Hybridization
Dr. Harris
Lecture 11 (Ch 9.5-9.13)
9/25/12
HW: Ch 9: 21, 23, 39, 43, *75
*(show all carbon atoms, refer to pg 215)
Introduction
• We now know that atoms can bond covalently through the sharing
of electrons
• VSEPR theory helps us predict molecular shapes. But, it does not
explain what bonds are, how they form, or why they exist.
• In ch 9, chemical bonding will be explained in terms of orbitals
Covalent Bonding Is Due to Orbital Overlap
• In a covalent bond, electron density is concentrated between the
nuclei.
• Thus, we can imagine the valence orbitals of the atoms overlapping
• The region of orbital overlap represents the covalent bond
Overlapping Valence Orbitals
• Recall s and p orbitals (ch 5)
• S orbitals are spherical. L = 0, mL = 0
• Max of 2 electrons
S
• P orbitals consist of two lobes of electron
density on either side of the nucleus.
• L= 1, mL = -1, 0, 1 (3 suborbitals)
• Max of 6 electrons
px
py
pz
Forming Sigma (σ) Bonds
Energy
+
+
+
H
H
H
σ
+
H
Covalent bond
1s1
1s1
• Two overlapping atomic orbitals form a
molecular bonding orbital.
• A sigma (σ) bonding orbital forms when
s-orbitals overlap.
σ
Hybridization
• Imagine the molecule BeH2. We know that Be has a valence configuration
of [He] 2s2 (Be-H bond is not ionic).
• However, when we fill our orbitals in order as according to Hund’s rule, we
notice that there are no unpaired electrons. Hence, we can not make any
bonds. Stay mindful of the fact that a covalent bond involves electron
sharing
ENERGY
Be
X
2p0
2H
2s2
1s1
1s1
Hybridization
• So how does BeH2 form? How can Beryllium make 2 bonds?
• To bond with 2 hydrogen atoms, Beryllium mixes (hybridizes) two of
its atomic orbitals. This creates two sp hybrid orbitals.
Energy
2p
2s2
sp hybridization
sp hybrid orbital
• Each sp orbital is 50% s character and 50% p character
Hybridization
pz
s
+
z
=
z
• The addition of an s-orbital to a pz orbital is
shown above. The s orbital adds constructively
to the (+) lobe of the pz orbital and adds
destructively to the lobe that is in the opposite
phase (-). The symbols indicate phase, not
charge.
• Remember, we are adding two atomic orbitals,
so we will obtain two hybrid orbitals
2 sp hybrid orbitals
sp-Hybridized Bonding
• Small negative lobes not shown. Recall that we have two unused p-orbitals
along the x and y axes.
• Now, 2 Hydrogen s-orbitals can overlap with the Be sp-hybrid orbitals to
form BeH2. We would expect BeH2 to be linear, as is predicted by VSEPR
sp2 Hybridization
• Whenever we mix a certain number of s and p atomic orbitals, we get the
same number of molecular orbitals. This is called the principle of
conservation of orbitals.
• The BH3 molecule gives us an example of sp2 hybrid orbitals.
• Once again, we have a situation where we don’t have enough bonding
sites to accommodate all of the hydrogens.
Energy
2p1
2s2
B
3H
1s1
1s1
1s1
Hybridization
ENERGY
• So, to make 3 bonding sites, 3 hybrid molecular orbitals are formed by
mixing the 2s-orbital with two 2p-suborbitals.
unused 2p suborbital
2p1
B
2s2
sp2 hybrid orbitals
• Each of these three hybrid orbitals are one-third s-character, and twothirds p-character.
sp2 orbitals
The result of adding the s
and p orbitals together is a
trigonal planar arrangement
of electron domains
This figure illustrates the 3
hybrid orbitals combined with
the unused 2p orbital, which
is perpendicular to the hybrid
orbitals.
sp2 Geometry and Bonding
empty 2p orbital
σ bond
H
+
H
+
H
+
H
B
H
H
sp3 Hybridization
• Involves the mixing of an s-orbital and 3 p-orbitals. The resulting
hybrid orbitals are one-fourth s-character and three-fourths pcharacter.
• Ex. CH4. To accommodate 4 hydrogen atoms, 4 hybrid orbitals are
created (C: [He] 2s22p2)
ENERGY
C
2p2
2s2
Four sp3 hybrid orbitals
Formation of sp3 Hybrid Orbitals
The four hybrid orbitals
arrange themselves
tetrahedrally. Do you
notice a trend yet?
4 σ-bonds
What about molecules with lone electron pairs?
ENERGY
• Ex. What is the hybridization of H2O?
• The valence electron configuration of O is [He]2s2 2p4
2p4
2s2
As you see, there are two unpaired
O electrons. Does this mean that
these two p-suborbitals can overlap
with the two H 1s orbitals without
hybridizing??
No. The reason is that we now have
two sets of lone pairs of electrons that
are substantially different in energy.
The orbitals will hybridize to form
degenerate (equal energy) sets of
electrons.
Water has sp3 hybridization
••
••
O
H
H
ENERGY
2p4
covalent
bonding
lone pair
2s2
bonding
electrons
2H
O
1s1
H2O
σ bonds
1s1
Water has sp3 hybridization
• The angle between the sp3 hybrid
orbitals in water is 104.5o, NOT
109.5o as expected in a normal
tetrahedron
• LONE PAIR REPEL THE ELECTRONS
IN THE O-H BONDS
Strength of Repulsion
Lone pair – Lone pair > Lone pair – Bonding pair > Bonding pair- Bonding pair
sp3d and sp3d2 hybridization
• Atoms like S, Se, I, Xe … etc. can exceed an octet because of sp3d and
sp3d2 hybridization (combination of an ns, np, and nd orbitals where n>3).
• This results in either trigonal bipyramidal or octahedral geometry
sp3d
sp3d2
Exceeding an Octet. Example: SF6
Energy
3d0
sp3d2 hybrid orbitals
3p4
Fluorine lone pair
3s2
S
sp3
6F
SF6
Exceeding an Octet. Example: SF6
unpaired electron
S
F
3 lone pair
x6
Look Familiar ???
Examples:
• What is the hybridization of the central atom?
• NH3
• NH4+
• PCl5
• CH3Cl
• SeF6
Double and Triple Bonding
• How can orbital overlap be used to explain double and triple bonds?
What kind of interactions are these?
• Lets look at ethene, C2H4
H
sp2
C
H
sp2
H
C
H
The hybridization of each
carbon is sp2 because each is
surrounded by three electron
domains. The geometry around
each C is trigonal planar.
Forming Double Bonds
H
Carbon atoms
H
C
C
H
H
unhybridized p-electron
2p2
sp2 hybrid orbitals
2s2
• We can see that for each carbon atom, the three sp2 electrons will
be used to make 3 bonds. But how is the double bond formed?
Double Bonds formed by simultaneous σ and π
interaction
•
•
+
+
+
+
H
H
H
H
The remaining electrons form a π bond. This bond forms due to
attraction between the parallel p-orbitals. The like-phase regions are
drawn toward one another and overlap.
All double bonds consist of 1 σ-bond and 1 π-bond
Triple Bonds formed by 1 σ-bond and 2 π-bonds.
Ex. HCN
H
sp
sp
C
N ••
• Can you draw the orbital diagram for this molecule?
Examples
• How many σ and π bonds are in each of the following molecules?
Give the hybridization of each carbon.
• CH3CH2CHCHCH3
• CH3CCCHCH2