Suggested HW: Ch 9: 25, 29, 39, 43, 72 (For 25 and 43, you are
illustrating the hybridization of the atomic orbitals into hybrid
orbitals and the overlapping of these hybrid orbitals as described
in the examples provided) Lecture 11 Covalent Bonding Pt 3:
Hybridization (Ch. 9.5-9.13)
Slide 2
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
Slide 3
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
Slide 4
Overlapping Valence Orbitals Recall s and p orbitals (ch 5) S
orbitals are spherical. L = 0, m L = 0 Max of 2 electrons P
orbitals consist of two lobes of electron density. L= 1, m L = -1,
0, 1 (3 suborbitals) Max of 6 electrons pxpx pypy S pzpz
Slide 5
Forming Sigma ( ) Bonds Covalent bond 1s 1 H + H + H + H + Two
overlapping atomic orbitals form a molecular bonding orbital. Plus
sign indicates phase of electron wave, NOT CHARGE A sigma ()
bonding orbital forms when s-orbitals overlap. stabilization
(energy drop) Energy
Slide 6
Introduction to Hybridization Imagine the molecule CH 4. We
know that carbon has 4 valence electrons (2s 2 2p 2 ). However,
when we fill our orbitals in order as according to Hunds rule, we
notice there are only enough unpaired electrons to make two bonds.
Stay mindful of the fact that a covalent bond involves the sharing
of unpaired electrons 2s 2 2p 2 C 1s 1 4 H ENERGY 1s 1 X
Slide 7
Four sp 3 hybrid orbitals sp 3 Hybridization So how does CH 4
form? How can carbon make 4 bonds? To make four bonds, carbon
hybridizes four of its atomic orbitals. This creates four
equivalent sp 3 hybrid orbitals, each containing one unpaired
electron. 2s 2 2p 2 The name sp 3 originates from the fact that the
hybrid orbitals form as a result of the mixture of 1 s-orbital and
3 p-orbitals. Thus, each sp 3 orbital is 25% s character and 75% p
character ENERGY
Slide 8
Formation of Sigma Bonding Orbitals sp 3 hybrid orbitals ENERGY
1s 1 C 4H bonding orbitals atomic s-orbitals
Slide 9
The addition of an s-orbital to a p z orbital is shown above.
The s orbital adds constructively to the (+) lobe of the p z
orbital and adds destructively to the lobe that is in the opposite
phase (-). The symbols indicate phase, not charge. 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. s pzpz + = z z Illustration of Orbital
Hybridization
Slide 10
Illustration of sp 3 Hybrid Orbitals and Orbital Overlap 4
-bonds The four hybrid orbitals arrange themselves
tetrahedrally.
Slide 11
sp 2 Hybridization The BH 3 molecule gives us an example of sp
2 hybrid orbitals. Once again, we have a situation where we dont
have enough bonding sites to accommodate all of the hydrogens.
(Remember, B is electron deficient!) 2s 2 2p 1 B 1s 1 3 H ENERGY
X
Slide 12
sp 2 Hybridization So, to make 3 bonding sites, 3 hybrid
molecular orbitals are formed by mixing the 2s-orbital with two
2p-suborbitals. This forms an sp 2 orbital. Each of these three
hybrid orbitals are one- third s-character, and two-thirds
p-character. 2s 2 2p 1 B Three sp 2 hybrid orbitals unused 2p
suborbital ENERGY
Slide 13
This figure illustrates the 3 hybrid orbitals combined with the
unused 2p orbital, which is perpendicular to the hybrid orbitals.
sp 2 orbitals The result of adding one s and two p orbitals
together is a trigonal planar arrangement of electron domains
Slide 14
H + empty 2p orbital H + H + B H bond H H sp 2 Geometry and
Bonding
Slide 15
sp Hybridization Imagine BeH 2 (the Be-H bond is covalent),
with Be having the electron configuration: [He]2s 2 Here, we have a
situation where no bonding electrons are available. To make two
Be-H bonds, Be must create two hybrid orbitals by mixing two atomic
orbitals (the 2s orbital and one of the 2p orbitals). This yields
sp hybrid orbitals (50% s, 50% p) 2s 2 ENERGY 2p 0 1s 1 Be 2 H
X
Slide 16
2s 2 ENERGY 2p 0 Be Two sp hybrid orbitals unused 2p
suborbitals sp Hybridization
Slide 17
Hybridization of Lone Electron Pairs Ex. What is the
hybridization of Oxygen in H 2 O? The valence electron
configuration of O is [He]2s 2 2p 4 2p 4 2s 2 As you see, there are
two unpaired O electrons. Does this mean that these two
p-suborbitals can overlap with the two Hydrogen 1s orbitals without
hybridizing?? ENERGY O 1s 1 2 H
Slide 18
No!! The reason is that we now have two sets of lone pairs of
electrons that are substantially different in energy (2s and 2p).
The orbitals will hybridize to form degenerate (equal energy) sets
of electrons. Lone pair must always be equal in energy with each
other, and with bonding electrons. 2p 4 2s 2 ENERGY O 1s 1 2 H X H
O H 2s electrons 2p electrons Hybridization of Lone Electron Pairs
BAD!!
Slide 19
Water has sp 3 hybridization 2p 4 2s 2 1s 1 ENERGY bonds H2OH2O
H O H sp 3 electrons O 2 H Four sp 3 hybrid orbitals Lone
pairBonding electrons
Slide 20
So What Do We Know So Far? Total Electron Domains Around Atom
(Bond + LP) Hybridization 2 sp 3 sp 2 4sp 3
Slide 21
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, C 2 H 4 C C H H H H The hybridization
of each carbon is sp 2 because each is surrounded by three electron
domains. The geometry around each C is trigonal planar. sp 2
Slide 22
2s 2 2p 2 sp2 hybrid orbitals unhybridized p-electron C C H H H
H Forming Double Bonds C We can see that for each carbon atom, we
need three sp 2 orbitals and three unpaired electrons to make three
sigma bonds. But how is the double bond formed?
Slide 23
All double bonds consist of 1 -bond and 1 -bond Double Bonds
formed by simultaneous and interaction The remaining p-electrons
form a bond. This bond forms due to attraction between the parallel
p-orbitals. The like-phase regions are drawn toward one another and
overlap. H + H + H + H +
Slide 24
Triple Bonds formed by 1 -bond and 2 -bonds. Ex. HCN HC N sp
Can you draw the orbital diagram for this molecule?
Slide 25
Examples How many and bonds are in each of the following
molecules? Give the hybridization of each carbon. CH 3 CH 2 CHCHCH
3 CH 3 CCCHCH 2
Slide 26
sp 3 d and sp 3 d 2 hybridization Atoms like S, Se, I, Xe etc.
can exceed an octet because of sp 3 d and sp 3 d 2 hybridization
(combination of ns, np, and nd orbitals where n>3). This results
in either trigonal bipyramidal or octahedral skeletal geometry sp 3
d sp 3 d 2
Slide 27
Exceeding an Octet. Example: SF 6 3p 4 3s 2 Energy 3d 0 sp 3 d
2 hybrid orbitals sp 3 6 F S SF 6 Fluorine lone pair
Slide 28
S Exceeding an Octet. Example: SF 6 F 3 lone pair unpaired
electron overlap x 6
Slide 29
Look Familiar ???
Slide 30
Examples: What is the hybridization of the central atom? CO 2 H
2 CO CH 3 CCH IF 5 PCl 5 SeOF 4