Formulas of Ionic Compounds

When a metal such as sodium (Na) or calcium (Ca) reacts with a nonmetal such as chlorine (Cl2), the product is ordinarily an ionic compound. The formula of that compound (e.g., NaCl, CaCl2) shows the simplest ratio between cation and anion (one Na+ ion for one Cl ion one Ca2+ ion for two Cl- ions). In that sense, the formulas of ionic compounds are simplest formulas. Notice that the symbol of the metal (Na, Ca) always appears first in the formula, followed by that of the nonmetal. 

Predict formulas of ionic compounds from charges of ions. 

The formulas of ionic compounds have a different meaning from those of molecular compounds. Each crystal of sodium chloride has a different total number of cations and anions. We cannot simply specify the numbers of ions present as the formula of this ionic compound, because each crystal would have a different formula and the subscripts would be enormous numbers. However, the ratio of the number of cations to the number of anions is the same in all the crystals, and the chemical formula shows this ratio. In sodium chloride, there is one Na+ ion for each Cl ion so its formula is NaCl. Sodium chloride is an example of a binary ionic compound, a compound formed from the ions of two elements. Another binary compound, CaCl2, is formed from Ca2+ and Cl- ions in the ratio 1 2, which is required for electrical neutrality. 

C02-0072. Nitrogen forms a few compounds in which it exists as an anion with - 3 charge. What are the chemical formulas of ionic compounds that form between nitrogen and (a) sodium (b) magnesium ... 

Here are some specific examples illustrating chemical formulas of ionic compounds. 

We learned to write formulas of ionic compounds in Chaps. 5 and 6. We balanced the charges to determine the number of each ion to use in the formula. We could not do the same thing for atoms of elements in covalent compounds, because in these compounds the atoms do not have charges. In order to overcome this difficulty, we define oxidation numbers, also called oxidation states. 

In writing the formulas of ionic compounds, make sure the subscripts are in the lowest ratio of whole numbers. 

Formulas of Ionic Compounds As we mentioned earlier, compounds (like atoms) must be electrically neutral. The net charge on an ionic compound must be zero. When a metal gives its electrons away to form a cation, there has to be some other species, often a nonmetal, present to accept the electrons. Thus, whenever a cation is forming, there is also a concurrent formation of an anion. Chemical systems have to remain electrically neutral. 

In formulas of ionic compounds, simple ions are represented by the element symbol Cl means Cl-, Na means Na+, and so on. 

SECTION THREE Names and Formulas of Ionic Compounds... 

The composition of an ionic compound is determined by overall charge neutrality. The total positive charge on the cations must exactly balance the total negative charge on the anions. The following names and formulas of ionic compounds illustrate this point ... 

Seawater contains many dissolved substances, mostly dissolved sodium chloride. In Chapter 4, you learned that sodium chloride is an ionic compound. Another ionic compound found dissolved in seawater is magnesium chloride. Some common ionic compounds used in everyday life are potassium chloride, a salt substitute used by people avoiding sodium for health reasons potassium iodide, added to table salt to prevent iodine deficiency and sodium fluoride, added to many toothpastes to strengthen tooth enamel. You will learn how to use the language of chemistry to name and write the formulas of ionic compounds. 

There is a tutorial on the textbook s Web site that provides practice converting between names and formulas of ionic compounds. 

Another way of figuring out the formulas of ionic compounds involves using the size of the charges on the two ions to get the subscript numbers for the formula. Consider the compound formed between Fe3+ and O2 ... 

Formulas of Ionic Compounds with Polyatomic Ions... 

Ionic compounds are composed of ions of opposite charge packed together in a highly organized way to form crystals. Molecules do not exist in ionic compounds. The formulas of ionic compounds give the simplest ratio of the ions in the crystal. Calcium chloride, CaCl2, is an ionic compound and its formula shows a 1 2 ratio of Ca2+ to Cl" ions. 

If you know the formula of a compound, you can calculate its percent composition. Just the reverse can be done too. If you know the percent composition of a compound, you can calculate a formula for the compound. A formula calculated from percent composition data is called an empirical formula (one calculated from experimental data). The formulas of ionic compounds are always empirical formulas. The formulas of molecular compounds may be the same as their empirical formulas or they may be some whole-number multiple of it. You will learn how to do composition-from-fbrmula and fbrmula-from-composition calculations in this chapter. 

The formulas of ionic compounds are always the same as their empirical formulas because ionic compounds do not consist of discrete molecular units. For example, a solid sample of sodium chloride (NaCl) consists of equal numbers of Na and Cl ions arranged in a three-dimensional network (Figure 2.11). In such a compound there is a 1 1 ratio of cations to anions so that the compound is electrically neutral. As you can see in Figure 2.11, no Na" ion in NaCl is associated with just one particular d ion. In fact, each Na ion is equally held by six surrounding Cl ions and vice versa. Thus NaCl is the empirical formula for sodium chloride. In other ionic compounds the actual structure may be different, but the arrangement of cations and anions is such that the compounds are all electrically neutral. Note that the charges on the cation and anion are not shown in the formula for an ionic compound. 

In order for ionic compounds to be electrically neutral, the sum of the charges on the cation and anion in each formula unit must be zero. If the charges on the cation and anion are numerically different, we apply the following rule to make the formula electrically neutral The subscript of the cation is numerically equal to the charge on the anion, and the subscript of the anion is numerically equal to the charge on the cation. If the charges are numerically equal, then no subscripts are necessary. This rule follows from the fact that because the formulas of ionic compounds are empirical formulas, the subscripts must always be reduced to the smallest ratios. Let us consider some examples. 

We normally write the empirical formulas of ionic compounds without showing the charges. The + and — are shown here to emphasize the transfer of electrons. 

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