05.1 - Naming Inorganic Compounds - ionic compounds · Chemical nomenclature The names and chemical...

Naming Inorganic Compounds Ionic compounds

Chemical nomenclature

The names and chemical formulas of compounds are essential in chemistry. The system used in naming substances is called chemical nomenclature, from the Latin words “nomen" (name) and “calare" (to call).

Many important substances that have been known for a long time, such as water (H2O) and ammonia (NH3), do have traditional names (called common names). For most substances, however, there are a set of rules that leads to a unique name for each substance.

The rules for chemical nomenclature are based on the division of substances into categories. The major division is between organic and inorganic compounds. Organic compounds contain carbon-carbon bonds and/or carbon-hydrogen bonds, often in combination with oxygen, nitrogen, or other elements. All others are inorganic compounds. Early chemists associated organic compounds with plants and animals and inorganic compounds with the nonliving portion of our world.

In this section we will see the basic rules for naming three categories of inorganic compounds:

• ionic compounds • molecular compounds • acids.

Ionic Compounds Many atoms gain or lose electrons to end up with the same number of electrons as the closest noble gas in the periodic table. We might deduce that this is because their electronic configuration is very stable. Nearby elements can obtain these same stable arrangements by losing or gaining electrons.

54 CHAPTER 2 Atoms, Molecules, and Ions

Picturing MoleculesThe molecular formula of a substance summarizes the composition of the substance butdoes not show how the atoms are joined together in the molecule. A structural formulashows which atoms are attached to which, as in the following examples:

The atoms are represented by their chemical symbols, and lines are used to represent thebonds that hold the atoms together.

A structural formula usually does not depict the actual geometry of the molecule,that is, the actual angles at which atoms are joined together. A structural formula can bewritten as a perspective drawing (! FIGURE 2.19), however, to give some sense of three-dimensional shape.

Scientists also rely on various models to help visualize molecules. Ball-and-stickmodels show atoms as spheres and bonds as sticks. This type of model has the advantageof accurately representing the angles at which the atoms are attached to one another inthe molecule (Figure 2.19). Sometimes the chemical symbols of the elements are super-imposed on the balls, but often the atoms are identified simply by color.

A space-filling model depicts what the molecule would look like if the atoms werescaled up in size (Figure 2.19). These models show the relative sizes of the atoms, but theangles between atoms, which help define their molecular geometry, are often more diffi-cult to see than in ball-and-stick models. As in ball-and-stick models, the identities ofthe atoms are indicated by color, but they may also be labeled with the element’s symbol.

G I V E I T S O M E T H O U G H T

The structural formula for ethane is

a. What is the molecular formula for ethane? b. What is its empirical formula? c. Which kind of molecular model would most clearly show

the angles between atoms?

2.7 | IONS AND IONIC COMPOUNDSThe nucleus of an atom is unchanged by chemical processes, but some atoms can read-ily gain or lose electrons. If electrons are removed from or added to an atom, a chargedparticle called an ion is formed. An ion with a positive charge is a cation (pronouncedCAT-ion); a negatively charged ion is an anion (AN-ion).

To see how ions form, consider the sodium atom, which has 11 protons and 11 elec-trons. This atom easily loses one electron. The resulting cation has 11 protons and 10electrons, which means it has a net charge of .

Na atom Na! ion

11e"

10e"11p!

11p!

Loses anelectron

1+

C CH

H

H

H

H

H

Hydrogen peroxideWater Methane

H HO

HO

HO C HH

H

H

Dashedwedge is bond behind page

Solid line is bond in plane of page

Wedge is bond out of page

Perspective drawing

Structural formula

Molecular formula

Space-filling model

Ball-and-stick model

H

HH

HC

H

H

CH4

HH C

" FIGURE 2.19 Differentrepresentations of the methane (CH4)molecule. Structural formulas, perspectivedrawings, ball-and-stick models, and space-filling models correspond to the molecularformula, and each helps us visualize theways atoms are attached to each other.

G O F I G U R EWhat advantage does a ball-and-stick model have over a space-filling model?

SECTION 2.7 Ions and Ionic Compounds 55

The net charge on an ion is represented by a superscript. The superscripts ,and , for instance, mean a net charge resulting from the loss of one, two, and threeelectrons, respectively. The superscripts , and represent net charges resultingfrom the gain of one, two, and three electrons, respectively. Chlorine, with 17 protonsand 17 electrons, for example, can gain an electron in chemical reactions, producing the

ion:

In general, metal atoms tend to lose electrons to form cations and nonmetal atoms tend togain electrons to form anions. Thus, ionic compounds tend to be composed of metals bondedwith nonmetals, as in NaCl.

