2. Recall : methane, CH4 – tetrahedral
shape (4 equal covalent bonds)
Carbon has an electron
arrangement 1s22s22p2
There is a contradiction
here . . . . . .
3. Recall: the s orbital shape
ONE s orbital in each energy level
4. Recall: the p orbital shape
THREE p orbitals in each energy level (beginning n = 2)
5. Carbon’s valence shell - 2nd energy level orbitals
The 4 orbitals are oriented like so:
Problem #1
This is not a tetrahedral
shape. The 3 p orbitals are at
900 to each other and the s
orbital is “non-directional”
6. Carbon ground state electron configuration
What is the expected orbital notation of carbon
in its ground state?
Problem #2?
Only 2 electrons appear to be available for bonding
7. Solution – require 4 unpaired electrons
Perhaps carbon “promotes”
one of its 2s electrons…
…to the empty 2p orbital.
8. BUT. . . this would mean that three of the bonds in a methane
molecule would be identical,
But what about the fourth
bond…?
Must have a lower bond energy
This was not observed by chemists
9. “hybridization” theory - What if the s orbital is combined
with three p orbitals to create four equal hybrid orbitals?
“hybridized”
These new orbitals have slightly MORE energy than
the 2s orbital… and slightly LESS energy than the 2p orbitals.
…
But all 4 hybrid orbitals have equal energy.
10. Since the 4 hybrid orbitals result from a combination of 1 s
and 3 p orbitals, these hybrids are termed sp3 orbitals.
11. 3 3 3 3
1s 2s 2px 2py 2pz sp sp sp sp
y
Hybridize
x
o
109.5
z
Summary of sp3 hybridization
14. Sigma (σ) bonds
The bonds formed in methane between carbon and
hydrogen atoms are called sigma bonds.
As shown in the diagram, a sigma bond is formed by the head
on overlap of two orbitals. THE OVERLAP REGION LIES
DIRECTLY BETWEEN THE TWO NUCLEI. Nuclei can rotate
about a sigma bond. Video tutorial link to ethane
15. sp2 hybridization
Consider the electron configuration of boron:
When boron bonds with hydrogen, VSEPR predicts a
trigonal planar geometry.
One s orbital hybridizes with 2
p orbitals to form 3 sp2 orbitals
16. sp2 hybridization also occurs in molecules that contain double
covalent bonds. For example, ethene C2H4
17. Each carbon : 3 hybridized sp2 orbitals and 1 p orbital
Sigma bond – single bond C-H
Sigma bond – sp2 orbitals: one half of double bond C=C
Pi bond – p orbitals: other half of double bond C=C
18. Pi bonds
A pi bond consists of two p orbitals that overlap above and
below a sigma bond. A double bond always consists of ONE
sigma bond and ONE pi bond.
The molecule cannot be twisted without breaking the pi bond.
Video tutorial ethene
19. Practice
Draw a diagram to show the orbital overlap in the sigma and pi
bonds in a molecule of formaldehyde CH2O. State the shape of
the molecule and the arrangement of negative charge centers
around the central atom.
20. Formaldehyde
2 Lone
Pairs
bond Sigma bond
trigonal planar (3 regions of electron density)
21. sp hybridization
Consider the electron configuration of beryllium:
When beryllium bonds with chlorine, VSEPR predicts a
linear geometry.
One s orbital hybridizes with 1
p orbitals to form 2 sp orbitals
23. In ethyne C2H2 each carbon has 2 sp orbitals and 2 un-hybridized
p orbitals.
The single bond between carbon and hydrogen is a sigma bond.
The triple bond between the two carbons consists of a sigma bond
between two sp2 orbitals and two pi bonds between the two sets of
p orbitals.
24. The two pi bonds are in different planes 900 to each other . A
triple bond always consists of ONE sigma bond and TWO pi
bonds. The molecule cannot be twisted without breaking the
pi bonds.
Show video tutorial acetylene