Transcript Chapter 9 Lecture 2

Chapter 9
Molecular Geometry
and Bonding Theories
Trigonal Bipyramidal Electron Domain
Table 9.3
– Trigonal
bipyramidal
– Seesaw
– T-shaped
– Linear
Shapes of Larger Molecules
• Consider the geometry
about a particular atom
rather than the
geometry of the
molecule as a whole
Shapes of Larger Molecules
• Larger molecules tend to
react at a particular site in the
molecule
• Called a functional group
acetic acid
Molecular Shape and Molecular Polarity
• A molecule possessing
polar bonds does not imply
that the molecule as a
whole will be polar
Fig 9.11 CO2, a nonpolar molecule
Molecular Shape and Molecular Polarity
• To determine the overall dipole moment for the
molecule, add the individual bond dipoles vectorially
Fig 9.12
Molecular Shape and Molecular Polarity
Fig 9.13 Molecules containing polar bonds
Polar
Polar
Nonpolar
Nonpolar
Polar
Covalent Bonding and Orbital Overlap
How does Lewis theory explain the bonds in H2 and HCl?
“Sharing of two electrons between the two atoms”
Overlap of: 2 1s orbitals
1s orbital and 3p orbital
Valence bond theory – bonds are formed by sharing
of e− from overlapping atomic orbitals (AOs)
Fig 9.15 Formation of the H2 molecule
74 pm
Hybrid Orbitals
• VSEPR theory allows prediction of molecular shapes
• How can tetrahedral, trigonal bipyramidal,
and other geometries arising from the atomic orbitals
we recognize?
Hybridization – mixing of two or more atomic
orbitals to form a new set of hybrid orbitals.
1. Mix at least 2 nonequivalent atomic orbitals (e.g. s and p).
Hybrid orbitals have very different shape from original
atomic orbitals.
2. Number of hybrid orbitals = number of pure atomic
orbitals used in the hybridization process.
3. Covalent bonds are formed by:
a) Overlap of hybrid orbitals with atomic orbitals
b) Overlap of hybrid orbitals with other hybrid orbitals
sp Hybrid Orbitals
Be
F
F
VSEPR predicts: Linear, 180°
Be
Assume Be absorbs the small amount of energy needed to
promote an electron from the 2s to the 2p orbital:
it can form
two bonds.
sp Hybrid Orbitals
F
Be
F
VSEPR predicts: Linear, 180°
Fig 9.16 formation of sp hybrid orbitals
• Mixing the s and p orbitals yields two degenerate orbitals that
are hybrids of the two orbitals:
– These sp hybrid orbitals have two lobes like a p orbital.
– One of the lobes is larger and more rounded as is the s
orbital.
sp Hybrid Orbitals
• These two degenerate orbitals would align themselves
180 from each other
Fig 9.17 Formation of two equivalent Be-F bonds in BeF2
• This is consistent with the observed geometry of beryllium
compounds: linear
sp2 Hybrid Orbitals
Using a similar model for boron leads to…
Fig 9.18
sp3 Hybrid Orbitals
With carbon we get…
Fig 9.19
Hybridization Involving d Orbitals
For geometries involving expanded octets on the
central atom, we must use d orbitals in our hybrids:
Hybridization Involving d Orbitals
This leads to five degenerate sp3d
orbitals…
…or six degenerate sp3d2 orbitals.
How do I predict the hybridization of the central atom?
Count the number of lone pairs AND the number
of atoms bonded to the central atom
# of Lone Pairs
+
# of Bonded Atoms
Hybridization
Examples
2
sp
BeCl2
3
sp2
BF3
4
sp3
CH4, NH3, H2O
5
sp3d
PCl5
6
sp3d2
SF6
Sigma () Bonds
Fig 9.14

Characterized by:
 Head-to-head overlap
 Single bonds are always  bonds
Pi () Bonds
 Pi bonds are characterized by:
 Side-to-side overlap
• In a multiple bond:
• one of the bonds is a  bond
and the rest are  bonds
Fig 9.22
Multiple bonds in ethylene
sp2
Fig 9.23
Molecular geometry of ethylene
Fig 9.24 The σ bonds in ethylene
sp2
Multiple Bonds
The π bond in ethylene
Fig
Fig9.25
9.25