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Chemical Bonding 2 MOLECULAR ORBITALS

Chemical Bonding 2 MOLECULAR ORBITALS. University of Lincoln presentation. 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…

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Chemical Bonding 2 MOLECULAR ORBITALS

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

  2. 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: H2, Li2, Be2, B2, C2, N2, O2, F2 • Using molecular orbital diagrams to rationalise 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

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

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

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

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

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

  8. Consider 2 Hydrogen atoms, A and B (1s1) B A Electron is most likely to be found within this volume This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  9. If 2 H atoms BOND to form the H2 molecule… Because A and B are bonded together, the electrons are more likely to be found in the shared space BETWEEN the nuclei This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

  11. Orbital summery ATOMIC Orbitals MOLECULAR Orbitals H + H H2 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  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

  13. TWO molecular orbitals are formed for every two atomic orbitals that overlap: • A bonding orbital (low energy) • An anti-bonding orbital (high energy) 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. Anti-bonding* ENERGY Bonding Atomic orbitals of Atom A Atomic orbitals of Atom B MOLECULAR ORBITALS This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  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

  17. 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 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 or Electrons in an antibonding orbital try to pull a bond apart, and result in bond weakening ψ bonding ψ bonding or ψ antibonding ψ antibonding This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

  20. Molecular Orbital Diagram1s orbital overlap – H2 Ψ (antibonding) σ*(1s) Energy Ψ (1s)A Ψ (1s)B σ*(1s) Ψ (bonding) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  21. Molecular Orbital Diagram2s orbital overlap – Li2 σ*(2s) Energy 2s 2s σ*(2s) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  22. Molecular Orbital Diagram2s orbital overlap – Be2 σ*(2s) Energy 2s 2s Be2 Be Be σ (2s) Be Be Be2 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

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

  26. Molecular Orbitals from p-Orbital Overlap • Pz-orbitals give -bonding and *-antibonding molecular orbitals • Px- and Py-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) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

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

  29. 2s and 2p overlap MO diagram σ*(2pZ) π*(2px) π*(2py) Energy π(2px) π(2py) 2p 2p σ(2pZ) σ*(2s) 2s 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. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  30. Molecular Orbital Diagram F2 σ*(2pZ) π*(2px) π*(2py) Energy π(2px) π(2py) 2p 2p σ(2pZ) σ*(2s) 2s 2s F σ(2s) F The formation of F2. The 1s atomic orbitals are emmited. The F nuclei lie on the z-axis This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  31. Molecular Orbital DiagramO2 σ*(2pZ) π*(2px) π*(2py) Energy π(2px) π(2py) 2p 2p σ(2pZ) σ*(2s) 2s 2s O σ(2s) O The formation of O2. The 1s atomic orbitals are emmited. The O nuclei lie on the z-axis This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  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

  33. Consider the homonuclear diatomics B2,C2 andN2 • The vapour phase of B2 contains PARAMAGNETIC B2 molecules • The C2 molecule is a gas phase species and is DIAMAGNETIC • The N2 molecule is DIAMAGNETIC and has a particularly high bond energy Experimental FACTS: This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

  35. Lets start with B2 FACT: The vapour phase of B2 contains PARAMAGNETIC B2 molecules Group 13 (3 valence electrons) The ATOM B 2 possible molecular structures: NOTE: Neither of these structures have unpaired electrons. They are therefore DIAMAGNETIC (not correct) B B B B B≡B B–B Bond Order =3 Bond order = 1 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  36. Consider the MO Diagram of B2 σ*(2pZ) π*(2px) π*(2py) Energy 2p 2p σ(2pZ) π(2px) π(2py) σ*(2s) 2s 2s B σ(2s) B This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

  38. Bond order in B2 No. bonding electrons = 4 No. anti-bonding electrons = 2 BOND ORDER = ½(4-2)= 1 (single bond) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  39. Molecular Structure of B2 B B FACT: The vapour phase of B2 contains PARAMAGNETIC B2 molecules B–B This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  40. What is the Molecular Structure of C2? FACT: The C2 molecule is a gas phase species and is DIAMAGNETIC The ATOM C Group 14 (4 valence electrons) Possible molecular structure: NOTE: This structure has no unpaired electrons and is therefore DIAMAGNETIC (correct) C C C=C Bond Order =2 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  41. MO Diagram of C2 σ*(2pZ) π*(2px) π*(2py) Energy 2p 2p σ(2pZ) π(2px) π(2py) σ*(2s) 2s 2s C σ(2s) C No unpaired electrons – therefore DIAMAGNETIC. No. bonding electrons = 6 No. anti-bonding electrons = 2 BOND ORDER = ½(6-2)= 2 (double bond) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  42. Molecular Structure of C2 FACT: The C2 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 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  43. What is the Molecular Structure of N2? FACT: The N2 molecule is DIAMAGNETIC and has a particularly high bond energy The ATOM N Group 15 (5 valence electrons) Possible molecular structure: NOTE: This structure has no unpaired electrons and is therefore DIAMAGNETIC (correct) N N N≡N Bond Order =3 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  44. MO Diagram of N2 σ*(2pZ) π*(2px) π*(2py) Energy 2p 2p σ(2pZ) π(2px) π(2py) σ*(2s) 2s 2s N σ(2s) N No. bonding electrons = 8. No. anti-bonding electrons = 2 BOND ORDER = ½(8-2)= 3 (triple bond). No unpaired electrons – therefore DIAMAGNETIC This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  45. Molecular Structure of N2 N N FACT: The N2 molecule is DIAMAGNETIC and has a particularly high bond energy N≡N Bond Order =3 Triple bond is very strong – hence would expect a high bond energy In this case, the MO diagram agrees with our initial structure This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

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

  48. In a covalent bond, atomic orbitals overlap to produce MOLECULAR ORBITALS How to draw MO diagrams:H2, Li2, Be2, B2, C2, N2, O2, F2 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) What should you know? This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

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