Lecture materials for the Introductory Chemistry course for Forensic Scientists, University of Lincoln, UK. See http://forensicchemistry.lincoln.ac.uk/ for more details.

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

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

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

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

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

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

11. Orbital summery This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License ATOMIC Orbitals MOLECULAR Orbitals H + H H 2

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

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

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

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

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

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

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

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

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

24.  -Bonds with p-Orbitals This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License 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 2 p Z 2 p Z σ (2 p Z )

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

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

28. 2s- and 2p-orbital overlap This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Energy A Diagram representing the energy solutions for n=1, 2 and 3 for the Schr ö dinger equation of a multi-electron atom 1 s 2 s 3 s 2 p 3 p 3 d 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. 2s and 2p overlap MO diagram This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Energy 2p 2p σ * (2 p Z ) σ (2 p Z ) π * (2p y ) π * (2p x ) π (2p y ) π (2p x ) 2s 2s σ * (2 s ) σ (2 s ) A molecular orbital diagram showing the approximate molecular orbitals when combining 2 s and 2 p orbitals. Suitable when forming homonuclear diatomic molecules involving O and F with the nuclai lying on the z-axis.

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

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

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

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

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

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

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

42. Molecular Structure of C 2 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

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

45. Molecular Structure of N 2 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 Triple bond is very strong – hence would expect a high bond energy

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