F' J' Grieman

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

• Ionic Character
• Ionic (NaCl) 9.00/12.0 (100) 75 (Cl2)
  0 Ionic (Cl2) 0
• considered ionic compound

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
example (gas phase!!!)
H?Cl ?(actual) 1.11 D r(HCl)
0.1275 nm ?(calc) ( )(
)/( ) 6.12 D
Ionic (HCl) (1.11/6.12)100
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Consider Ionic Compounds Again Last time all
gas phase What energies involved in ionic
combination?




Coulombic Attraction
Ep
Consider more
complete story of ionic solids

Incorrect state crossing
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Example Na(s) ½F2(g) ? NaF(g) 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)
109
Na(g) ? Na(g) e-
495
77
½ F2 (g) ? F(g)
F(g) e- ? F-(g)
-328
Na(g) F-(g) ? NaF(s)
-923!!!
Na(s) ½ F2 (g) ? NaF(s)
-570!!!
Lattice Energy ? Bond Energy because of ionic
solid structure
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NaF Ionic Solid Structure
Bond length longer But!!!

?
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• Another binary compound MgO
• Mg2 O2-
• Why 2 and not 1??
• 2180
  kJ/mol
• 
  Huge!!!
• 737 kJ/mol
• very endothermic!!
• Must be
• -3916 kJ/mol
• Why? (Remember LE(NF) 923 kJ/mol)

Why not (IE1IE2)Na 5055
kJ/mol F2-(EA) cant be measured
Lattice energy cannot compensate Ions tend to
form Non-Ionic compounds?
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• Non-Ionic Compounds???
• Again tend to form
• But now
• Examples
• F2 F 7 valence e-s, needs 1 e-, but
  cant be transfer, must share!!

Both have
2) O2 6 valence e-s, needs 2
Again, both have
Polyatomics
, each F share 2 e-s
O shares 4 e-s
OF2
Now configurations, H shares 2 e-s,
O shares 4 e-s
OH2
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CH4
C has all shared with Hs
Noble gas configuration stability - Why? What
about OF?
O trys to form double bond, but sharing with F,
Z(F) 9 not enough!!!
O
NeO?
NeO obeys octet rule!? But!!!
O

• Bond Concept Molecules atoms held together by
  
• bonds are
• polyatomics
• 2) doubly shared, triply shared
  between atoms

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Represent electron pair bond by a line
H
H

• Bond Characteristics
• multiple bonds have
  higher order
• F F O O
  N N
• 
  

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
(?Ed) Usually
measured at constant pressure, so

(RT usually adds very little) Example
?Hd (kJ/mol)
(Bond Enthalpy) energy ? F F
158 energy ? O O
498 energy ? N N
945 CH4 By symmetry
all 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)
Average for C?H, D 416 kJ/mol
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Average over many molecules
Trends