VSEPR

1. VSEPR Valence Shell Electron-pair Repulsion model

2. VSEPR • This model is useful in predicting the geometry of molecules. • Remember: molecules are nonmetal & nonmetal. • The geometry around a given atom in a molecule is determined by minimizing electron pair repulsions.

3. Linear • Electron Pair Repulsion. • Note there are two electron pairs surrounding Be. • The charge of an electron is negative. • The position that reduces that repulsion is 180 ° or linear.

4. Three pairs or Trigonal Planar • Consider three bonds such as BH3. • What arrangement will minimize electron pair repulsion? • 120 ° or trigonal planar.

5. What about 4 pairs? • With 4 pairs in a plane, the largest angle available is 90°. • Is that the largest angle possible? • What if you go to three dimensions?

6. Tetrahedral • By arranging the 4 bounds around the central atom in three dimensions you can increase the bond angle to 109.5° and reduce the electron pair repulsion.

7. Trigonal Bipyramidal • 5 bonds • This is the only electron arrangement in which there are two different angles between electron pairs to reduce repulsion. • Example: PCl5

8. Octahedral • 6 electron pairs. • Each angle is 90° • Examples include PCl5-

9. Steps to apply VSEPR • Draw the Lewis Dot structure for the molecule. • Count the electron pairs and arrange them in the way that minimizes repulsions. • Determine the positions of the atoms from the way the electron pairs are shared. • Determine the name of the shape from the position of the atoms.

10. Lone pairs and Bond angles • Lone pairs have a greater repulsion than bound electrons. • This repulsion has the effect of “squishing” the bond angle in the molecule.

11. Repulsion of lone pair electrons

12. Question • What are the bond angles of CH3CO2CH3?

13. Answer • How close were you? • Is the angle between the C-O-C really 109°? • Why or why not?