Chem 150
Week 11
Handout 1
Valence bond method and Hybrid orbitals
What a good bonding theory should do: Describe the set of observed properties a molecule possesses
(bond length, bond energy bond angle...)
Valence-bond method: A description of covalent bonding in terms of overlap of
singly occupied atomic orbitals.
Example H2S
2H atoms
+
1S atom
CH4 is tetrahedral, but how can we explain this with valence bond theory?
Algebraically combine 2s and 2p orbitals to give hybrid orbitals (sp3)
Note that the four sp3 hybrid orbitals all have the same energy (degenerate) so the four valence
electrons of carbon occupy them singly. Since they all have the same energy and are singly occupied
they like to be as far as possible from each other (same as VSEPR prediction for 4 electron groups) giving
rise to tetrahedral geometry. Now we can overlap the four sp3 hybrid orbitals of carbon with four singly
occupied 1s orbitals of four hydrogen atoms.
Methane as described by valence bond method.
How about BF3?
Note: there is no physical evidence for hybrid orbitals. It is an after the fact realization (e.g. methane
is tetrahedral so let’s use sp3 hybrid orbitals to explain the shape!) Nevertheless it is a very powerful
theory and is used by many chemists to explain the bonding in thousands of molecules. Here is how
you use this theory:
How to predict the shape of a molecule and a hybridization scheme consistent with this prediction.
1. Write the Lewis structure
2. Use VSEPR to establish the geometry of the electron-groups.
3. Use VSEPR to establish the molecular geometry.
4. Select a hybridization scheme that corresponds to the VSEPR prediction (sp3 for tetrahedral,
sp2 for trigonal planar and sp for linear geometry).
Multiple covalent bonds:
•
Consider the bonding in C2H4
•
VSEPR predicts 120° H-C-H and H-C-C bond angles
What is the shape of the whole molecule? Planar or twisted? Consider the geometry of Carbon!
Bonding in acetylene: VSEPR predicts linear structure:
Class exercise:
Describe the bonding scheme and molecular geometry of acetic acid, CH3COOH? What is the maximum
number of atoms in CH3COOH that can sit in a plane? How many sigma (σ) and how many pi (π) bonds
have you formed?