CHM 1045: General Chemistry and
Qualitative Analysis
Unit 10:
Molecular Geometries
and Bonding Theories
Dr. Jorge L. Alonso
Miami-Dade College –
Kendall Campus
Miami, FL
Textbook Reference:
•Chapter # 8 (skip 9)
Molecular
Geometries
and Bonding
•Module #12 (skip 13)
Molecular Shape and Reactivity:
Lock & Key Theory
The shape of a molecule plays an important role in its reactivity
Receptor, active site
{MolecularShape&Reactivity}
t-RNA
Molecular Shape determines function!
Molecular
Geometries
and Bonding
Molecular Shape:
Effect of Cocaine on CNS
Presynaptic
Neuron
synapse
Dopamine binding to
receptors and uptake pumps.
Cocaine concentrates in areas
of the brain that are rich in
dopamine synapses.
Postsynaptic
Neuron
Cocaine binding to uptake
pumps; inhibition of
dopamine uptake when
cocaine is present in the
synapse. Cocaine (turquoise) binds to the uptake
pumps and prevents them from removing dopamine from the
synapse. This results in more dopamine in the synapse, and
Molecular
Geometries
and Bonding
Molecular Shape determines function!
Dopamine
Cocaine
Molecular
Geometries
and Bonding
Molecular Shapes
1. By noting the number of
bonding and
nonbonding electron
pairs we can easily
predict the shape of the
molecule.
2. electron pairs, whether they be bonding or nonbonding,
repel each other.
3. By assuming the electron pairs are placed as far
as possible from each other.
Molecular
Geometries
and Bonding
Bonding Theories
(1) Valence Shell Electron Pair Repulsion
(VSEPR) Theory: considers only the valence
electrons and their geometry when they repel
each other when they form bonds.
(2) Valence Bond (VB) Theory: considers the
valence electrons and the quantum mechanical
orbitals and how they hybridize to form bonds.
(3) Molecular Orbital (MO) Theory: uses
bonding and antibonding molecular orbitals
Molecular
which best explain the energy characteristics
Geometries
and Bonding
of molecules.
Electron Domains
• We can refer to the
electron pairs as 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.
• This molecule has four electron domains.
Molecular
Geometries
and Bonding
Valence Shell Electron Pair
Repulsion Theory (VSEPR)
“The best arrangement of a given number of
electron domains is the one that minimizes the
repulsions among them.”
Molecular
Geometries
and Bonding
ElectronDomain
Geometries
These are the
electron-domain
geometries for two
through six electron
domains around a
central atom.
{VSEPRTheory
ElectrDomain}
Molecular
Geometries
and Bonding
Electron-Domain Geometries
How would the structure of CH4 and NH3 differ?
• Count the number of electron domains in the Lewis structure.
• The geometry will be that which corresponds to that number of
electron domains.
Molecular
Geometries
and Bonding
Molecular Geometries
• The electron-domain geometry is often not the
shape of the molecule, however.
• The molecular geometry is that defined by
the positions of only the atoms in the
molecules, not the nonbonding pairs.
Molecular
Geometries
and Bonding
Linear Electron Domain
• In this 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 is.
Molecular
Geometries
and Bonding
Trigonal Planar Electron Domain
• There are two molecular geometries:
 Trigonal planar, if all the electron domains are
bonding
 Bent, if one of the domains is a nonbonding pair.
Molecular
Geometries
and Bonding
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.
Molecular
Geometries
and Bonding
Tetrahedral Electron Domain
Molecular
Geometries
and Bonding
• There are three molec
Molecular
Geometries
and Bonding
Nonbonding Pairs and Bond Angle
• Nonbonding pairs are physically
larger than bonding pairs.
• Therefore, their repulsions are
greater; this tends to decrease
bond angles in a molecule.
Molecular
Geometries
and Bonding
Trigonal Bipyramidal Electron
Domain
• There are two
distinct positions in
this geometry:
 Axial
 Equatorial
Where will the nonbonding
electrons be for SF4?
SF4
Molecular
Geometries
and Bonding
Trigonal Bipyramidal Electron
Domain
Lower-energy conformations result from
having nonbonding electron pairs in
equatorial (where there is more space),
rather than axial, positions in this geometry.
Molecular
Geometries
and Bonding
Trigonal Bipyramidal Electron- Domain
Molecular
Geometries
and Bonding
Octahedral Electron Domain
Molecular
Geometries
and Bonding
Electron Domains determined from
Molecular Geometries
What is the electron domain
geometries that led to the
formation of these molecular
structures?
