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Molecular Structure and Bonding
Dr
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A pair of ions does not stay alone but form crystals

Covalent Bond
Two non-metals share
(valence) electrons:

(Remark: Transition metals can form covalent bonds also !)

Polar Covalent Bond
Two non-metals share electrons unevenly because
of electronegativity difference
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This results on partially negative and positive charges on the atoms

Metallic Bond
Metal atoms share all
their valence electrons,
which freely move
between all atoms which
form a network
...
Pauling – he
estimated EN from bond energies

Allred-Rochow EN

Example:
Flourine (r = 72 pm)
Carbon (r = 77 pm)
Calculate the AR
electronegativity

Shielding of 2p electrons:
Flourine:
S = 6 * 0
...
85 = 3
...
8 = 5
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2/(72 2) + 0
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35
Carbon:
S = 3 * 0
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85 = 2
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75 = 3
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25 / (77 2) ) + 0
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71

Calculate Dipole moments

Polar Molecules
Electrons are not equally shared in a bond,
which can lead to a dipole moment of the whole molecule

Polar Bonds and Geometry

Which bond type ?

(exception:
Transition
metals !)

Electron counting

Formal Charge
Split all bonds in the middle
=> “real” charge on atoms
(2) Octet Rule
Count all bonding electrons for
one atom
=> 8 is most stable
(3) Oxidation Number
Give all bonding electrons to
the more electronegative atom

Special Cases

6 VE:
Especially
common for
B and Al !

“Extended
octet”

Especially P and
S can use dorbitals to make
more than 3
resp
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2
...

4
...

6
...

8
...

10
...

• Hybridization is determined by the electron domain
geometry
...
2

The sp hybrid orbitals in gaseous BeCl2
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2

The sp hybrid orbitals in gaseous BeCl2(continued)
...
3

The sp2 hybrid orbitals in BF3
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4

The sp3 hybrid orbitals in CH4
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5

The sp3 hybrid orbitals in NH3
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5 continued

The sp3 hybrid orbitals in H2O
...
6

The sp3d hybrid orbitals in PCl5
...
7

The sp3d2 hybrid orbitals in SF6
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Use the Lewis structures to ascertain the arrangement of groups and
shape at each central atom
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sp2

SOLUTION:

sp2

sp3 hybridized
O
sp2

O

sp3

hybridized H
H

C
H

C

H
C

sp3

H

H
sp2 hybridized

H sp

2
sp2 C sp

C
3

sp H

2

sp3
H sp3

O

H
C

sp3
sp

bonds

sp3
sp3
H
3H

C
H3C

CH3

bond

Tasks
• Draw the Lewis Structures and the Hybrid
Orbitals for Ethane, Ethene and Ethyne
(mark the hybrid orbitals)
• Which hybridization has the central atom in:
H2O, O2, NH3, NH4+, N in pyridine, O in THF,
S in SOCl2, C in HCHO compared to CO

Chemical Reactivity
From the hybrid orbitals we can estimate if a
molecule acts as Lewis acid or base
(if there is an electrophilic or nucleophilic center)

Consider the “empty” pz orbital of C in HCHO vs
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position so have
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Atomic wave functions are summed to obtain molecular wave functions
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If wave functions cancel each other, an antibonding MO is formed (a node of
zero electron density occurs between the nuclei)
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14

An analogy between light waves and atomic wave functions
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Molecular Orbitals
• Molecular orbitals:
• each contain a maximum of two electrons
• have definite energies
• can be visualized with contour diagrams
• are distributed over the whole molecule
(not only in between 2 atoms)
• When two AOs overlap, two MOs form
...
15

The MO diagram for H2
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Energy

*1s

1s

1s

H2 bond order
= 1/2(2-0) = 1

1s

AO
of H

Prentice Hall © 2003

MO
of H2

Chapter 9

AO
of H

Electron Configurations and Molecular
Properties
• Two types of magnetic behavior:
• paramagnetism (unpaired electrons in molecule):
strong attraction between magnetic field and
molecule;
• diamagnetism (no unpaired electrons in molecule):
weak repulsion between magnetic field and
molecule
...
chem
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edu
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shtml

Example CO
LUMO

HOMO

“lone
pair” on C

Chemical Reactivity
Important are the HOMO and LUMO (“frontier orbitals”)

http://www
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com/webbook/12_lab/lab
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anti-bonding)

BH3 molecule

Combination of 3 H
orbitals to
3 group orbitals

Compare HOMO/LUMO to BH3 !
=> what is an acid / base ?

Homework (3)
http://firstyear
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usyd
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au/calculators/mo_diagrams

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