CHAPTER 9
AP/HONORS CHEMISTRY
MOLECULAR GEOMETRY








VSEPR - valence-shell electron pair repulsion
IDEAL GEOMETRIES
Type
geometry bond angle
AB2 (2 e- pairs) Linear
180o
example on board
AB3 (3 e- pairs) Triangular
120o
example on board
Planar
AB4 (4 e- pairs) Tetrahedral 109.5o example on board
AB5(5 e- pairs)
Trigonal 120o & 180o example on board
bipyramidal
AB6(6 e- pairs)
Octahedral
90o
example on board
EFFECT OF UNPAIRED
ELECTRONS





Type
geometry bond angle
AB2E (2 e- pairs,
Bent
example on board
1 lone pair)
AB2E2 (2 e- pairs,
Bent
105o example on board
2 lone pairs)
AB3E (3 e- pairs,
Triangular 107o example on board
1 lone pair) pyramidal
the electron pair geometry approximately the same as that
observed when only single bonds exist
CONTINUED
the molecular geometry is quite different
with lone pairs. Why?
 lone pairs take up more space than when
bonded
 consider NH3 it has a electronic pair
geometry of tetrahedral but the angle is
107o
 the water molecule is nonlinear because
of lone pairs 105o

OTHER LONE PAIR GEOMETRIES
AB4E
see-saw
SF4
 AB3E2
T-shaped
ClF3
 AB2E2
linear
XeF2
 AB5E
square pyramidal
BrF5
 AB4E2
square planar
XeF4
 when dealing with geometries multiple bonds
behave like single bonds

CONTINUED
the geometry depends upon how many
terminal atoms the central atom has
around it and the number of unshared
electrons
 the VSEPR model can be extended to
molecules which do not have a single
central atom. C2H2 and C2H4

POLARITY OF MOLECULES





Polar
 large (not large enough to form ionic bonds)
electronegative difference
 unequal share of the electrons
nonpolar equal sharing
the degree of polarity is measured by its dipole
moment
π = Qr, Q = charge at either end, r = distance
1 debye = 3.33 x 10-30 coulomb-meters H-Cl
1.03D bond length 1.36 Ǻ
CONTINUED




is it easy to determine whether a diatomic molecule
is polar. Why?
check if the elements are the same or different
if a molecule contains more than two atoms, we
must decide whether the whole molecule is polar or
nonpolar
the are two criteria for determining the polarity of a
molecule: bond polarity and molecular geometry
ATOMIC ORBITALS

Sigma bonds σ


single lobe where the electron density is
concentrated in the region directly between two
bonded atoms
pi bond π

this orbital has two lobes one above the bond
axis and one below. The electron density is
zero
all single bonds are sigma bonds and any
other bond in a double or triple bond will be
pi bonds
 on board

DELOCALIZE BONDING
The bonds are given to all atoms i.e.
benzene on board makes it stable
(aromaticity)
 magnetism - molecules with one or more
paired electrons are attracted to a
magnetic field
 the more unpaired electrons, stronger
attraction- paramagnetism
 with no unpaired electrons, weak repulsion
- dimagnetism
