At the end of this lesson, students should be able to : (a) Explain Valence Shell Electron Pair Repulsion theory (VSEPR) (b) Draw the basic molecular shapes: linear, planar, tetrahedral, and octahedral. (c) Predict and explain the shapes of molecule and bond angles in a given species.
.. :F.. - N - F:.. :F:.... Lewis structure No. of e surrounding a Assumption: e pairs/bonding pair in the valence shell repel one another VSEPR!! Study of molecular geometry Gives the position of all atoms
- The 3D arrangement of atoms in a molecule. - affects physical and chemical properties. - predicted by using Valence Shell Electron Pair Repulsion (VSEPR) model. - The study of molecular geometry states The valence electron pairs surrounding a repel one another. Thus, the orbital containing those electron pairs are oriented to be as far apart as possible.
Divide molecules into 2 categories: Central atom has No Lone Pairs.... O = C = O.... Central atom has 1 Lone Pairs...... : F - N - F :.... : F :..
BASIC MOLECULAR SHAPE Linear Trigonal Planar Tetrahedral Trigonal Octahedral
Class # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 2 2 0 linear linear B B
Bond angle = 180 Cl Be Cl Atoms arranged in a straight line 2 no atoms lone pairs bonded to atom Eg: BeCl 2, HgCl 2
VSEPR Class # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 2 2 0 linear linear AB 3 3 0 planar planar
Bond angle = 120 Central atom has 3 terminal atoms 120 Eg: BF 3
VSEPR Class # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 2 2 0 linear linear AB 3 3 0 planar planar AB 4 4 0 tetrahedral tetrahedral
Eg: CH 4 Bond angle = 109.5 Central atom has 4 terminal atoms
VSEPR Class # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 2 2 0 linear linear AB 3 3 0 planar planar AB 4 4 0 tetrahedral tetrahedral AB 5 5 0
Eg: PCl 5 Bond angle = 90, 120 Central atom has 5 terminal atoms
VSEPR Class # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 2 2 0 linear linear AB 3 3 0 planar planar AB 4 4 0 tetrahedral tetrahedral AB 5 5 0 AB 6 6 0 octahedral octahedral
Eg: XeF 6 Bond angle = 90 Central atom has 6 terminal atoms
-Geometry of molecule is more complicated if the has both lone pairs and bonding pairs. -3 types of repulsive forces: those between bonding pairs those between lone pairs those between a bonding pair & lone pair
bonding-pair vs. bonding pair repulsion lone-pair vs. bonding < < pair repulsion lone-pair vs. lone pair repulsion
Class # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 3 AB 2 E 3 0 2 1 planar planar planar bent
VSEPR Class # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 4 4 0 tetrahedral tetrahedral AB 3 E 3 1 tetrahedral pyramidal
VSEPR Class # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 4 4 0 tetrahedral tetrahedral AB 3 E 3 1 tetrahedral pyramidal AB 2 E 2 2 2 tetrahedral bent H O H
VSEPR Class # of atoms bonded to # lone pairs on AB 5 5 0 AB 4 E 4 1 Arrangement of electron pairs Molecular Geometry distorted tetrahedron
VSEPR Class # of atoms bonded to # lone pairs on AB 5 5 0 AB 4 E 4 1 AB 3 E 2 3 2 Arrangement of electron pairs Molecular Geometry distorted tetrahedron T-shaped F F Cl F
VSEPR Class # of atoms bonded to # lone pairs on AB 5 5 0 AB 4 E 4 1 AB 3 E 2 3 2 AB 2 E 3 2 3 Arrangement of electron pairs Molecular Geometry distorted tetrahedron T-shaped linear I I I
VSEPR Class # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 6 6 0 octahedral octahedral AB 5 E 5 1 octahedral square pyramidal F F F F Br F
VSEPR Class # of atoms bonded to # lone pairs on Arrangement of electron pairs Molecular Geometry AB 6 6 0 octahedral octahedral AB 5 E 5 1 octahedral AB 4 E 2 4 2 octahedral square pyramidal square planar F F Xe F F
Predicting Molecular Geometry 1. Draw Lewis structure for molecule. 2. Count number of lone pairs on the and number of atoms bonded to the. 3. Use VSEPR to predict the geometry of the molecule. e.g: What are the molecular geometries of SO 2 and SF 4? O S O F AB 2 E bent F S F F AB 4 E distorted tetrahedron
Excercise: Use the VSEPR model to predict the geometry of the following molecules and ions: a)c 2 H 4 b) OF 2 c) AlCl - 4 d) AsH 3