Ionic Bonding
Chapter 8
Ionic Compound Review
• Formed between cation (positive metal
ion) and anion (negative nonmetal ion)
• Formed by elements on opposite sides of
periodic table
Ionic Compound Structure
• Cations and anions in an ionic crystal are
arranged in a systematic, periodic 3-D array that
maximizes the attractive forces among cations
and anions while minimizing the repulsive forces
• Determined by forces of Coulomb’s Law
• Larger charged ions, lead to stronger ionic compounds
• Smaller ions, lead to stronger forces holding the compound
together
Energetics of Ionic Bonding
• it takes 495 kJ/mol
to remove electrons
from sodium.
• We get 349 kJ/mol
back by giving
electrons to
chlorine.
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!
• 495- 349 = 146
(endothermic
Energetics of Ionic Bonding
• There must be a
third piece to the
puzzle.
• What is as yet
unaccounted for is
the electrostatic
attraction between
the newly formed
sodium cation and
chloride anion.
Lattice Energy
• This third piece of the puzzle is the lattice
energy:
The energy required to completely separate a mole of
a solid ionic compound into its gaseous ions.
• The energy associated with electrostatic
interactions is governed by Coulomb’s law:
Q 1Q 2
Eel = 
d
Lattice Energy
• Lattice energy, then,
increases with the charge
on the ions.
– Greater force between the
ions (Coulomb’s Law)
• It also increases with
decreasing size of ions.
– Greater forces between
ions when their distance
decreases (Coulomb’s
Law)
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.
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.
Ionic Compound Properties
• High melting and Boiling points
– High temperatures (amounts of energy) are required to
overcome the attraction between positive and negative ions
• Hard
– Crystals are hard because the positive and negative ions are strongly attracted to
each other and difficult to separate
• Brittle, lack ductility and malleability
– When pressure is applied the electrostatic repulsion can be
enough to split the crystal
• Soluble in water
– When ionic compounds dissolve in water, the water can
dissociate the ions making the lattice structure fall apart and
overcome the positive and negative attractions holding the
compound together
Ionic Compound Properties
• Low volatility (does not want to become a vapor)
– High temperatures (amounts of energy) are required to
overcome the attraction between positive and negative ions
• Lack conductivity as solid (only conductive in aqueous
solution)
– Ions are bound tightly to each other impeding electrons to flow and conduct heat
and electricity
• The stronger the coulombic forces between the ions in
an ionic compound the more these properties will be
expressed