole moment polar molecule S O O dipole moment polar molecule H O C O no dipole moment nonpolar molecule H C H H no dipole moment nonpolar molecule