Molecular Geometry (Shapes of Molecules)

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-

3 Electron Pairs • 2 Bonded • 1 Un-Bonded Other Examples: • SO2, O3, PbCl2, SnBr2 Bent Shape • 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 About 117o

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

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

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

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

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 Predicting Molecular Shapes with Two, Three, or Four Electron Groups 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

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

Molecular Geometry (Shapes of Molecules)