2.7 Ions and Ionic Compounds


The nucleus of an atom is unchanged by ordinary chemical processes, but atoms can readily gain or lose electrons. If electrons are removed from or added to a neutral atom, a charged particle called an ion is formed. An ion with a positive charge is called a cation (pronounced CAT-ion); a negatively charged ion is called an anion (AN-ion). The sodium atom, for example, which has 11 protons and 11 electrons, easily loses one electron. The resulting cation has 11 protons and 10 electrons, so it has a net charge of The net charge on an ion is represented by a superscript; and mean a net charge resulting from the loss of one, two, or three electrons, respectively. The superscripts and represent net charges resulting from the gain of one, two, or three electrons, respectively. The formation of the ion from an Na atom is shown schematically as follows:


Chlorine, with 17 protons and 17 electrons, often gains an electron in chemical reactions, producing the ion.


In general, metal atoms tend to lose electrons to form cations, whereas nonmetal atoms tend to gain electrons to form anions.


SAMPLE EXERCISE 2.7

Give the chemical symbols, including mass numbers, for the following ions: (a) The ion with 22 protons, 26 neutrons, and 19 electrons; (b) the ion of sulfur that has 16 neutrons and 18 electrons.

Solution

(a) The number of protons (22) is the atomic number of the element, so the element is Ti (titanium). The mass number of this isotope is (the sum of the protons and neutrons). Because the ion has three more protons than electrons, it has a net charge of Thus, the symbol for the ion is

(b) By referring to a periodic table or table of elements, we see that sulfur (symbol S) has an atomic number of 16. Thus, each atom or ion of sulfur has 16 protons. We are told that the ion also has 16 neutrons, so the mass number of the ion is Because the ion has 16 protons and 18 electrons, its net charge is Thus, the complete symbol for the ion is

In general, we will focus on the net charges of ions and ignore their mass numbers unless the circumstances dictate that we specify a certain isotope.

PRACTICE EXERCISE

How many protons and electrons does the ion possess?

Answer: 34 protons and 36 electrons



In addition to simple ions such as Na+ and Cl-, there are polyatomic ions such as NO3- (nitrate ion) and SO42- (sulfate ion). These ions consist of atoms joined as in a molecule, but they have a net positive or negative charge. We will consider further examples of polyatomic ions in Section 2.8.

The chemical properties of ions are very different from those of the atoms from which they are derived. The difference is like the change from Dr. Jekyll to Mr. Hyde: Although the body may be essentially the same (plus or minus a few electrons), the behavior is much different.

Predicting Ionic Charges

Many atoms gain or lose electrons so as to end up with the same number of electrons as the noble gas closest to them in the periodic table. The members of the noble-gas family are chemically very nonreactive and form very few compounds. We might deduce that this is because their electron arrangements are very stable. Nearby elements can obtain these same stable arrangements by losing or gaining electrons. For example, loss of one electron from an atom of sodium leaves it with the same number of electrons as the neutral neon atom (atomic number 10). Similarly, when chlorine gains an electron, it ends up with 18, the same as argon (atomic number 18). We will use this simple observation to explain the formation of ions in Chapter 8, where we discuss chemical bonding.

The periodic table is very useful for remembering the charges of ions, especially those of the elements on the left and right sides of the table. As Figure 2.22 shows,


Figure 2.22 Charges of some common ions found in ionic compounds. Notice that the steplike line that divides metals from nonmetals also separates cations from anions.



SAMPLE EXERCISE 2.8

Predict the charges expected for the most stable ions of barium and oxygen.

Solution

We will assume that these elements form ions that have the same number of electrons as the nearest noble-gas atom. From the periodic table, barium has atomic number 56. The nearest noble gas is xenon, atomic number 54. Barium can attain a stable arrangement of 54 electrons by losing two of its electrons, forming the cation.

Oxygen has atomic number 8. The nearest noble gas is neon, atomic number 10. Oxygen can attain this stable electron arrangement by gaining two electrons, thereby forming the anion.

