6.2: Ionic Compounds

Properties of Ionic Compounds

In an ionic compound, one or more electrons are transferred from metals to nonmetals,

which form positive and negative ions. The attraction between these ions is called an ionic

bond.

The physical and chemical properties of an ionic compound such as NaCl are very

different from those of the original elements. For example, the original elements of NaCl

were sodium, a soft, shiny metal, and chlorine, a yellow-green poisonous gas. However, when they react and form positive and negative ions, they produce NaCl, which is ordinary table

salt, a hard, white, crystalline substance that is important in our diet.

In a crystal of NaCl, the larger Cl^- ions are arranged in a three-dimensional structure

in which the smaller Na^+ ions occupy the spaces between the Cl^- ions. In

this crystal, every Na^+ ion is surrounded by six Cl^- ions, and every Cl^- ion is surrounded by

six Na^+ ions. Thus, there are many strong attractions between the positive and negative ions,

which account for the high melting points of ionic compounds. For example, the melting

point of NaCl is 801 °C. At room temperature, ionic compounds are solids.

Chemical Formulas of Ionic Compounds

The chemical formula of a compound represents the symbols and subscripts in the lowest

whole-number ratio of the atoms or ions. In the formula of an ionic compound, the sum of

the ionic charges in the formula is always zero. Thus, the total amount of positive charge is

equal to the total amount of negative charge. For example, to achieve a stable electron configuration, one Na atom (metal) loses its one valence electron to form Na^+, and one Cl atom

(nonmetal) gains one electron to form a Cl^- ion. The formula NaCl indicates that the compound has charge balance because there is one sodium ion, Na^+, for every chloride ion, Cl^-. The ionic charges are not shown in the formula of the compound.

Subscripts in Formulas

Consider a compound of magnesium and chlorine. To achieve a stable electron configuration, one Mg atom (metal) loses its two valence electrons to form Mg^+2. Two Cl atoms

(nonmetals) each gain one electron to form two Cl^- ions. The two Cl^- ions are needed to

balance the positive charge of Mg^+2. This gives the formula MgCl2, magnesium chloride,

in which the subscript 2 shows that two Cl^- ions are needed for charge balance.

Writing Ionic Formulas from Ionic Charges

The subscripts in the formula of an ionic compound represent the number of positive and

negative ions that give an overall charge of zero. Thus, we can now write a formula directly

from the ionic charges of the positive and negative ions. Suppose we wish to write the

formula for the ionic compound containing Na^+ and S^-2 ions. To balance the ionic charge

of the S^-2 ion, we will need to place two Na^+ ions in the formula. This gives the formula

Na2S, which has an overall charge of zero. In the formula of an ionic compound, the cation

is written first, followed by the anion. Appropriate subscripts are used to show the number

of each of the ions. This formula is the lowest ratio of ions in the ionic compound. This

lowest ratio of ions is called a formula unit.