Intramolecular Bonds
Ionic
• Electrostatic attraction between ions
Polar Covalent
• Unequal sharing of electrons
NonPolar Covalent
• Equal sharing of electrons
Chemical
Bonding
© 2009, Prentice-Hall, Inc.
Ionic Bonding
It takes 495 kJ/mol to
remove electrons
from sodium.
(ionization energy)
We get 349 kJ/mol
back by giving
electrons to
chlorine.
(electron affinity)
Chemical
Bonding
© 2009, Prentice-Hall, Inc.
Energetics of Ionic Bonding
But these numbers
don’t explain why
the reaction of
sodium metal and
chlorine gas to form
sodium chloride is
so exothermic!
Chemical
Bonding
© 2009, Prentice-Hall, Inc.
Energetics of Ionic Bonding
• There must be a
third piece to the
puzzle.
• What is unaccounted
for so far is the
electrostatic
attraction between
the newly-formed
sodium cation and
chloride anion.
Chemical
Bonding
© 2009, Prentice-Hall, Inc.
Lattice Energy
• This third piece of the puzzle is the lattice
energy:
The change in energy that takes place when
separated gaseous ions are packed together to
form an ionic solid.
• The energy associated with electrostatic
interactions is governed by Coulomb’s law:
Q 1Q 2
Eel = 
d
Chemical
Bonding
© 2009, Prentice-Hall, Inc.
Lattice Energy
• Lattice energy, then, increases with the charge
on the ions.
• It also increases with decreasing size of ions.
Chemical
Bonding
© 2009, Prentice-Hall, Inc.
Predicting charges
• Position on periodic table can be used to predict charges on
ions.
Representative elements tend to gain/lose enough electrons to
attain noble gas configurations.
Transition metals rarely do. Transition metals lose outermost s
electron before outermost d electrons.
Mg: [Ne]3s2
Mg+: [Ne]3s1 not stable
Mg2+: [Ne] stable
Cl: [Ne]3s23p5
Cl-: [Ne]3s23p6 = [Ar]
stable
Chemical
Bonding
© 2009, Prentice-Hall, Inc.
Energetics of Ionic Bonding
Li(s) + ½ F2(g)  LiF(s)
1.Li(s)  Li(g)
sublimation
2.Li(g)  Li+(g) + e- I.E.
+161kJ
+520kJ
3.½ F2(g)  F(g)
Dissociation
+77kJ
4.F(g) + e-  F-(g)
E.Affinity
-328kJ
5.Li+(g) + F-(g)  LiF(s)
Lattice Energy -1047kJ
-617 kJ
Chemical
Bonding
© 2009, Prentice-Hall, Inc.
Energetics of Ionic Bonding
By accounting for all
three energies
(ionization energy,
electron affinity, and
lattice energy), we
can get a good idea
of the energetics
involved in such a
process.
Chemical
Bonding
© 2009, Prentice-Hall, Inc.
Energetics of Ionic Bonding
• These phenomena
also helps explain the
“octet rule.”
• Metals, for instance, tend to stop losing electrons
once they attain a noble gas configuration
because energy would be expended that cannot
be overcome by lattice energies.
Chemical
Bonding
© 2009, Prentice-Hall, Inc.
Practice with Lattice Energy
1. Arrange the following ionic compounds
in order of increasing lattice energy:
NaF, CsI, and CaO.
2. Which substance would you expect to
have the greatest lattice energy, MgF2,
CaF2, or ZrO2?
3. Predict the ion generally formed by (a)
Sr, (b) S, (c) Al
Chemical
Bonding
© 2009, Prentice-Hall, Inc.