12.4 Molecular Geometry

1. 12.4 Molecular Geometry • Lewis Dot Structures do not represent the actual shape of molecules. • However, Lewis Dot Structures do allow us to predict the shape of the molecule. • Predicting the 3-dimensional shape of a molecule is called molecular geometry. • To correctly predict the shape of a molecule, a correct Lewis structure is essential, including non-bonded electrons.

2. Molecular Geometry • The model used to predict molecular geometry is called VSEPR • Valence • Shell • Electron • Pair • Repulsion • The valence electron pairs surrounding an atom (both bonding and unbonded) repel one another. Consequently, the orbitals containing those electron pairs are oriented to be as far away from each other as possible.

3. Simple Molecular Geometry AXn • Molecular geometry, even for the most difficult structures, can be broken down into a central atom surrounded by terminal atoms. • Tables 7.5 and 7.8 give the possible molecular geometry and angles between bonds for molecules fitting the AXn format. • In order to determine the correct format of AXn, you must make a correct Lewis structure.

4. Primary Molecular Geometries • BeH2 is in format AX2 • OH2 is in format AX2E2 • The difference is the unshared pairs which changes the molecular geometry. Linear molecule Bent molecule

5. Primary Molecular Geometries • BF3 is in format AX3 • NH3 is in format AX3E • The difference is the unshared pairs which changes the molecular geometry. Triangular planar Triangular pyramidal

6. Fun With Models • Go to a lab station and inventory the model kits according to the Atom Parts in the ATOMOD 2 Brochure. Let me know if you have less or extras!

7. Draw then make the following molecules • CH4 methane • C2H4 ethene • NH4+ ammonium • CO2 carbon Dioxide