Transcript Chapter 9 Lecture 1

Chapter 9
Molecular Geometry
and Bonding Theories
Molecular Shapes
• Shape of a molecule plays an important role in its reactivity
• The number of bonding and nonbonding electron pairs is
used to predict the shape of the molecule
Lewis structure of carbon tetrachloride, CCl4
Fig 9.1 Tetrahedral geometry of CCl4
Fig 9.2 Some common molecular shapes
Fig 9.3 Shapes of ABn molecules
Fig 9.3 Derivatives from ABn geometry
CH4
NH3
H2O
What Determines the Shape of a Molecule?
• Electron pairs, whether they be bonding or nonbonding,
repel each other
• By assuming the electron pairs are placed as far as
possible from each other, we can predict the shape of the
molecule
Electron Domains
• Electron pairs are referred to as electron domains.
• Central atom, A, in this
molecule, has four
electron domains
• In a double or triple bond, all electrons shared between
those two atoms are on the same side of the central atom;
therefore, they count as one electron domain.
Valence shell electron pair repulsion (VSEPR) model:
The best arrangement of a given number of electron domains
is the one that minimizes the repulsions among them.
Fig 9.5 Balloon analogy for electron domains
Table 9.1
Electron-domain
geometries as a
function of the
number of electron
domains
Fig 9.6 The molecular geometry of NH3
• Electron-domain geometry is often not the
shape of the molecule, however.
• Molecular geometry is that defined by the
positions of only the atoms in the
molecules, not the nonbonding pairs.
The molecular geometry of CO2
Linear; 180°
The molecular geometry of O3
Bent; 120°
BeCl2
Cl
Be
Cl
0 lone bonded
pairs on to
central
atom
2 atoms
central
atom
BF3
CH4
PCl5
equatorial
axial
axial
SF6
Linear Electron Domain
Table 9.2
• In the linear domain, there is only one molecular
geometry: linear
• NOTE: If there are only two atoms in the
molecule, the molecule will be linear no matter
what the electron domain
Trigonal Planar Electron Domain
Table 9.2
• There are two molecular geometries:
– Trigonal planar, if all the electron domains are bonding
– Bent, if one of the domains is a nonbonding pair
Tetrahedral Electron Domain
Table 9.2
• There are three molecular geometries:
– Tetrahedral, if all are bonding pairs,
– Trigonal pyramidal if one is a nonbonding pair,
– Bent if there are two nonbonding pairs.
Nonbonding Pairs and Bond Angles
• Nonbonding pairs are physically
larger than bonding pairs.
• Therefore, their repulsions are
greater; this tends to decrease bond
angles in a molecule.
Multiple Bonds and Bond Angles
• Double and triple bonds place greater electron
density on one side of the central atom than do
single bonds:
• Therefore, they also affect bond angles.
Trigonal Bipyramidal Electron Domain
• There are two distinct
positions in this
geometry:
– Axial
– Equatorial
Trigonal Bipyramidal Electron Domain
SF4
Lower-energy conformations result from having
nonbonding electron pairs in equatorial, rather
than axial, positions in this geometry.
Trigonal Bipyramidal Electron Domain
• There are four distinct
molecular geometries
in this domain:
– Trigonal
bipyramidal
– Seesaw
– T-shaped
– Linear
Table 9.3
Octahedral Electron Domain
Table 9.3
• All positions are
equivalent in the
octahedral domain.
• There are three
molecular
geometries:
– Octahedral
– Square pyramidal
– Square planar
Predicting Molecular Geometry
1. Draw Lewis structure for molecule.
2. Count number of lone pairs on the central atom and
number of atoms bonded to the central atom.
3. Use VSEPR to predict the geometry of the molecule.
What are the molecular geometries of SO2 and SF4?
O
S
AB2E
bent
F
O
F
S
F
AB4E
F
distorted
tetrahedron