CO2: two double bonds
SO2: trigonal planar
SO3: trigonal planar
(1 double bond)
NF3: tetrahedral
ClF3: trigonal
bipyramidal (5 epairs on Cl)
Molecular
Geometries
and Bonding
Larger Molecules: HC2H3O2
In larger
molecules, it
makes more sense
to talk about the
geometry about a
particular atom
rather than the
geometry of the
molecule as a
whole.
3
1
2
4
Molecular
Geometries
and Bonding
Larger Molecules
This approach makes
sense, especially
because larger
molecules tend to react
at a particular site in the
Molecular
molecule.
Geometries
{OtherLargeMolecules}
and Bonding
Molecular Structure
Molecular
Geometries
and Bonding
Polarity
Just because a molecule
possesses polar bonds
does not mean the
molecule as a whole will be
polar.
Molecular
Geometries
and Bonding
Polarity
By adding the
individual bond
dipoles, one can
determine the
overall dipole
moment for the
molecule.
Molecular
Geometries
and Bonding
Polarity
Molecular
Geometries
and Bonding
Valence Bond Theory:
Orbital (s,p,d,f) Overlap and Bonding
• Covalent bonds are formed by the sharing of electrons from
adjacent atoms whose orbitals overlap.
• Which orbitals are involved in the bonding of the molecules
shown below?
H2
HCl
Cl2
Molecular
Geometries
and Bonding
Atomic Orbitals & Molecular Geometries?
It is hard to imagine that from the
atomic orbitals we know (s, px, py & pz)
Atomic Orbitals
Molecular
Geometries
sp
sp2
p orbitals have Angles that are 900 from each other
……..could arise the molecular
geometries we know to exist: tetrahedral,
trigonal planar, etc.
sp3Molecular
Geometries
and Bonding
Hybrids: Mixing Orbitals
BeF2 = : F¨: Be : F
¨:
¨
sp hybrid
¨
(Be & B, accept less than octet)
• Consider beryllium (Be): In its ground
electronic state, it would not be able to
form bonds because it has no singlyoccupied orbitals.
• But if it absorbs the small amount of
energy needed to promote an electron
from the 2s to the 2p orbital, it can form
two bonds.
• With hybrid orbitals the orbital diagram
for beryllium would look like this.
Molecular
• The sp orbitals are higher in energy
than
Geometries
the 1s orbital but lower than the and
2p.Bonding
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.
Molecular
Geometries
and Bonding
sp hybrid orbitals
for Beryllium
Hybrid Orbitals
• These two degenerate orbitals would align
themselves 180 from each other.
• This is consistent with the observed geometry of
beryllium compounds: linear.
BeF2 = : F¨: Be : F
¨:
¨
:
¨
¨
(Be & B, accept less than octet)
¨
¨
¨
:
Molecular
Geometries
and Bonding
Hybrid Orbitals
Using a similar model for boron (B) leads to…
BF3
Molecular
Geometries
and Bonding
Hybrid Orbitals
…three degenerate sp2 orbitals.
BF3
Molecular
Geometries
and Bonding
Hybrid Orbitals
With carbon we get…
CH4
Molecular
Geometries
and Bonding
Hybrid Orbitals
…four degenerate
sp3 orbitals.
CH4
Molecular
Geometries
and Bonding
Hybrid Orbitals
For geometries involving expanded octets on
the central atom, we must use d orbitals in
our hybrids.
PF5
Molecular
Geometries
and Bonding
Hybrid Orbitals
PF5
This leads to five degenerate
sp3d orbitals…
…or six degenerate sp3d2
orbitals.
SF6
Molecular
Geometries
and Bonding
Hybrid Orbitals
(1)
(2)
Once you know the
electron-domain
geometry, you know
the hybridization
state of the atom.
(3)
(4)
(5)
{HybridOrbitals}
Molecular
Geometries
and Bonding
Valence Bond Theory: Bond Types
• Hybridization is a major player in this
approach to bonding.
• There are two ways orbitals can overlap to
form bonds between atoms.
Molecular
Geometries
and Bonding
Sigma () Bonds
H2
HCl
Cl2
s-s bond
s-px bond
px-px bond
• Sigma bonds are characterized by
 Head-to-head overlap.
 Cylindrical symmetry of electron density about the
internuclear axis.
Molecular
Geometries
and Bonding
Pi () Bonds
• Pi bonds are
characterized by
 Side-to-side overlap.
 Electron density
above and below the
internuclear axis.
Molecular
Geometries
and Bonding
Single Bonds
Single bonds are always  bonds, because 
overlap is greater, resulting in a stronger bond
and more energy lowering.
