Ionic Bonds and Ionic Compounds

We have seen that elements may lose or gain electrons to become isoelectronic with the nearest noble gas. Where do the electrons go when an element loses them to become a cation? Where do electrons come from when an element gains them to become an anion? For an atom to gain or lose electrons, there must be an interaction between two different chemical species. If electrons are fully exchanged, then we consider this interaction to be ionic. The resulting attractions between the positively charged cations and the negatively charged anions are referred to as ionic bonds.

•Ionic Bonds

•As we saw earlier, the electrons in the outermost (valence) shell of an atom are largely responsible for the ways in which that atom will interact with other elements. For example, this image shows the electron configurations of sodium (11 e−, 1 valence e−), neon (10 e−, 8 valence e−), and fluorine (9 e−, 7 valence e−).

Our model of ionic bonding and chemical reactivity states that sodium and fluorine have a strong driving force to become isoelectronic with the nearest noble gas, neon. Because sodium needs to lose one electron and fluorine needs to gain one for this to occur, one atom of sodium can give up its valence electron to a fluorine atom, resulting in two ions with noble gas configurations matching that of neon. The positive and negative ion are held tightly together by electrostatic forces, which are strong forces between oppositely charged particles. When large groups of sodium and fluorine atoms react in this way, the result is the ionic compound, sodium fluoride.

Crystal Lattices

Any ionic compound is composed of extremely large numbers of cations and anions. Each cation is attracted by all of the anions but repelled by all the other cations, and vice versa. In order to maximize the number of attractive interactions while minimizing the repulsive ones, the ions form a three-dimensional structure known as a crystal lattice. There are a variety of lattice forms that ionic compounds can exhibit, but all of them involve a regular, repeating pattern in which cations and anions are held rigidly in place by various neighboring ions. For example, sodium fluoride takes the form of a cubic lattice, shown here:

Some properties of the crystal form that are exhibited at the atomic level can also be seen at the macroscopic level. Due to the cubic arrangement of ions in sodium fluoride, a single pure crystal of this compound will tend to have smooth faces at right angles to one another.

Lattice Energy

There are a number of different ways to measure the strength of a given crystal lattice. One way would be to measure the amount of energy needed to completely pull apart an ionic substance into isolated ions. This value, known as the lattice energy, cannot be measured directly, but it can be calculated based on measured energy changes for other more feasible processes.

: Lattice Energies for Some Ionic Compounds

Properties of Ionic Compounds

Ionic compounds exhibit certain properties, some of which are listed below:

•All ionic compounds form crystals.•Ionic compounds tend to have high melting points and boiling points.•Ionic compounds are very hard and very brittle.•Ionic compounds conduct electricity when dissolved in water.The last property above requires some additional explanation.

We are all familiar with the process of dissolution on a large scale. If you stir a spoonful of salt into a glass of water, the salt crystals are broken down and seem to disappear into the water. On the atomic level, the dissolution of an ionic compound occurs when water interacts with the ions in the crystal lattice, causing the lattice to break apart.

Once the ions are dissolved, the presence of charged particles distributed throughout the liquid allows the solution to conduct electricity. The more ions that are freed from the lattice, the more conductive the solution will be.

Below is a summary of some common ionic compounds and their practical applications:

•Ionic bonds are electrostatic attractions between two oppositely charged ions. Ions can be formed and then bonded when metal atoms donate their valence electrons to nonmetal atoms.

•The ions in ionic compounds are arranged in rigid three-dimensional patterns called crystal lattices. The crystal lattice that is formed is a characteristic property of a given compound.

•We can indirectly measure the energy necessary to break apart a given lattice into its isolated ions. We call this value the lattice energy, and it gives us one way to measure the strength of the ionic bonds in that compound.

•Ionic compounds have the following properties: (1) they form crystals; (2) they have high melting/boiling points; (3) they are hard and brittle; (4) they can conduct electricity when dissolved in water.

•Dissolution is a process in which water interacts with the ions in a crystal lattice, causing the lattice to break apart.

Lesson Summary