Molecular Geometry (Shapes of Molecules) VSEPR Theory CP Chemistry http://www.scl.ameslab.gov/MacMolPlt/Surface.JPG

VSEPR Theory Electron groups around the central atom will be most stable when they are as far apart as possible – we call this valence shell electron pair repulsion theory because electrons are negatively charged, they should be most stable when they are separated as much as possible The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule

Electrons vs. Molecular Geometry The geometry of electron pairs around a central atom is called the electron geometry. The arrangement of bonded nuclei around a central atom forms the molecular geometry. Lone pair electrons on a central atom will repel other pairs but will not be visible in the molecular geometry (no nuclei) If there are lone pairs on the central atom the electron geometry and the molecular geometry will differ.

Two electron pairs on central atom Examples: CS2, HCN, BeF2

3 electron pairs on central atom All are in bonds Trigonal Planar Shape Examples: SO3, BF3, NO3-, CO32-

Bent Shape 3 Electron Pairs 2 Bonded 1 Un-Bonded Un-bonded electron pair takes up more space and “repels” more. The bond angle will change to less than the original 120o to about 117o-115o Other Examples: SO2, O3, PbCl2, SnBr2 About 117o

Four electron pairs on central atom All 4 in bonds Examples: CH4, SiCl4, SO42-, ClO4-

Trigonal Pyramidal Shape 4 Electron Pairs 3 bonded 1 un-bonded (lone pair) Trigonal Pyramidal Shape Examples: NH3, PF3, ClO3. H3O+ Bond angles are reduced from 109.5o to 107o due to extra repulsion by lone pair

2 Un-bonded (lone pairs) BENT SHAPE 4 Electron Pairs 2 Bonded 2 Un-bonded (lone pairs) BENT SHAPE Examples: H2O, OF2, SCl2 Bond angles are reduced a little more due to repulsion To 104.5o

The steps in determining a molecular shape Molecular formula Lewis structure Electron-group arrangement (electron geometry) Bond angles Molecular geometry Count all e- pairs around central atom Note lone pairs and double bonds Consider bonding e- pairs only Step 1 Step 2 Step 3 Step 4

Representing 3-Dimensional Shapes on a 2-Dimensional Surface One of the problems with drawing molecules is trying to show their dimensionality By convention, the central atom is put in the plane of the paper Put as many other atoms as possible in the same plane and indicate with a straight line For atoms in front of the plane, use a solid wedge For atoms behind the plane, use a hashed wedge

Predicting Molecular Shapes with Two, Three, or Four Electron Groups PROBLEM: Draw the molecular shape and predict the bond angles (relative to the ideal bond angles) of (a) PF3 SOLUTION: (a) For PF3 - there are 26 valence electrons, 1 nonbonding pair The shape is based upon the tetrahedral arrangement. The F-P-F bond angles should be <109.50 due to the repulsion of the nonbonding electron pair. The final shape is trigonal pyramidal. < 109.50

Predicting Molecular Shapes with More Than One Central Atom PROBLEM: Determine the shape around each of the central atoms in acetone, (CH3)2C=O. Find the shape of one atom at a time after writing the Lewis structure. SOLUTION: tetrahedral tetrahedral trigonal planar >1200 <1200