Chemical Bonding and Molecular Structure (Ch. 10)

1. Electronic Configuration of Atoms Chemical Bonding and Molecular Structure (Ch. 10) Molecular Structure • General Summary -- Structure and Bonding Concepts Lewis Electron Dot Formula of Molecule octet rule VSEPR Theory 3-D Shape of Molecule Electronegativity and Bond Polarity Molecular Orbital Theory OR Bonding Description of Molecule Polarity of Molecule Valence Bond Theory Intermolecular Forces and Bulk Properties Chemical Reactivity

2. Valence Shell Electron Pair Repulsion Theory Hypothesis -- The structure of a molecule is that which minimizes the repulsions between pairs of electrons on the central atom. “Electron Groups” (EG) = (# of atoms attached to central atom) + (# of lone pairs, or single electrons, on central atom)

3. Valence Shell Electron Pair Repulsion Theory (cont.) (A = central atom, X = terminal atom, E = lone pair) Related Aspects: -- In trigonal bipyramidstructures, lone e- pairs adopt equatorial positions (e) -- Order of repulsions: Lp-Lp > Lp - Bp > Bp - Bp (predicts distortions from ideal geometries)

4. Sample Problems • Use VSEPR Theory to predict (and draw) 3-D structures of: NH3 SF4 BrF4–

5. Sample Problems • Use VSEPR Theory to predict (and draw) 3-D structures of: NH3 SF4 BrF4–

6. Polarity of Molecules • Can predict from molecular shape • Polar or Non-Polar? • In very symmetrical structures (e.g. CO2 or CF4), the individual bond dipoles effectively cancel each other and the molecule is non-polar. • In less symmetrical structures (e.g. SO2 and SF4), the bond dipoles do not cancel and there is a net dipole moment which makes the molecule polar.

7. Polarity of Molecules, cont.

8. Valence Bond Theory • Basic Concept • Covalent Bonds result from overlap of atomic orbitals • For example, consider the H2 and HF molecules:

9. Two Types of Covalent Bonds s(sigma) bond “head-to-head” overlap along the bond axis p(pi) bond“side-to-side” overlap of p orbitals: • Single bond -- always a s bond • Double bond -- combination of one s bond and one p bond • Triple bond -- combination of one s bond and two p bonds + 2p 2p p bond

10. s (sigma) bonds “head-to-head” overlap along the bond axis

11. p (pi) bonds “side-to-side” overlap of p orbitals:

12. Hybrid Atomic Orbitals Question: Description of bonding in CH4 molecule? • Experimental fact -- CH4 is tetrahedral (H-C-H angle = 109.5º) Problem If only s and p orbitals are used, angles ought to be 90º since the p orbitals are mutually perpendicular! Solution Modify the theory of atomic orbitals and use: Hybridization Combination of 2 or more atomic orbitals on the same atom to form a new set of “Hybrid Atomic Orbitals” used in bonding.

13. Types of Hybrid Orbitals {Note: combination of n AO’s yields n Hybrid Orbitals) Example: in CH4, C is sp3 hybridized: ground state - valence shell orbital diagram (predicts 90º angles -- wrong!) C hybridized state (predicts 109.5º angles -- right!) C

14. Examples • Use valence bond theory to describe the bonding in the following (use clear 3-D pictures showing orbital overlap, etc) H2O NH3 CH4 PF3 --simple s bonds and lone pairs H2CNH --double bond like H2CCH2ethene and H2COformaldehyde) HCN --triple bond like HCCHethyne and N2nitrogen)

15. Formaldehyde

16. Nitrogen

17. Comparison of VB and MO Theory • Valence Bond Theory (“simple” but somewhat limited) • e– pair bonds between two atoms using overlap of atomic orbitals on two atoms • Molecular Orbital Theory (more general but “complex”) • All e–’s in molecule fill up a set of molecular orbitals that are made up of linear combinations of atomic orbitals on two or more atoms MO’s can be: “localized” -- combination of AO’s on two atoms, as in the diatomic molecules “delocalized” -- combination of AO’s on three or more atoms as in benzene (C6H6)

18. Molecular Orbitals for Simple Diatomic Molecules • In H2 the 1s atomic orbitals on the two H atoms are combined into: • a bonding MO -- s1s and an antibonding MO --s*1s MO energy level diagram for H2 (only the bonding MO is filled):

19. Simple MO Diagrams • In contrast, the MO diagram for the nonexistent molecule, He2, shows that both bonding and antibonding MO’s are filled: Bond Order = 1/2[(# of bonding e–’s) - (# of antibonding e–’s)] • For H2 = 1/2 [2-0] = 1 (a single bond) • For He2 = 1/2 [2-2] = 0 (no net bonding interaction)

20. MO’s for 2nd Row Diatomic Molecules • (e.g. N2, O2, F2, etc) • AO combinations -- from s orbitals and from p orbitals (p. 438-9)

21. MO Energy Level Diagram

22. Examples • e.g. Fill in MO diagram for C2, N2, O2, F2 and Ne2 and determine bond order for each: • General “rules” • Electrons fill the lowest energy orbitals that are available • Maxiumum of 2 electrons, spins paired, per orbital • Hund’s rule of maximum unpaired spins applies* *(accounts for paramagnetism of O2 (VB theory fails here!)

23. MO diagram for oxygen, O2

24. MO diagram for oxygen, O2

25. Delocalized Molecular Orbitals • By combining AO’s from three or more atoms, it is possible to generate MO’s that are “delocalized” over three or more atoms e.g. Resonance in species like formate ion HCO2– and benzene C6H6 can be “explained” with a single MO description containing delocalized bonds. VB description: MO description:

26. Benzene

27. Sample Problem • Fully describe the bonding in NaHCO3 using valence bond theory.

28. Sample Problem • Fully describe the bonding in NaHCO3 using valence bond theory. Answer: The Lewis structure is (and its resonance equivalent). VSEPR theory gives a trigonal planar structure at carbon (3 EG), and a bent structure at the central O (4 EG). At C, the expected hybridization is sp2, with 120° bond angles. For the terminal O with a single bond, there will be one s bond between the p orbital (terminal O) and the sp2 orbital (central C). For the C=O bond, there will be a similar s bond, but also one p bond from the side-on interaction of one p orbital on each atom. continued…

29. Sample Problem, cont. For the C-OH bond there will be one s bond between the O sp3 orbital and the C sp2 orbital. At the central oxygen there is sp3 hybridization (EG = 4). Each lone pair lies in one sp3 hybrid orbital. The O-H bond is a s bond between the O sp3 hybrid and the H 1s orbital.

30. Sample Problem • Write the MO diagram for HCl. Predict the bond order and sketch the bonding and antibonding MO’s. [note: H 1s energy = -13 eV, Cl 3s energy = -25 eV, Cl 3p energy = -14 eV]

31. Sample Problem • Write the MO diagram for HCl. Predict the bond order and sketch the bonding and antibonding MO’s. [note: H 1s energy = -13 eV, Cl 3s energy = -25 eV, Cl 3p energy = -14 eV] • Answer: The bond order is 1.