Ch 9: Polarity of Molecules
Polarity of polyatomic molecules
polarity of a polyatomic molecule depends on:
individual bond polarity
molecular geometry
e.g., CO2, H2O, NH3: are these molecules polar?
for each polar bond in a molecule, we must look at the
individual bond dipole
then, for all of the bond dipoles, we must ask what overall
effect results from adding up the individual bond dipoles
bond dipoles are vector quantities - they possess both
direction and magnitude
194
The overall dipole moment of a polyatomic molecule is the
sum of its bond dipoles
e.g., CO2:
e.g., H2O:
e.g., NH3
In general: for ABn type molecules in which all B atoms
are the same, the following symmetrical geometries lead
to nonpolar molecules regardless of how polar the
individual bonds are:
linear (AB2)
trigonal planar (AB3)
tetrahedral and square planar (AB4)
trigonal bipyramid (AB5) octahedral (AB6)
195
E.g., are the following molecules polar?
CCl4
CS2
SO3
SF4
PF5
196
Now: we’ve learned to predict the geometries and
polarities of molecules – these will have a major influence
on physical properties of substances……
We now want to examine more closely the nature of the
chemical bond and relate what we know about electronic
structure of atoms to the electronic structure and
geometries of molecules
Two models:
Valence Bond (VB) Model
Molecular Orbital (MO) Model
Valence bond (VB) model
Combines the notion of electron-pair bonding (Lewis
valence) with the idea of atomic orbitals
Review atomic orbitals from Ch 6!
197
Buildup of e- density between nuclei occurs when
valence AOs overlap, or share a region of space
orbital overlap allows two unpaired e- of opposite spin
to share space between nuclei, forming a covalent
bond
e.g., 1s + 1s case for H2
The equilibrium H-H bond length is a compromise
between:
AO overlap (attraction)
nuclear-nuclear repulsion
this can be shown with a potential energy curve
198
Can s & p -type atomic orbitals overlap? can atomic p
orbitals overlap with other p-orbitals?
e.g., HF, N2, O2, F2
VB model successful in explaining bonding in diatomics in
terms of atomic orbitals…….
How to reconcile observed and VSEPR geometries of
polyatomic molecules using VB model?
199
Hybrid orbitals
VB model helps explain why covalent bonds form
Overlap of AOs on atoms permits sharing of
electrons
How to reconcile observed molecular geometries
with formation of bonds from atomic orbitals?
e.g., bonding in BeF2: How does Be bond to 2 F atoms?
Be: 1s22s2
F: 1s22s22p5
Each F can accommodate an additional electron in its 2p
subshell, but how can Be bind 2 fluorine atoms when it
has no unpaired electrons?
According to the VB model a bond results from
sharing of unpaired electrons via overlap of AOs…
What if we promote a 2s electron of Be to the 2p level....
200
Now, according to VB theory, Be can bind 2 F atoms....
Would the 2 Be-F bonds be identical?
Experimentally, both Be - F bonds in BeF2 are
equivalent (same length)
Can we propose another model?
allow the occupied 2s and one
2p orbital of Be to mix and
form two hybrid orbitals:
201
What are the characteristics of these sp
"hybrid orbitals" ?
like p-orbitals: have two ‘lobes’
unlike p-orbitals, one ‘lobe’ is larger
than the other
large lobes of the sp hybrid orbitals
point in opposite directions
Why should orbital hybridization occur? Isn't energy
required to promote a 2s e- of Be into a 2p orbital?
The increased overlap between the sp hybrid orbitals of
Be and the 2p orbitals of F makes up for energy required
to promote the 2s e-
The formation of BeF2 from overlap of 2sp and 2p orbitals
is more exothermic than promotion of a 2s e- is
endothermic
202
Similarly, how does C [1s22s22p2] combine with 4
hydrogens to form CH4 ?
how to get 4 unpaired e- on C to share with 4 H atoms?
promote a 2s e- to the 2p subshell; hybridize the 2s and all
3 2p orbitals: gives 4 sp3 hybrid orbitals
sp3 hybridization of C in CH4 results in 4 equivalent hybrid
atomic orbitals which point to the vertices of a tetrahedron
So the bonding in CH4 is described (in the VB theory) as
the overlap between 4 equivalent sp3 hybrid AOs of C and
1s AOs of H
203
What about the bonding in BF3? sp2 hybridization of B:
Mixing a 2s AO with 2-2p AOs produces 3 equivalent sp2
hybrid orbitals
each sp2 orbital has a large lobe and a small lobe
the large lobe of each sp2 hybrid AO points to the corner
of an equilateral triangle
204
Based upon our knowledge of geometries (the VSEPR
model), can we predict which set of hybrid orbitals an
atom uses in bond formation?
note the following:
sp hybridization
2 equivalent sp hybrid orbitals; point in opposite
directions (i.e., linear)
sp2 hybridization
3 equivalent sp2 hybrid orbitals; point to corners
of an equilateral triangle (i.e., trigonal planar)
sp3 hybridization
4 equivalent sp3 AOs; point to the vertices of a
tetrahedron
To determine which hybrid orbital set the center
atom uses, count the total number of e- pairs on
the atom; then….
Total # e- pairs
hybrid orbitals
2
sp
3
sp2
4
sp3
5
____
205
Problems du Jour
What set of hybrid orbitals is used by the central atom in
each of the following?
CS2
XeF4
PF6
H3O+
NH3
BH4206