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 HOMONUCLEAR COVALENT BONDS University of Lincoln presentation

5. Covalent Homonuclear Molecules This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Examples of covalent homonuclear molecules Hydrogen (H 2 ) Oxygen (O 2 ) Ozone (O 3 ) Iodine (I 2 ) Phosporous (P 4 ) Sulphur (S 6 ) Sulphur (S 8 )

6. Molecules with Homonuclear Bonds This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Molecules with one homonuclear bond Ethane (C 2 H 6 ) Hydrazine (N 2 H 4 ) Hydrogen peroxide (H 2 O 2 )

7. Making a Covalent Bond – sharing valence electrons This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License In order to share valence electrons, 2 atoms have to come into close contact with each other 2 hydrogen atoms 1 hydrogen molecule, H 2 He 1 s 1 1 s 1 1 s 2 H H H H

8. Bringing 2 atoms together is not easy – there are FOUR forces in play… This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License + + – ATOM A ATOM B

9. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The Four Forces Internuclear separation + + - - (2) (3) (4) (1)

11. Non-bonded vs Bonded Radii This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Note: the internuclear distance is SMALLER when atoms are bonded together 71 103 73 75 118 77 88 37 Covalent X–X radius (pm) BONDED 142 206 146 150 236 154 176 74 Covalent Bond Length (pm) (2 x r cov ) 135 F 185 S 140 O 154 N 210 Si 185 C 208 B 120 H Van der Waals radius (pm) NON-BONDED Element

12. … Hence, atoms must overlap to form a bond 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 The bigger the overlap, the SHORTER the bond. The shorter the bond, the STRONGER it is. Bond length

14. Breaking Bonds… This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Breaking the C-C bond produces two radicals Breaking the S-S bond opens up the ring structure C 2 H 6 S 6 2CH 3 · ·S-S-S-S-S-S ·

15. Bond Order This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The larger the bond order, the STRONGER the bond 3 Triple X ≡ X 2 Double X=X 1 Single X–X Bond Order Name of Bond Type of Bond

16. Some Bond Energies This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Group 17 I–I Br–Br Cl–Cl F–F S=S S–S O=O O–O Bond 490 P ≡P 151 200 P–P 193 945 N ≡ N 242 400 N=N 159 159 N–N 425 813 C ≡ C 266 598 C=C 498 346 C–C 146 436 H–H Bond Energy (kJmol -1 ) Bond Energy (kJmol -1 ) Bond

17. Bond Energy & Bond Length This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The shorter the bond, the higher the bond energy F is anomalous due to its small size. Bond energy would be expected to be ~275 kJmol-1 267 151 I–I 228 193 Br–Br 199 242 Cl–Cl 141 159 F–F Bond Length (pm) Bond Energy (kJmol -1 )

18. Adjacent Lone Pair Effect This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License veryclose Because F is a small atom (look at its position on the Periodic Table – it is the smallest of the 1 st row elements) its valence electrons are very close and tend to repel each other. The two atoms are forced apart and the bond is weakened This anomalous behaviour is common in 1 st row elements, particularly, N, O and F F F

19. Group Trends in Homonuclear Single Bond Energies This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Note the anomalous behaviour of N–N, O–O and F–F. Group 14 show the expected trend

20. Formation of Multiple bonds This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Can only make a single bond Could make a double bond (sharing both of its unpaired electrons with another atom) Could make a double or a triple bond. A triple bond would be stronger (sharing all three unpaired electrons with another atom) N O F

21. Bond Energies for X 2 Molecules in Group 15 (in their natural state) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License N 2 has very high bond energy…why? Bond Energy (kJmol -1 )

22. Formation of the N 2 Molecule This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License N is small enough to overlap with another N atom sufficiently to share all three of its unpaired electrons and make a very strong TRIPLE BOND LINEAR Molecule N N N

24. … Nitrogen forms a triple bond This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Other elements in Group 15 can only form single bonds X = N X = P, As, Sb or Bi

25. Periodic Trends in Bond Length, Bond Energy & Bond Order This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License X-X bond distances X-X bond dissociated enthalpy for X 2 molecules containing the first row elements

26. Bond Orders of the 1 st Row Elements This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License 1 F–F 2 O=O 3 N ≡N 2 C=C 1 B–B Bond Order Homonuclear Diatomic

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