Molecular Geometry and Polarity Part I: Molecular Geometry – Valence Bond Theory (B)

1. Molecular Geometry and Polarity Part I: Molecular Geometry – Valence Bond Theory (B) Dr. Chin Chu River Dell Regional High School

2. Valence Bond Theory Combines bonding with geometry. Addresses the equality of bonds in a molecule as far as length and energy are concerned.

3. Valence Bond Theory • Explains the structures of covalently bonded molecules • ‘how’ bonding occurs • Principles of VB Theory: • Bonds form from overlapping atomic orbitals and electron pairs are shared between two atoms • A new set of hybridized orbitals may form out of necessity. • Lone pairs of electrons are localized on one atom

6. Hybridization • When atomic orbitals on the central atom do not provide sufficient number of orbitals with single electron for proper bonding with surrounding atoms, mixing of atomic orbitals to produce hybrid orbitals with proper orientation in 3D space provides a viable solution. • The mixing of several atomic orbitals to form the same number of hybrid orbitals. • All the hybrid orbitals that form are the same in shape and energy but differing in orientation in 3D space.

7. sp3 hybridization One -s orbitals and 3 – p orbitals blended to give four identical hybrid sp3 orbitals

8. sp3 hybridization

9. sp3 hybridization

11. Geometry?

12. Hybridization • We blend the s and p orbitals of the valence electrons and end up with the tetrahedral geometry. • We combine one s orbital and 3 p orbitals. • sp3hybridization has tetrahedral geometry.

13. sp3 geometry • This leads to tetrahedral shape. • Every molecule with a total of 4 atoms and lone pair is sp3 hybridized. • Gives us trigonal pyramidal and bent shapes. 109.5º

18. sp2 Hybridization We mix 1 – s orbital with 2- p orbitals to produce three identical hybrid sp2 orbitals

21. Examples? BF3, C2H4, AlCl3

22. Where is the P orbital? • Perpendicular • The overlap of orbitals makes a sigma bond (s bond)

24. sigma bonds in ethene H H pi bonds in ethene C C H H

25. Ethene, sp2 hybridization Geometry?

26. sp2 hybridization in C2H4 • trigonal planar • 120º angle • one p bond • One sigma and one pi bond between the C-C atoms

28. sp Hybridization We combine an s – orbital with a – p orbital to form two identical hybrid sp orbitals

29. Bonding in BeCl2: sp Orbital overlap Lone pairs of Cl stay in the p- atomic orbitals

30. What about sp • one s and one p hybridize • Linear • C2H2

31. sp hybridization • end up with two lobes 180º apart. • p orbitals are at right angles • makes room for two p bonds and two sigma bonds. • a triple bond or two double bonds possible

32. Ethyne sp hybridization + pi bonds

33. sp3d Hybridization Geometry?

34. sp3d2 Hybridization Geometry?

35. To predict the hybridization of the central atom: Count the number of lone pairs AND the number of atoms bonded to the central atom. What is the GEOMETRY of electron pairs? # of Lone Pairs + # of Bonded Atoms Hybridization Examples 2 sp BeCl2 3 sp2 BF3 4 sp3 CH4, NH3, H2O 5 sp3d PCl5 6 sp3d2 SF6

36. Hybridization and Geometry of Electrons • sp3: tetrahedral • sp2: trigonal planar • sp: linear • sp3d: trigonal bipyramid • sp3d2: octahedral • Geometry of molecule determined by number of shared and unshared electrons.

37. Types of Hybrid Orbitals: Summary • sp3:1 s and 3 p orbitals mix to form 4 sp3 orbitals. • sp2:1 s and 2 p orbitals mix to form 3 sp2 orbitals leaving 1 p orbital intact. • sp: s and 1 p orbitals mix to form 4 sp orbitals leaving 2 p orbitals intact. • sp3d: five orbitals • sp3d2 : six orbitals