Valence Bond Theory Chapter 1

1. VALENCE BOND THEORY VALENCE BOND THEORY Valence Bond TheoryChapter 1 Dr. Shuchita Agrawal BTIRT Sironja, Sagar

2. Valence Bond Theory • Why do atoms form bonds with other atoms? • To achieve a more stable (lower) energy state. • Atoms are happiest (most stable) when their orbitals are full. • They do not like being half full.

3. + = ? H H Sharing Electrons • Consider hydrogen: • Electron configuration: • Hydrogen would be much happier with another electron. • If hydrogen bumped into another hydrogen, its electron configuration would be identical.

4. H H Sharing Electrons • The shared electrons automatically assume opposite spins. • Thus, the atoms are sharing not just electrons, but orbitals.

5. Consider a graph of intermolecular distance vs. energy:

6. + = + = Sharing Electrons • The 1s orbitals are overlapped to share electrons. • The new combined orbital is called a sigma (s) bond.

7. Sharing Electrons • Now consider hydrofluoric acid, HF. • Draw energy level diagrams for hydrogen and fluorine in their ground states. • Propose a theory for how they form a bond. • Draw a picture of the resulting bond.

8. H F HF Sharing Electrons n=2 n=1 + =

9. Sharing Electrons • What orbitals are being shared? • 1s and 2pz (or 2py, or 2px) • What does this look like?

10. Sharing Electrons + =

11. Sharing Electrons + =

12. Orbital Hybridization • One of the most influential chemistry books ever written was The Nature of the Chemical Bond by Linus Pauling (1901-1994). • Published in 1939, Pauling was awarded the Nobel prize in 1954. Pauling received a second Nobel prize for peace in 1962.

13. Orbital Hybridization • Pauling made tremendous contributions to the field of chemistry, and was an outspoken activist against war and nuclear weapons. Pauling in 1987.

14. Orbital Hybridization • Consider the energy level diagram for carbon: • Carbon appears to have only 2 unpaired electrons, yet it is capable of forming 4 covalent bonds. C

15. Orbital Hybridization • Carbon is able to form four bonds by hybridizing its s and p sublevels into an sp3 orbital. The resulting orbitals consist of 1 s orbital, and 3 p orbitals, so they are called sp3. sp3 2p 2s Carbon promotes an electron from its 2s orbital to the empty 2p orbital 1s C

16. sp3 orbitals are shaped like balloons.

17. sp3 orbitals form (s) sigma bonds with other orbitals.

18. Orbital Hybridization • Other hybridizations are possible: • Boron may form 3 sigma (s) bonds with each sp2 orbital. The resulting orbitals consist of 1 s orbital, and 2 p orbitals, so they are called sp2. sp2 2p 2s 1s B

19. Orbital Hybridization • What hybrid orbitals can Be form? • Beryllium forms the hybrid orbital sp (the 1 is assumed). The resulting orbitals consist of 1 s orbital, and 1 p orbitals, so they are called sp. sp 2p 2s 1s Be

20. Orbital Hybridization • It should be remembered that these atoms only form hybrid orbitals for bonding. • The shape of the hybrid orbital is also very important. • sp3 = tetrahedral, bond angle = 109.5o • sp2 = trigonal planar, bond angle = 120o • sp = linear, bond angle = 180o

21. Problem • Phosphorous can form 2 different molecules with chlorine: PCl3 and PCl5. • Nitrogen can only form NCl3. • Can you explain why? • Hint – How does P differ from N in its electron configuration? • Hint – What does P have available to it that N does not?

22. Orbital Hybridization • What hybrid orbital(s) will N form? • Nitrogen forms 4 sp3 orbitals, but only has 3 unpaired orbitals with which to form covalent bonds. sp3 2p 2s 1s N

23. Orbital Hybridization • Can Phosphorous form hybrid orbitals? • It must! How? 3d 3p 3s Phosphorous forms 5 hybrid orbitals using the d sub shell. 2p 2s The resulting hybridized orbitals consist of 1 s, 3 p, and 1 d, and are therefore called sp3d. 1s P

24. Double & Triple Bonds • Consider the molecule ethane, C2H6. • Each carbon forms 4 sp3 hybrid orbitals in order to bond to 3 hydrogens and to each other. • Now consider ethene, C2H4. • What is the Lewis diagram for C2H4?

25. H H H H C C Double & Triple Bonds • Ethene forms a double bond between the carbons to satisfy the octet rule. • The sp3 hybrid orbital cannot explain this bond formation.

26. Double & Triple Bonds • The carbon atoms instead form 3 sp2 hybrid orbitals, retaining the normal 2pz orbital. sp2 2pz 2p 2s 1s C

27. Double & Triple Bonds • 3 sp2 hybrid orbitals take the shape of trigonal planar.

28. Double & Triple Bonds • 3 sp2 orbitals + 1 2pz orbital

29. Double & Triple Bonds • When two p orbitals overlap, this allows a new type of bond to form. • A bond formed by two overlapping p orbitals is called a p (pi) bond. • A p bond is weaker than a s bond, due to the greater distances involved.

30. Double & Triple Bonds • When the sp2 orbitals form a s bond between carbons, the 2pz orbitals of each carbon overlap. s

32. Double & Triple Bonds • Now consider ethyne, or C2H2. • The Lewis diagram predicts a triple bond between the carbons. • Can you hypothesize what a triple bond consists of?

33. Double & Triple Bonds • Carbon forms 2 sp hybrid orbitals. sp 2p 2s 2py2pz 1s C

34. Double & Triple Bonds • The resulting s bond between the sp orbitals brings both 2py and 2pz orbitals together to form two p bonds.

35. Double & Triple Bonds

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