Principles of Chemical Science_Valence Bond Theory and Hybridization - Lec14

5.111 Lecture Summary #14 Wednesday, October 8, 2014

Readings for today: Sections 3.4, 3.5, 3.6 and 3.7 (Same sections in 4th and 5th ed) –
Valence Bond Theory.
Read for Lecture #16: Sections 6.13, 6.15, 6.16, 6.17, 6.18, and 6.20 (Same sections in 4th
and 5th ed) – The Enthalpy of Chemical Change.
Topics: I. Valence bond theory and hybridization
A. Sigma and pi bonds
B. Hybridization of atomic orbitals
i. sp3 hybridization
ii. sp2 hybridization
iii. sp hybridization
I. VALENCE BOND THEORY AND HYBRIDIZATION
1s! 1s!
In valence bond theory, bonds result from the pairing of
unpaired electrons in atomic orbitals.

H! H!
A. SIGMA AND PI BONDS
σ (sigma) bond: cylindrically symmetric with nodal plane across the bond axis.
π (pi) bond: a bond with e- density in two lobes, one on each side of the bond axis. A
pi bond has a nodal plane along the bond axis.
We can describe multiple bonds according to valence-bond theory.
• single bond:
• double bond: one σ-bond plus one
• triple bond: one σ-bond plus π-bonds

B. HYBRIDIZATION OF ATOMIC ORBITALS
i) sp3 hybridization
A carbon atom has four unpaired electrons available for bonding once a 2s-electron is
to an empty 2-p orbital.
E!


2pz! 2px! 2py!
electron 2sp3! 2sp3! 2sp3! 2sp3!
promotion!

2s! _________ orbitals!
C
(4 valence e-s)!

hybridization!


1

, The sp3 hybrid orbitals are equivalent and degenerate. They differ only in their
in space.




+
- - -
-




+
+
+
four sp3 hybrid orbitals


For carbon, each sp3 orbital contains a single electron, allowing four bonds.




What provides the energy for the initial electron promotion?
!


Each bond is labeled based on the bond type
(σ or π) and atomic orbital composition: (C ,H )




Consider ethane, C2H6.

H! 109.5° H!


H! C C H!
2sp3! 2sp3! 2sp3! 2sp3! 2sp3! 2sp3! 2sp3!
H! H!
σ bond!

Two bond types in ethane: and .




2