F. J. Grieman

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F. J. Grieman
Molecular Bonding (3) Chemical Bonding
(2) Ionic bonding energy in solids Covalent
Bonding sharing electrons Electron pair bond
concept Bond Characteristics bond order bond
enthalpy (energy)
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Bonding Ionic ? Polar covalent ?
covalent Introduced dipoles
Dipole moment ? (e?)R 1 Debye
3.336E-30Cm

• Ionic Character ?(actual)/?(calc) x 100
• Ionic (NaCl) 9.00/12.0 (100) 75
  ?(Cl2) 0 Ionic (Cl2) 0
• Ionic gt 50 considered ionic compound

Electronegativity indicator of ionic extent -
the greater ?EN, the more ionic Molecule Cl-Cl
HCl NaCl EN 3.0/3.0
2.1/3.0 0.9/3.0 ?EN
0 0.9 2.1 covalent polar ionic Polar
covalent example HCl ? H?Cl ?- ?(actual)
1.11 D r(HCl) 0.1275 nm ?(calc)
(1.60E-19 C)(1.275E-10 m)/(3.336E-30 Cm/D)
6.12 D Ionic (HCl) (1.11/6.12)100 18.2
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Consider Ionic Compounds Again Last time all
gas phase What energies involved in ionic
combination?

Ionization energy (IE)

Electron Affinity (EA)

Coulombic Attraction
Ep (e)(-e)/r(bond
distance)
Consider more complete story of
ionic solids
Elements in standard states ?
solid
Incorrect state crossing
Solid Formation
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Example Na(s) ½F2(g) ? NaF(s) Reaction Pro
cess Energy (kJ/mol)
C O M P A R E M A G N I T U D E S
Na(s) ? Na(g)
Sublimation, ?Esub(Na)
109
Na(g) ? Na(g) e-
Ionization, IE1(Na)
495
Dissociation, ½ ?Ed(F2) (Break Bond)
77
½ F2 (g) ? F(g)
F(g) e- ? F-(g)
-328
Electron Affinity, -EA(F)
Na(g) F-(g) ? NaF(s)
Lattice Energy, LE(NaF)
-923!!!
Na(s) ½ F2 (g) ? NaF(s)
Energy of formation, ?Ef (NaF)
-570!!!
Lattice Energy ? Bond Energy because of ionic
solid structure (-408
kJ/mol NaCl)
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NaF Ionic Solid Structure
Bond length longer But!!! 6 ionic bonds Maximize
attractions Minimizes repulsions
?
Na (0.95 Å)
F- (1.36 Å)
Lattice
Ionic bond
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• Another binary compound MgO
• Mg2 O2-
• Why 2 and not 1??
• Mg ? Mg2 2e- (IE1 IE2) 2180 kJ/mol
• 
  Huge!!!
• O 2e- ? O2- -EA 737 kJ/mol
• very endothermic!!
• Must be lattice energy !
• Mg2(g) O2-(g) ? MgO(s)
• LE(MgO) -3916 kJ/mol
• Why? (Remember LE(NaF) 923 kJ/mol)

Why not Na2 and F2-? (IE1IE2)Na 5055 kJ/mol
really, really huge F2-(EA) cant be
measured really, really endothermic Lattice
energy cannot Compensate Ions tend to form
Noble Gas configurations Non-Ionic compounds?
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• Non-Ionic Compounds???
• Again tend to form Noble gas configurations
• But now Sharing valence shell e-s
• Examples
• F2 F 7 valence e-s, needs 1 e-, but
  cant be transfer, must share!!

Both have Ne configuration
2) O2 6 valence e-s, needs 2
valence electron represented by a dot
Again, both have Ne configuration
Polyatomics
Ne configurations, each F share 2 e-s
O shares 4 e-s
OF2
Now Ne He configurations, H shares 2 e-s, O
shares 4 e-s
OH2
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H
CH4
C has 4 e-s all shared with Hs
H
Noble gas configuration stability - Why? Pauli
allows e- placed in low energy orbital close to
atom nuc. e- attraction What about OF?
If O forms double bond, but sharing with F, it
would have to go into high energy 3s! Z(F) 9
not enough!!!
O
NeO?
NeO obeys octet rule!? But!!! Ne(2p) ? O(2p)
ENe(2p) ltlt EO(2p)
O

• Bond Concept Molecules atoms held together by
  bonds
• bonds are pairs of
  shared e-s that lead to lower Ep
• polyatomics atoms can have more than one bond,
  bond to
• more than one atom
• 2) doubly shared, triply shared multiple bond
  between atoms

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Represent electron pair bond by a line
H
H
O
O
C
H
H
H
H

• Bond Characteristics
• Bond Order of shared pairs between atoms
• multiple bonds have
  higher order
• F ? F O ? O
  N ? N
• 1 2
  3

H H \ / H?C?C?H / \ H
H
H H \ / C?C /
\ H H
H?C ? C?H
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2) Bond Energies how much energy does it take
to break a bond?
dissociation energy (?Ed) Usually measured at
constant pressure, so ?Hd ?Ed RT

(RT usually adds very little) Example
?Hd (kJ/mol)
(Bond Enthalpy) F?F energy ? F F
158 O?O energy ? O O 498
stronger as expected N?N energy ? N
N 945 even stronger as
expected Party analogy couple
demonstration CH4 By symmetry all 4 bonds
should be equal CH4 ? CH3 H ?
CH2 2H ? CH 3H ? C 4H kJ/mol
438 456
428 342 Different
environments cause bond energies to
change But!!! Usually not very much (? 5)
Can use average
over many, many
molecules Average for C?H, ?Hd 416 kJ/mol
Couple selector
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Average over many molecules
Trends B.O. ?, ?Hd ?
Next Bond distances
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Bond Order of shared pairs between atoms
Multiple bonds have higher order
F ? F O ? O N ? N 1 2
3 Cl ? Cl 1 Br ? Br
1 I ? I 1
Bond Length Distance between
atoms in molecule Å 1 x
10-8 cm
Increases as bond order decreases
?
Increases as atom size increases
?