MOLECULAR SHAPES Valence Shell Electron Pair Repulsion Theory VSEPR – a model for predicting 3-D Molecular Shapes

Importance of Molecular Shapes Our universe is three-dimensional – true properties of molecules can only be understood by looking at actual arrangements of molecules in space. Example application: Chiral molecules – most drugs contain a carbon bonded to 4 different other atoms or groups of atoms.

In 2-D molecules look identical, in 3-D they are not. Link to chiral molecules In 2-D, It looks as though the left structure can be converted into the right by rotating 180o. In 3-D, we can see that these 2 molecules are actually mirror images of each other and are not identical.

Chiral Molecules – binding surfaces in your body recognize one arrangement but not the other

Chiral blockbuster drugs C&E News, June 14, 2004, p. 47

Effect of thalidomide mutations Thalidomide – 1 form of drug treats morning sickness, the other causes mutations Effect of thalidomide mutations

VSEPR Theory VSEPR (Valence Shell Electron Pair Repulsion) Theory allows us to predict the shape of molecules. Each molecule has pairs of electrons- The bonding pairs form covalent bonds. The nonbonding pairs occupy the same orbital. Each of these pairs can be thought of as an electron cloud- a cloud of negative charge. Link to electrical charges. Link to Phet Simulation

Like Charges Repel These clouds of electrons, areas of negative charge repel each other in such a way as to get as far apart from each other as possible. When there are only two electron clouds, the farthest they can get away from each other is 180o, a straight line. Link to tutorial

Key ideas Molecular geometry is determined by the arrangement of bonding and non-bonding electron pairs around the central atom. The bonding and nonbonding valence electron pairs arrange themselves as far APART in space as possible in order to minimize ELECTROSTATIC REPULSION between negative charges.

Effect of the number of electron pairs around the central atom 4 charge clouds, tetrahedral 3 charge clouds, trigonal planar 2 charge clouds, linear 6 charge clouds, Octahedral 5 charge clouds, trigonal bipyramidal

Steps for Applying VSEPR: Draw Lewis Dot structure (be sure to include all nonbonding (lone) pairs on central atom). Count the number of bonding and nonbonding electron pairs on the central atom. Double and triple bonds count as only ONE bonding pair (one bonding “region”). Consult chart from your notes below that describes arrangement of electron pairs in space that minimizes electrostatic repulsions.

Effect of Nonbonding Electrons Pairs on Bond Angles Nonbonding electrons pairs are more spreadout and therefore exert a greater repulsion than bonding pairs. Example: Bond angles for CH4, NH3, H2O VSEPR animations

Conventions for Drawing in 3-D Use to represent bonds in plane of paper Use to represent bonds receding into plane of paper Use to represent bonds coming out of plane of paper Use to represent nonbonding (lone) electron pairs on central atom

Linear 2 bonding electrons pairs (2 atoms attached to center atom) 0 Non Bonding e- pairs (lone pairs) Bond angle = 180o Type: AB2 Ex. : BeF2, CO2 Link to Linear Shape Animation

Three Clouds With three electron clouds, the farthest the electron clouds can get away from each other is 120o, the corners of an equilateral triangle. This shape is known as trigonal planer. Link to Triangular Planar Animation

Trigonal Planar 3 bonding electron pairs (3 atoms attached to center atom) 0 nonbonding pairs Bond angle = 120o Type: AB3 Ex. : BCl3

Bent (3 total e- pairs) 2 Bonding e- pairs 1 nonbonding e- pairs Bond angle < 120 o Type: AB2E2 Ex. : O3 Link to 2,1 Bent

Four clouds When four clouds are attached to a central atom, the farthest they can get away from each other is the four corners of a tetrahedron. The tetrahedral angle is 109.5o. Link to animation

Tetrahedral 4 Bonding electron pairs 0 Non-Bonding e- pairs Bond angle = 109.5o Type: AB4 Ex. : CH4

Trigonal Pyramidal 3 Bonding e- pairs 1 Non-bonding e- pair Bond angle = 107o Type: AB3E Ex. : PH3 Link to video

Bent (4 total e- pairs) 2 Bonding e- pairs 2 Non Bonding e- pairs Bond angle 104.5o Type: AB2E2 Ex. : H2S Link to 2,2, bent

Trigonal Bipyramidal 5 atoms attached to center atom 0 lone pairs Bond angle = equatorial -> 120o axial -> 90o Type: AB5 Ex. : PBr5

Octahedral 6 atoms attached to center atom 0 lone pairs Bond angle = 90oType: AB6 Ex. : SF6

Molecules with multiple centers A central atom is any atom with more than one atom bonded to it Perform exercise individually for each atom Molecular shape will refer only to the atoms/lone pairs immediately attached to that atom