1. PowerPoint to accompany Chapter 8 Part 2 Molecular Geometry and Bonding Theories

2. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Covalent Bonding and Orbital Overlap  We think of covalent bonds forming through the sharing of electrons by adjacent atoms.  In such an approach, this can only occur when orbitals on the two atoms overlap. Figure 8.14

3. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Covalent Bonding and Orbital Overlap  Increased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron- electron repulsion.  However, if atoms get too close, the internuclear repulsion greatly raises the energy. Figure 8.15

4. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Hybrid Orbitals  To explain geometries, we often assume that the atomic orbitals of an atom mix to form new orbitals called hybrid orbitals.  The process of mixing orbitals as atoms approach each other is called hybridisation.

5. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia sp Hybrid Orbitals Consider beryllium:  In its ground electronic state, it would not be able to form bonds because it has no singly-occupied orbitals.  But if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds.

6. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia sp Hybrid Orbitals  Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals:  These sp hybrid orbitals have two lobes like a p orbital.  One of the lobes is larger and more rounded as is the s orbital, this is the bonding lobe. Figure 8.16

7. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia sp Hybrid Orbitals  These two degenerate orbitals would align themselves 180  from each other.  This is consistent with the observed geometry of beryllium compounds: linear. Figure 8.17

8. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia sp Hybrid Orbitals  With hybrid orbitals, the orbital diagram for beryllium would look like this.  The sp orbitals are higher in energy than the 1s orbital but lower than the 2p.

9. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia sp 2 and sp 3 Hybrid Orbitals Using a similar model for boron leads to sp 2 hybridisation, i.e.

10. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia sp 2 and sp 3 Hybrid Orbitals …three degenerate sp 2 orbitals. Figure 8.18

11. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

12. sp 2 and sp 3 Hybrid Orbitals With carbon we get sp 3 hybridisation, i.e. … …four degenerate sp 3 orbitals.

13. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

15. Hybridisation involving d orbitals For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids. What element is this?

16. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Hybridisation involving d orbitals This leads to: - five degenerate sp 3 d orbitals or - six degenerate sp 3 d 2 orbitals.

17. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Hybrid Orbitals Once the electron- domain geometry is known, the hybridisation state of the atom is known. Table 8.4

18. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Multiple Bonds sigma,  - overlap of two orbitals along the internuclear axis. pi,  - sideways overlap of two p orbitals. Figure 8.21

19. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecular Orbital (MO) Theory Though valence bond theory effectively conveys most observed properties of ions and molecules, there are some concepts better represented by molecular orbitals. Figure 8.22

20. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecular Orbital of H 2  In H 2 the two electrons go into the bonding molecular orbital.  The bond order is half the difference between the number of bonding and antibonding electrons, i.e. bond order = ½(no. of bonding electrons - no. of antibonding electrons) Figure 8.23

21. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecular Orbital of H 2  For hydrogen, with two electrons in the bonding MO and none in the antibonding MO, the bond order is: 1212 (2 - 0) = 1 Figure 8.23

22. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecular Orbital of He 2  In the case of He 2, the bond order would be:  Therefore, He 2 does not exist. 1212 (2 - 2) = 0 Figure 8.23

23. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

24. Scotty! There’s no dilithium!

25. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecular Orbitals from 2p Orbitals  For atoms with both s and p orbitals, there are two types of interactions:  The s and the p orbitals that face each other overlap in  fashion.  The other two sets of p orbitals overlap in  fashion. Figure 8.26

26. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia This is the 2 nd half of the p-block With paired p electons.

27. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecular Orbitals for Li 2 and Be 2  The MO diagram looks like this:  There are both  and  bonding molecular orbitals and  * and  * antibonding molecular orbitals. Figure 8.27

28. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

32. Molecular Orbitals for B 2 to Ne 2  The smaller p-block elements in the second period have a sizeable interaction between the s and p orbitals.  This flips the order of the s and p molecular orbitals in these elements. Figure 8.31 Figure 8.32

33. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecular Orbitals of Second-Row Diatomic Molecules Figure 8.33

34. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

37. Electron Configurations and Molecular Properties  Paramagnetic molecules have one or more unpaired electrons and are attracted into a magnetic field.  Diamagnetic molecules have no unpaired electrons and are weakly repelled by a magnetic field. (It is a much weaker effect than paramagnetism.)

38. Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Chapter 8 End of Part 2  Molecular Orbitals  Hybrid Orbitals  Special Molecular Properties