1. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Chemical Bonding 2 MOLECULAR ORBITALS University of Lincoln presentation

2. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecular Orbitals What you need to know… In a covalent bond, atomic orbitals overlap to produce MOLECULAR ORBITALS Drawing molecular orbital diagrams for the homonuclear diatomics: H 2, Li 2, Be 2, B 2, C 2, N 2, O 2, F 2 Using molecular orbital diagrams to rationalise observed trends in the properties of molecules

3. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The Covalent Bond – Recap Non-bonded atoms – NO OVERLAP of atomic orbitals Bonded atoms – OVERLAP of atomic orbitals TWO ATOMSONE MOLECULE

4. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Which orbitals will overlap? Rules: Only orbitals with the same symmetry (shape) will overlap – s-orbitals overlap with s-orbitals – p-orbitals overlap with p-orbitals The more similar their energy, the better the overlap (and hence, the better the bond)

5. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Extent of Overlap, S No overlap No bond Weak overlap Weak bond Good overlap Good bond S is negligibleS is small S is large Same symmetry Different energy Same symmetry Some difference in energy Same symmetry Similar energy

6. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecular Orbital Theory Where are the electrons most likely to be found in a molecule? Link to “Molecular orbitals” video

7. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Remember… Heisenberg’s Uncertainty Principle “Electrons are so small, it is impossible to be sure where they are at any given time” Schrödinger “It is possible to define volumes of space where the electrons are most likely to be found” – s p d and f atomic orbitals

8. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Consider 2 Hydrogen atoms, A and B (1s 1 ) A B Electron is most likely to be found within this volume

9. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License If 2 H atoms BOND to form the H 2 molecule… Because A and B are bonded together, the electrons are more likely to be found in the shared space BETWEEN the nuclei

10. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License ATOMIC ORBITALS are volumes of space in which you are most likely to find an electron in an atom MOLECULAR ORBITALS are volumes of space in which you are most likely to find an electron in a molecule MOLECULAR ORBITAL THEORY states that in a molecule, all electrons are housed within molecular orbitals Definitions

11. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License ATOMIC Orbitals MOLECULAR Orbitals H + HH2H2 Orbital summery

12. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License How many molecular orbitals are formed when two atoms overlap?

13. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License TWO molecular orbitals are formed for every two atomic orbitals that overlap: 1.A bonding orbital (low energy) 2.An anti-bonding orbital (high energy)

14. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Remember Only the occupied atomic orbitals are relevant (i.e. those containing electrons)

15. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License ENERGY Bonding Anti-bonding* MOLECULAR ORBITALS Atomic orbitals of Atom A Atomic orbitals of Atom B

16. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecular Orbitals from s-orbital Overlap

17. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License When the overlap is directly in-line with the two nuclei the resulting bond is called a SIGMA (  ) bond s-orbitals produce sigma bonds Therefore, the two molecular orbitals are called:  bonding  * anti-bonding

18. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License  (1s) Bonding and  *(1s) Anti- bonding orbitals for the Hydrogen Molecule Electrons in an antibonding orbital try to pull a bond apart, and result in bond weakening or ψ bonding ψ antibonding ψ bonding ψ antibonding

19. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Bonding and antibonding video Link to “Bonding and antibonding” video

20. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecular Orbital Diagram 1s orbital overlap – H 2 Energy Ψ (antibonding) Ψ (bonding) Ψ (1s) A Ψ (1s) B σ * (1s)

21. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecular Orbital Diagram 2s orbital overlap – Li 2 Energy 2s σ * (2s)

22. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecular Orbital Diagram 2s orbital overlap – Be 2 Energy 2s σ * (2s) σ (2s) Be Be 2 Be Be 2

23. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecular Orbitals from p-orbital Overlap

24. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License  -Bonds with p-Orbitals By convention, the z-axis always runs along the main axis of the molecule P z -orbitals produce  -bonds End-on overlap produces a  -bond 2pZ2pZ 2pZ2pZ σ(2p Z )

25. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License  -Bonds with p-Orbitals P x - and P y -orbitals produce  -bonds Sideways overlap results in a bond called a Pi (  ) bond 2px2px 2px2px π(2p x )

26. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecular Orbitals from p-Orbital Overlap P z -orbitals give  -bonding and  *-antibonding molecular orbitals P x - and P y -orbitals give  -bonding and  *-antibonding orbitals Since  -overlap is better than  -overlap, the  -bonding orbital is the lowest in energy (most stable) and conversely the  *- antibonding is the highest in energy (least stable)

27. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecular Orbital Diagram 2p-orbital overlap Sometimes the  (2p z ) is higher in energy than the  (2p x ) and  (2p y ) Energy 2p σ * (2p Z ) σ (2p Z ) π * (2p y )π * (2p x ) π(2p y )π(2p x )

28. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License 2s- and 2p-orbital overlap Energy A Diagram representing the energy solutions for n=1, 2 and 3 for the Schrödinger equation of a multi-electron atom 1s1s 2s2s 3s3s 2p2p 3p3p 3d3d N = 1 N = 2 N = 3 The 2s atomic orbital is lower in energy than the 2p atomic orbitals Sometimes the  (2p z ) is higher in energy than the  (2p x ) and  (2p y ) Link to “Energy level diagrams” video

29. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License 2s and 2p overlap MO diagram Energy 2p σ * (2p Z ) σ (2p Z ) π * (2p y ) π * (2p x ) π(2p y ) π(2p x ) 2s σ * (2s) σ (2s) A molecular orbital diagram showing the approximate molecular orbitals when combining 2s and 2p orbitals. Suitable when forming homonuclear diatomic molecules involving O and F with the nuclai lying on the z-axis.

30. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecular Orbital Diagram F 2 Energy 2p σ * (2p Z ) σ (2p Z ) π * (2p y ) π * (2p x ) π(2p y ) π(2p x ) 2s σ * (2s) σ (2s) F F The formation of F 2. The 1s atomic orbitals are emmited. The F nuclei lie on the z-axis

31. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecular Orbital Diagram O 2 Energy 2p σ * (2p Z ) σ (2p Z ) π * (2p y ) π * (2p x ) π(2p y ) π(2p x ) 2s σ * (2s) σ (2s) O O The formation of O 2. The 1s atomic orbitals are emmited. The O nuclei lie on the z-axis

32. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Using Molecular Orbital Diagrams to Rationalise (explain) Observed Trends in the Properties of Molecules

33. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Consider the homonuclear diatomics B 2, C 2 andN 2 Experimental FACTS: The vapour phase of B 2 contains PARAMAGNETIC B 2 molecules The C 2 molecule is a gas phase species and is DIAMAGNETIC The N 2 molecule is DIAMAGNETIC and has a particularly high bond energy

34. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Definitions… A PARAMAGNETIC molecule contains one or more unpaired electrons A DIAMAGNETIC molecule contains no unpaired electrons

35. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Lets start with B 2 FACT: The vapour phase of B 2 contains PARAMAGNETIC B 2 molecules B B B B Group 13 (3 valence electrons) B The ATOM 2 possible molecular structures: B ≡ B B–B Bond Order =3 Bond order = 1 NOTE: Neither of these structures have unpaired electrons. They are therefore DIAMAGNETIC (not correct)

36. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Consider the MO Diagram of B 2 Energy 2p σ * (2p Z ) σ (2p Z ) π * (2p y ) π * (2p x ) π(2p y )π(2p x ) 2s σ * (2s) σ (2s) B B

37. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License What about the bonding? MO diagrams can give us the BOND ORDER: BOND ORDER =½ [( Number of bonding electrons )-( Number of anti- bonding electrons )]

38. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Bond order in B 2 No. bonding electrons = 4 No. anti-bonding electrons = 2 BOND ORDER = ½(4-2)= 1(single bond)

39. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecular Structure of B 2 B B–B FACT: The vapour phase of B 2 contains PARAMAGNETIC B 2 molecules

40. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License What is the Molecular Structure of C 2 ? FACT: The C 2 molecule is a gas phase species and FACT: The C 2 molecule is a gas phase species and is DIAMAGNETIC C C C Group 14 (4 valence electrons) The ATOM Possible molecular structure: C=C Bond Order =2 NOTE: This structure has no unpaired electrons and is therefore DIAMAGNETIC (correct)

41. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License MO Diagram of C 2 No unpaired electrons – therefore DIAMAGNETIC. No. bonding electrons = 6 No. anti- bonding electrons = 2 BOND ORDER = ½(6-2)= 2(double bond) Energy 2p σ * (2p Z ) σ (2p Z ) π * (2p y ) π * (2p x ) π(2p y )π(2p x ) 2s σ * (2s) σ (2s) C C

42. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License FACT: The C 2 molecule is a gas phase species and is DIAMAGNETIC C C C=C Bond Order =2 In this case, the MO diagram agrees with our initial structure Molecular Structure of C 2

43. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License What is the Molecular Structure of N 2 ? FACT: The N 2 molecule is DIAMAGNETIC and has a particularly high bond energy N N N Group 15 (5 valence electrons) The ATOM Possible molecular structure: N ≡ N Bond Order =3 NOTE: This structure has no unpaired electrons and is therefore DIAMAGNETIC (correct)

44. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License MO Diagram of N 2 No. bonding electrons = 8. No. anti-bonding electrons = 2 BOND ORDER = ½(8-2)= 3 (triple bond). No unpaired electrons – therefore DIAMAGNETIC Energy 2p σ * (2p Z ) σ (2p Z ) π * (2p y ) π * (2p x ) π(2p y )π(2p x ) 2s σ * (2s) σ (2s) N N

45. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License FACT: The N 2 molecule is DIAMAGNETIC and has a particularly high bond energy N N N ≡ N Bond Order =3 In this case, the MO diagram agrees with our initial structure Molecular Structure of N 2 Triple bond is very strong – hence would expect a high bond energy

46. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Summary

47. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Definitions Molecular Orbital Theory Molecular Orbitals Paramagnetism Diamagnetism

48. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License What should you know? In a covalent bond, atomic orbitals overlap to produce MOLECULAR ORBITALS How to draw MO diagrams:H 2, Li 2, Be 2, B 2, C 2, N 2, O 2, F 2 How to use the MO diagram to determine molecular structure (including the bond order) How to use the MO diagram to rationalise magnetic behaviour (paramagnetism or diamagnetism)

49. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acknowledgements JISC HEA Centre for Educational Research and Development School of natural and applied sciences School of Journalism SirenFM http://tango.freedesktop.org