Chapter 10 – Hybridization and Molecular Orbitals
Three ways to view bonding (Simple Complex)
I. Atomic Orbital Overlap (chapter 9)
II. Hybrid Atomic Orbitals
III. Molecular Orbitals (MO theory)
Which model best explains the bonding, depends on the
molecule. MO theory is the most general and the most
complicated.
Problem- Ssimple overlap of atomic orbitals does not
always explain the bonding in real molecules.
Modify our bonding theory to try to explain reality.
Example: Methane, CH4
From experiment: All C-H bonds are identical in length
and strength.
Regular Electron configuration of Carbon = 1s22s22p2
- only has 2 unpaired electrons to use for bonds
This implies that carbon could only form 2 bonds to
other atoms, which doesn't jive with reality.
One simple modification – assume 1 e- in 2s is promoted
C = 1s22s12p3
- improvement –
have 4 unpaired ecould form 4 bonds
- remaining deficiency – not all bonds alike
bond with 2s not the same as with 2p
Need to modify our model some more.
Hybrid Atomic Orbitals
Modification done with math - add and subtract the 's for
the s and p orbitals together)
- mix the 2s and 2p orbitals (Orbital Blender)
- yields orbitals with intermediate character
- yields degenerate orbitals (same energy)
- they are called Hybrid Atomic Orbitals
Number of atomic orbitals into blender = Number of
hybrid atomic orbitals out of blender
- Orbital name tells what kind and how many of each type
went into the blender
• Carbon
Recipe:
Mix one 2s orbital with three 2p orbitals
Blend thoroughly with math.
Result:
four hybrid atomic orbitals
all have equal energy
called: sp3 hybrid orbitals
Electron configuration is now: 1s22(sp3)4
- improvement – 4 equal energy unpaired e- ’s
- Now explains the 4 identical bonds in CH4
Hybrid atomic orbitals can overlap with orbitals on
another atom to form bonds.
Each sp3 orbital on carbon can overlap with a 1s orbital
on hydrogen to make CH4.
Other kinds of hybrids
Note: in multiple bonds, hybrid atomic orbitals are only
needed to explain the first bond
Mixture
Hybrids
# Hybrids
e- Geometry
one s + one p
sp
2
one s + two p’s
sp2
3
trigonal planar
one s + three p’s
sp3
4
tetrahedral
one s + three p’s + one d
sp3d
5
one s + three p’s + two d’s
sp3d2
6
linear
trigonal
bipyriamidal
octahedral
Second bond or third bonds in double or triple bonds can
be explained using the simple overlap of unmodified
atomic (p) orbitals. (The first way we looked at bonding.)
III. Molecular Orbitals
- Some aspects of bonding are not explained by Lewis
structures, VSEPR theory or hybridization.
- Molecular Orbital Theory (MO theory)
- Orbitals are made for the molecule as a whole
The Hydrogen Molecule
- Mix the Atomic orbitals (AO’s) with math to make two
new MO’s (different math than for hybrids)
- One bonding MO ()
- electron density is between the nuclei
- contributes to bond formation
- One antibonding MO (*)
- little electron density between nuclei
- goes against bond formation
Molecular Orbital Diagrams
- Atomic orbitals used to make MO’s shown left and
right
- resulting MO’s shown in the middle
- Follow same rules for filling with electrons as with
atomic orbitals
- H2 has two bonding electrons.
The Helium Molecule, He2 (hypothetical)
-2 He atoms, together they have 4 electrons
- mix the 1s orbitals as with hydrogen
-2 bonding electrons
-2 nonbonding electrons
Bond Order = ½(bonding e- - antibonding e-).
Bond order = 0 for NO BOND
Bond order = 1 for single bond
Bond order = 2 for double bond
Bond order = 3 for triple bond
H2 = ½(bonding electrons - antibonding electrons)
= ½(2 - 0) = 1.
H2 has a single bond.
He2 = ½(bonding electrons - antibonding electrons)
= ½(2 - 2) = 0.
He2 makes no bond and is not a stable molecule.
Second-Row Diatomic Molecules
Molecular Orbitals from mixing 2p AO’s
- Two ways for two p orbitals to overlap (be mixed to
make MO's)
- end on ( orbital)
- side-to-side ( orbital)
- For two atoms bonding
- each brings three p-orbitals pxpypz
- yields 6 MO’s:
- one  and two  orbitals
- one * and two *orbitals
- The relative energies of these  and  orbitals depends
on the atoms involved.
Electron Configurations for B2 through Ne2
From experiment it is known that:
- For B2, C2 and N2 the 2p orbital is higher in energy
than the 2p
-
For O2, F2 and Ne2 the 2p orbital is higher in energy
than the 2p
Two types of magnetic behavior:
- Paramagnetism, unpaired electrons in molecule
- Diamagnetism, no unpaired electrons in molecule
- N2 is diagmagnetic, bond order = 3
- O2 is paramagnetic, bond order = 2