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

2. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Chemical Bonds A CHEMICAL BOND joins atoms together There are 4 types of chemical bond: COVALENT BONDS Ionic bonds Coordinate bonds Metallic bonds

3. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Homonuclear Covalent Bonding What you need to know… Covalent bond formation Bond length Bond energy Bond order Relationship between bond length, bond energy and bond order Trends in the periodic table

4. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Definitions… A MOLECULE is a discrete neutral species resulting from the formation of a covalent bond or bonds between two or more atoms A HOMONUCLEAR BOND is a covalent bond between 2 identical atoms

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

7. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Making a Covalent Bond – sharing valence electrons 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 HHHH He 1s11s1 1s11s1 1s21s2

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

10. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License How close do the atoms have to be to form a bond? The VAN DER WAALS RADIUS (r v ) of an atom X is measured as half of the distance of closest approach of 2 NON-BONDED atoms of X The COVALENT RADIUS (r cov ) of an atom X is taken as half of the internuclear distance (r) in a HOMONUCLEAR X–X bond. The internuclear distance (r) in a bonded pair of atoms is called the BOND LENGTH r

11. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Non-bonded vs Bonded Radii ElementVan der Waals radius (pm) NON-BONDED Covalent X–X radius (pm) BONDED Covalent Bond Length (pm) (2 x r cov ) H1203774 B20888176 C18577154 Si210118236 N15475150 O14073146 S185103206 F13571142 Note: the internuclear distance is SMALLER when atoms are bonded together

12. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License …Hence, atoms must overlap to form a bond 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

13. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Bond Energy Sometimes called the BOND ENTHALPY The BOND ENERGY is the amount of energy required to break a bond: The larger the bond energy, the STRONGER the bond H–H 2H The bond energy is, therefore, a measure of how strong a bond is:

14. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Breaking Bonds… Breaking the C-C bond produces two radicals Breaking the S-S bond opens up the ring structure C2H6C2H6 S6S6 2CH 3 · ·S-S-S-S-S-S·

15. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Bond Order Type of BondName of BondBond Order X–XSingle1 X=XDouble2 X≡XX≡XTriple3 The larger the bond order, the STRONGER the bond

16. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Some Bond Energies BondBond Energy (kJmol -1 ) BondBond Energy (kJmol -1 ) H–H436O–O146 C–C346O=O498 C=C598S–S266 C≡CC≡C813S=S425 N–N159F–F159 N=N400Cl–Cl242 N≡NN≡N945Br–Br193 P–P200I–I151 P≡PP≡P490Group 17

17. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Bond Energy & Bond Length Bond Energy (kJmol -1 ) Bond Length (pm) F–F159141 Cl–Cl242199 Br–Br193228 I–I151267 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

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

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

22. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Formation of the N 2 Molecule 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

23. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Other elements in Group 15… P, As, Sb and Bi are TOO BIG to form multiple bonds – they can’t get close enough to overlap sufficiently These elements form SINGLE BONDS with three other atoms forming TETRAHEDRAL molecules

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

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

26. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Bond Orders of the 1 st Row Elements Homonuclear Diatomic Bond Order B–B1 C=C2 N≡NN≡N3 O=O2 F–F1

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

28. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Things to Remember… 1.The covalent bond is formed by overlapping atomic orbitals

29. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License 2.Bond Order (single, double, triple) 3.Bond Energy (energy to break bond, kJmol -1 ) (measure of bond strength) 4.Bond Length (internuclear distance, pm) 5.Trends in the periodic table…

30. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License TRENDS: –Bond energy increases as bond order increases –Bond length decreases as bond order increases –Bond energy decreases as bond length increases The shorter the bond, the stronger it is

31. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Definitions Molecule Homonuclear bond van der Waals radius Covalent radius Bond length Bond energy Bond order

32. 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