AP CHEMISTRY NOTES
p. 1
IONIC BONDS
 As mentioned, this occurs between a metal and a nonmetal. Metals, having low ionization energy, can easily lose
electrons. Nonmetals, having high electronegativities, can easily gain electrons. Therefore, these atoms can
become ions and then attract each other.
 The amount of interaction between two ions can be determined by Coulomb’s law:
Q Q 
E = 2.31 x 10−19 J•nm  1 2 
 r 
Where E is measured in J, Q1 and Q2 are the numerical ionic charges of the ions, and r is the between the ion
centers in nanometers.
 Example: In NaCl, the distance between the ion centers is 2.76 Å (0.276 nm). Find the ionic energy per ion pair
for NaCl.
 (1)(1) 
E = 2.31 x 10−19 J•nm 
 = −8.37 x 10−19 J
 0.276 nm 
Since the losing of energy results in a more stable situation, E will be a negative quantity when the two ions
are oppositely charged and there is an attraction between the ions. E will be a positive quantity whenever
the ions are of the same type of charge—this indicates that there will be repulsion forces between the two
ions and energy is needed to make them come together.
 Since E is negative here, that indicates the ion pair of NaCl has less energy than the separated Na + and Cl−
ions.
 Ionic compounds form lattices where the ions pack together in such a way to minimize the positive-positive and
negative-negative repulsions and maximize the positive-negative attractions. A picture of LiF is on p. 364.

 Ionic size also plays a large part in determining lattice structure and stability. How the ions pack will be largely
influenced by the size of the ions when regarding the forces mentioned in the previous statement.
 An important aspect of ionic bonding is the lattice energy:
 This is the change in energy that takes place when separated gaseous ions are packed together to form an ionic
solid.
 This is always extremely exothermic.
 Energy changes involved in the formation of ionic compounds.
 There are several steps involved in the formation of ionic compounds that correspond to the energy involved in
the process. Since a lattice forms when two gaseous ions come together to form an ionic solid, the following
steps will occur for both reactants:
 both must become individual atoms
 both must become ions
 both must become gases
 To do these things, the following will occur:
 Sublimation or vaporization of the atoms. The atoms must form a gas. This step is endothermic.
 Dissociation of molecules into atoms. Many nonmetals are diatomic (or polyatomic) and some metals (such
as Hg) can also be diatomic. These must break down into individual atoms in order to react. This step is
endothermic.
 Ionization of metal atom to form an ion. Metals form cations by losing electrons, so this involves the
ionization energy needed to do so. This includes all the successive ionizations needed for the stable ion of
the metal to form. This step is endothermic for all ionizations.
 Formation of nonmetallic ion. Since nonmetals form anions, they must gain electrons. This involves the
electron affinity of the atom. This step will be exothermic for the first electron gained by the atom. Any
additional electrons will cause the electron affinity to be endothermic.
 Formation of solid from the gaseous ions. This corresponds to the lattice energy. This step is extremely
exothermic and will cause the formation of any ionic bond to be exothermic overall.
 See the page on “Energy Changes Involved in the Formation of LiF(s)” for an example of this.
AP CHEMISTRY NOTES
p. 2
 The amount of lattice energy needed to form ion pairs corresponds to:
lattice energy ∝
Q 1 Q2
r
where ∝ means proportional to.
 Comparisons can be made between different ion pairs in regards to the amount of lattice energy involved. You
can do this by checking the Q1Q2:r ratio of the different pairs.
 Check the Q’s first. If they are different, the pair with the higher charges will have more lattice energy.
 If the charges are the same between the two pairs, then check “r”. The pair with the smaller r-value will
have the higher lattice energy. This means the smaller ions will lead to more lattice energy.
 Example: In the following pairs, which will have more lattice energy?
NaCl and MgO → MgO has about 4 times as much lattice energy since NaCl is (+1)(-1) and MgO is (+2)(-2)
NaCl and KCl → NaCl has more lattice energy since Na is smaller than K and r will be smaller for NaCl