PRACTICE EXERCISE

Predict the charge of the most stable ion of aluminum.

Answer: 3+


The periodic table is very useful for remembering the charges of ions, especially those of the elements on the left and right sides of the table. As Figure 2.22 shows, the charges of these ions relate in a simple way to their positions in the table. On the left side of the table, for example, the group 1A elements (the alkali metals) form 1+ ions, and the group 2A elements (the alkaline earths) form 2+ ions. On the other side of the table the group 7A elements (the halogens) form 1- ions, and the group 6A elements form 2- ions. As we will see later in the text, many of the other groups do not lend themselves to such simple rules.

Ionic Compounds

A great deal of chemical activity involves the transfer of electrons between substances. Ions form when one or more electrons transfer from one neutral atom to another. Figure 2.23 shows that when elemental sodium is allowed to react with elemental chlorine, an electron transfers from a neutral sodium atom to a neutral chlorine atom. We are left with an Na+ ion and a Cl- ion. Objects of opposite charge attract, however, so the Na+ and the Cl- ions bind together to form the compound sodium chloride (NaCl), which we know better as common table salt. Sodium chloride is an example of an ionic compound, a compound that contains both positively and negatively charged ions.


Figure 2.23 (a) The transfer of an electron from a neutral Na atom to a neutral Cl atom leads to the formation of an Na+ ion and a Cl- ion. (b) Arrangement of these ions in solid sodium chloride (NaCl) is pictured at the right.



We can often tell whether a compound is ionic (consisting of ions) or molecular (consisting of molecules) from its composition. In general, cations are metal ions, whereas anions are nonmetal ions. Consequently, ionic compounds are generally combinations of metals and nonmetals, as in NaCl. In contrast, molecular compounds are generally composed of nonmetals only, as in H20.


SAMPLE EXERCISE 2.9

Which of the following compounds would you expect to be ionic:

Solution

We would predict that and are ionic compounds because they are composed of a metal combined with a nonmetal. The other two compounds, composed entirely of nonmetals, are predicted (correctly) to be molecular compounds.

PRACTICE EXERCISE

Which of the following compounds are molecular: FeS,

Answer: CBr4 and P4O6


The ions in ionic compounds are arranged in three-dimensional structures. The arrangement of Na+ and Cl- ions in NaCl is shown in Figure 2.23. Because there is no discrete molecule of NaCl, we are able to write only an empirical formula for this substance. In fact, only empirical formulas can be written for most ionic compounds.

We can readily write the empirical formula for an ionic compound if we know the charges of the ions of which it is composed. Chemical compounds are always electrically neutral. Consequently, the ions in an ionic compound always occur in such a ratio that the total positive charge equals the total negative charge. Thus, there is one Na+ to one Cl- (giving NaCl), one Ba2+ to two Cl- (giving BaCl2), and so forth.

As you consider these and other examples, you will see that if the charges on the cation and anion are equal, the subscript on each ion will be 1. If the charges are not equal, the charge on one ion (without its sign) will become the subscript on the other ion. For example, the ionic compound formed from Mg (which forms Mg2+ ions) and N (which forms N3- ions) is Mg3N2:

formula to come


SAMPLE EXERCISE 2.10

What are the empirical formulas of the compounds formed by (a) and ions; (b) and ions; (c) and ions?

Solution

(a) Three ions are required to balance the charge of one ion. Thus, the formula is

(b) Two ions are required to balance the charge of three ions (that is, the total positive charge is and the total negative charge is ). Thus, the formula is

(c) Two ions are needed to balance the charge of one Thus the formula is In this case the formula for the entire polyatomic ion must be enclosed in parentheses so that it is clear that the subscript 2 applies to all the atoms of that ion.

PRACTICE EXERCISE

Write the empirical formulas for the compounds formed by the following ions: (a) and (b) and (c) and

Answers: (a) Na3PO4; (b) ZnSO4; (c) Fe2(CO3)3