Lesson 8.2 Ionic Bonds
Suggested Reading
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Zumdahl Chapter 8 Section 8.1, 8.4, 8.5
Essential Question

What are the basic characteristics of ionic bonds?
Learning Objective



List and define the three types of bonding.
Explain why chemical bonds occur.
Define and calculate lattice energy.
Describing Ionic Bonding
Recall that an ionic bond is a chemical bond formed by the electrostatic
attraction between positive and negative ions. The bond forms between two
atoms when one or more of the electrons are transferred from the valence
shell of one atom to the valence shell of the other. The atom that that loses
electrons becomes a cation (positive ion), and the atom that gains electrons
becomes an anion (negative ion). Ions tend to attract as many neighboring
ions of opposite charge as possible. When large numbers of ions gather
together, they form an ionic solid. The solid normally has a regular, crystalline
structure that allows for the maximum attraction of ions, given their particular
sizes.
Why Do Ionic Bonds Occur?
The electron transfer that occurs during the formation of an ionic bond results
in the formation of ions, each of which has a noble-gas configuration. Thus,
atoms will form ionic bonds in order to achieve a more stable electron
configuration.
Note that after the ionic bond is formed, Na+ has the noble gas electron
configuration of [Ne] while Cl- has the configuration of [Ar]. Recall that such
noble-gas configurations are particularly stable. This stability of the ions
accounts in part for the formation of ionic solids such as NaCl.
The formation of the ionic bond between Na and Cl can be represented using
Lewis symbols as follows. When writing Lewis symbols to illustrate the
formation of an ionic bond, an arrow is usually used to show the direction of
transfer.
Energy Involved in Ionic Bonding
Looking at the energy changes associated with the formation of ionic bonds
can help you to understand why certain atoms bond ionically and others do
not. If atoms bond, there should be a net decrease in energy, because the
bonded state should be more stable and therefore at a lower energy level.
Consider again the formation of NaCl. You can think of this as occurring in two
steps.
1.
2.
An electron is transferred to give ions.
The ions attract one another forming a bond.
In reality, these events occur simultaneously. However, the net quantity of
energy involved is the same whether the steps occur one after the other or
simultaneously (Hess's law).
In the first step Na loses a 3s electron. Removing this electron requires
energy equal to the first ionization energy of the Na atom, which is 496 kJ/mol.
Adding the electron to the chlorine atom releases energy equal to the electron
affinity of the chlorine atom, which is -349 kJ/mol. The formation of the ions is
not by itself energetically favorable since it requires more energy to remove an
electron from the Na atom than is released when the electron is added to the
chlorine atom.
(496 - 349)kJ/mol = + 147 kJ/mol, endothermic
Additional energy of at least 147 kJ/mol is required to form ions. However,
more than enough energy is released to meet this need when the ions attract
one another in step 2. Thus, the energy released during bond formation
comes from the attraction of the oppositely charged ions. You can estimate
this energy using Coulomb's law, which states the energy obtained when ions
attract to form a bond is as follows
where E has units of joules, r is the distance between nuclei in nm (10-9), and
Q1 and Q2 are the charges on the ions. Note that Coulomb's law takes other
forms and that the energy E is sometimes denoted by F or Fe to denote
electrostatic force. For the formation of NaCl this energy is
The negative sign indicates that energy is released and that an attractive force
exists. This mean that the bonded ions are lower in energy than the separated
ions. Coulomb's law can also be used to calculate the repulsive energy
associated with bringing two like ions together. In this case the energy will be
positive.
Just how strongly the ions attract each other in the solid state is indicated by
the lattice energy of the solid. The lattice energy is the change in energy that
takes place when ionic solids are separated into isolated gaseous ions.
Zumdahl defines the lattice energy as the change in energy that takes place
when separated gaseous ion are packed together to form an ionic solid.
Both definitions are valid although in my experience the former is more
commonly used.
Lattice Energies from the Born-Haber Cycle
Direct experimental determination of the lattice energy of an ionic solid is
difficult, so this quantity is usually calculated using Hess's law,
thermochemical data, and a thermochemical cycle called the Born-Haber
cycle developed by Max Born and Fritz Haber in 1919. In this method, we
break the formation of an ionic solid down into five steps. We then sum the
energy change associated with each step using Hess's law in order to
calculate the lattice energy for the ionic compound as shown below.
Again lets consider the formation of NaCl.
Step 1: Sublimation of Na: Sublimation is the transformation of a solid to a
gas. Thus, in the first step Na(s) is vaporized to a Na(g). The enthalpy change
is 107.3 kJ/mol.
Step 2: Dissociation of Cl: Cl molecules are dissociated to Cl atoms. The
enthalpy change for this step equals the bond dissociation energy, which is
122 kJ/mole.
Step 3: Ionization of Na: Sodium ions are ionized to Na+. This is equal to the
first ionization energy of Na, which is 495.8 kJ/mol.
Step 4: Formation of Cl-: The electrons from the ionization of Na are
transferred to Cl. The enthalpy change for this is the electron affinity for Cl,
which equals -349 kJ/mol.
Step 5: Formation of NaCl(s): The ions formed in steps 3 & 4 combine. For
NaCl(s) this value is -787 kJ/mol.
If we write these five step and add them along with their corresponding
enthalpy values using Hess's law we obtain:
Note that what we call lattice energy is technically the enthalpy of formation
for the ionic solid. However, this value is called lattice energy by convention
since it refers to the energy change associated with the formation of a crystal
lattice.. The more energy released during the formation of the ionic bond, the
more stable the ionic solid, and the more likely it will form.