Lesson 8.2 Ionic Bonds
Suggested Reading- Zumdahl Chapter 8 Section 8.1, 8.4, 8.5
- 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.
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.
- An electron is transferred to give ions.
- The ions attract one another forming a bond.
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 Eis sometimes denoted by F orFeto 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 energyis 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 ionare 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.