Molecular
Geometries
and Bonding
Multiple Bonds
In a multiple bond one of the bonds is a  bond
and the rest are  bonds.
Molecular
Geometries
and Bonding
Multiple Bonds
• In a molecule like
formaldehyde (shown
at left) an sp2 orbital
on carbon overlaps in
 fashion with the
corresponding orbital
on the oxygen.
• The unhybridized p
orbitals overlap in 
fashion.
Molecular
Geometries
and Bonding
Multiple Bonds
In triple bonds,
as in acetylene,
two sp orbitals
form a  bond
between the
carbons, and two
pairs of p orbitals
overlap in 
fashion to form
the two  bonds.
H C
C H
Molecular
Geometries
and Bonding
Delocalized Electrons: Resonance
When writing Lewis structures for species like
the nitrate ion, we draw resonance structures to
more accurately reflect the structure of the
molecule or ion.
Molecular
Geometries
and Bonding
Delocalized Electrons: Resonance
• In reality, each of the
four atoms in the
nitrate ion has a p
orbital.
• The p orbitals on all
three oxygens
overlap with the p
orbital on the centralMolecular
Geometries
and Bonding
nitrogen.
Delocalized Electrons: Resonance
This means the  electrons
are not localized between the
nitrogen and one of the
oxygens, but rather are
delocalized throughout the ion.
Molecular
Geometries
and Bonding
Resonance
The organic
molecule
benzene has
six  bonds
and a p orbital
on each
carbon atom.
Molecular
Geometries
and Bonding
Resonance
• In reality the  electrons in benzene are not localized,
but delocalized.
• The even distribution of the  electrons in benzene
makes the molecule unusually stable.
or
Molecular
Geometries
and Bonding
Molecular Orbital (MO) Theory
Considers the wave nature of electrons.
If waves interact destructively, the resulting orbital is higher in
energy: an antibonding molecular orbital.
Molecular
Geometries
and Bonding
If waves interact constructively, the resulting orbital is lower in
energy: a bonding molecular orbital.
MO Theory
In H2 the two electrons go into the bonding molecular orbital.
BOND ORDER:
1
(2 - 0) = 1
2
Molecular
Geometries
and Bonding
The bond order is one half the difference between the number of
bonding and antibonding electrons.
MO Theory
• In the case of He2, the bond order would be
BOND ORDER:
1
(2 - 2) = 0
2
{MO Theory}
• Therefore, He2 does not exist.
Molecular
Geometries
and Bonding
MO Theory
• For atoms with both
s and p orbitals,
there are two types
of interactions:
 The s and the p
orbitals that face
each other overlap in
 fashion.
 The other two sets of
p orbitals overlap in
 fashion.
Molecular
Geometries
and Bonding
MO Theory
• The resulting
MO diagram
looks like
this.
• There are
both  and 
bonding
molecular
orbitals and
* and *
antibonding
molecular
orbitals.
Molecular
Geometries
and Bonding
MO Theory
• The smaller p-block elements in
the second period have a
sizeable interaction between the
s and p orbitals.
• This flips the order of the s and p
molecular orbitals in these
elements.
Molecular
Geometries
and Bonding
Magnetism: Ferro-, Dia- & ParaAll substances are magnetic. The science of physics has
divided substances into 3 magnetic categories, they are:
1.
Ferromagnetic, e.g., Fe, Ni, Permalloy (alloy 20% Fe and
80% Ni)
Exert strong lines of force when exposed to a magnetic field,
for example, the familiar magnetic compass.
2.
Diamagnetic (weakly repelled)
e.g., Cu, Ag, H2O, Liquid N2
3.
Paramagnetic (weakly attracted)
e.g., Air, Liquid Oxygen, Platinum.
Magnetic effects are so
minute, that they need
instrumentation for
detection.
Molecular
Geometries
and Bonding
Molecular
Geometries
and Bonding
Paramagnetism
• Unpaired electrons have their spins aligned
 or 
This increases the magnetic field of the atom.
• Atoms with unpaired electrons are called
paramagnetic .
Paramagnetic atoms are attracted to a magnet.
Molecular
Geometries
and Bonding
Diamagnetism
• Paired electrons have their spins unaligned
 .
Paired electrons have no net magnetic field.
• Atoms with unpaired electrons are called
diamagnetic.
Diamagnetic atoms are repelled by a magnet.
{para Vs dia}
Molecular
Geometries
and Bonding
Second-Row MO Diagrams
Molecular
Geometries
and Bonding