Molecular Geometry and VSEPR Theory

VSEPR Theory Valence Shell Electron Pair Repulsion Theory States that electron pairs repel each other and assume an orientation about an atom to minimize repulsion – the shape a molecule takes is due to this repulsion

Valence shell electron pair repulsion (VSEPR) model: Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs. AB 2 20 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry 10.1 linear B B

Cl Be 2 atoms bonded to central atom 0 lone pairs on central atom 10.1

AB 2 20linear Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar 10.1

AB 2 20linear Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar 10.1 AB 4 40 tetrahedral

10.1

AB 2 20linear Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar 10.1 AB 4 40 tetrahedral AB 5 50 trigonal bipyramidal trigonal bipyramidal

10.1

AB 2 20linear Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar 10.1 AB 4 40 tetrahedral AB 5 50 trigonal bipyramidal trigonal bipyramidal AB 6 60 octahedral

10.1

bonding-pair vs. bonding pair repulsion lone-pair vs. lone pair repulsion lone-pair vs. bonding pair repulsion >>

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 30 trigonal planar AB 2 E21 trigonal planar bent 10.1

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 3 E31 AB 4 40 tetrahedral trigonal pyramidal 10.1

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR AB 4 40 tetrahedral 10.1 AB 3 E31tetrahedral trigonal pyramidal AB 2 E 2 22tetrahedral bent H O H

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR 10.1 AB 5 50 trigonal bipyramidal trigonal bipyramidal AB 4 E41 trigonal bipyramidal See-Saw (distorted tetrahedron)

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR 10.1 AB 5 50 trigonal bipyramidal trigonal bipyramidal AB 4 E41 trigonal bipyramidal See-Saw AB 3 E 2 32 trigonal bipyramidal T-shaped Cl F F F

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR 10.1 AB 5 50 trigonal bipyramidal trigonal bipyramidal AB 4 E41 trigonal bipyramidal See-Saw AB 3 E 2 32 trigonal bipyramidal T-shaped AB 2 E 3 23 trigonal bipyramidal linear I I I

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR 10.1 AB 6 60 octahedral AB 5 E51 octahedral square pyramidal Br FF FF F

Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry VSEPR 10.1 AB 6 60 octahedral AB 5 E51 octahedral square pyramidal AB 4 E 2 42 octahedral square planar Xe FF FF

10.1

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 CCl 4 ? (In SF 4, S uses an expanded octet of 10.) AB 2 Ebent C Cl AB 4 Tetrahedral 10.1 S O O S O O

Parent shapes for EX n molecules (n = 2-5) Formula n shapeshapes of structures EX 2 2 linear EX 3 3 trigonal planar EX 4 4 tetrahedral EX 5 5 trigonal bipyramidal

Parent shapes for EX n molecules (n = 6-8) Formula n shapeshapes of structures EX 6 6 octahedral EX 7 7 pentagonal bipyramidal EX 8 8 square antiprismatic

Activity Go around to each lab station At each station there are two molecules In your data table, –write the chemical formula (determine the class) –draw the Lewis structure –determine the molecular geometry Example: CompoundLewis StructureMolecular Geometry NH 3 AB 3 E Trigonal Bipyramidal

Activity CompoundLewis StructureMolecular Geometry CO 2 AB 2 Linear CH 2 O AB 3 Trigonal Planer NO 2 - AB 2 E Bent CH 4 AB 4 Tetrahedral

Activity CompoundLewis StructureMolecular Geometry NH 3 AB 3 E Trigonal Pyramidal H 2 O AB 2 E 2 Bent PCl 5 AB 5 Trigonal Bipyramidal SI 6 AB 6 Octohedral