SAMPLE EXERCISE 2.7 Writing Chemical Symbols for Ions

Give the chemical symbol, including superscript indicating mass number, for (a) the ion with22 protons, 26 neutrons, and 19 electrons; (b) the ion of sulfur that has 16 neutrons and18 electrons.

SOLUTION(a) The number of protons is the atomic number of the element. A periodic table or list ofelements tells us that the element with atomic number 22 is titanium (Ti). The mass number(protons plus neutrons) of this isotope of titanium is . Because the ion has threemore protons than electrons, it has a net charge of : .

(b) The periodic table tells us that sulfur (S) has an atomic number of 16. Thus, each atom orion of sulfur contains 16 protons. We are told that the ion also has 16 neutrons, meaning themass number is . Because the ion has 16 protons and 18 electrons, its net chargeis and the ion symbol is .

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

PRACTICE EXERCISEHow many protons, neutrons, and electrons does the ion possess?

Answer: 34 protons, 45 neutrons, and 36 electrons

In addition to simple ions such as and , there are polyatomic ions, such as(ammonium ion) and (sulfate ion). These latter ions consist of atoms

joined as in a molecule, but they have a net positive or negative charge. We considerpolyatomic ions in Section 2.8.

It is important to realize that the chemical properties of ions are very different fromthe chemical properties of the atoms from which the ions are derived. Although a givenatom and its ion may be essentially the same (plus or minus a few electrons), the behav-ior of the ion is very different from that of its associated atom.

Predicting Ionic ChargesMany atoms gain or lose electrons to end up with the same number of electrons as thenoble gas closest to them in the periodic table. Noble-gas elements are chemically non-reactive and form very few compounds. We might deduce that this is because their elec-tron arrangements are very stable. Nearby elements can obtain these same stablearrangements by losing or gaining electrons. For example, the loss of one electron froman atom of sodium leaves it with the same number of electrons as in a neon atom (10).Similarly, when chlorine gains an electron, it ends up with 18, the same number of elec-trons as in argon. We will use this simple observation to explain the formation of ionsuntil Chapter 8, where we discuss chemical bonding.

SO42-NH4

+Cl-Na+

79Se2-

32S2-2-16 + 16 = 32

48Ti3+3+22 + 26 = 48

Cl atom Cl! ion

17e!

Gains anelectron

17p" 17p" 18e!

Cl-

3-- , 2-3+

+ , 2+

For example, the loss of one electron from an atom of Na leaves it with the same number of electrons as in a Ne atom.

Similarly, when Cl gains an electron, it ends up with 18, the same number of electrons as in Ar.

Ionic Compounds When elemental Na is allowed to react with elemental Cl, an electron transfers from Na to Cl, forming a Na+ ion and a Cl- ion.

Because objects of opposite charge attract, the Na+ and the Cl- ions bind together to form the compound sodium chloride (NaCl). Sodium chloride (table salt), is an example of an ionic compound, a compound made up of cations and anions.


SAMPLE EXERCISE 2.9 Identifying Ionic and Molecular Compounds

Which of these compounds would you expect to be ionic: N2O, Na2O, CaCl2, SF4?

SOLUTIONWe predict that Na2O and CaCl2 are ionic compounds because they are composed of a metalcombined with a nonmetal. We predict (correctly) that N2O and SF4 are molecular com-pounds because they are composed entirely of nonmetals.

PRACTICE EXERCISEWhich of these compounds are molecular: CBr4, FeS, P4O6, PbF2?

Answer: CBr4 and P4O6

The ions in ionic compounds are arranged in three-dimensional structures, asFigure 2.21(b) shows for NaCl. Because there is no discrete “molecule” of NaCl, we areable to write only an empirical formula for this substance. This is true for most otherionic compounds.

We can write the empirical formula for an ionic compound if we know the chargesof the ions. This is true because chemical compounds are always electrically neutral.Consequently, the ions in an ionic compound always occur in such a ratio that the totalpositive charge equals the total negative charge. Thus, there is one to one (giv-ing NaCl), one to two (giving BaCl2), and so forth.

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


Why don’t we write the formula for the compound formed by and asCa2O2?

O2-Ca2+

Mg Mg3N22 ! N 3 "

N3-Mg2+

Cl-Ba2+Cl-Na+

Cl# ion

17e#

11e#

e!

Na! ion

10e#11p!

11p!

Loses anelectron

Gains anelectron

17p! 17p!

Cl atom

Na atom

(b) (c)(a)

18e#

! FIGURE 2.21 Formation of an ionic compound. (a) The transfer of an electron from a Na atom to a Cl atom leads to the formation of a ion and a ion. (b) Arrangement of these ions in solid sodium chloride, NaCl. (c) A sample of sodium chloride crystals.

Cl-Na+In general ionic compounds are combinations of metals and nonmetals. In contrast, molecular compounds are generally composed of nonmetals only, as in H2O.

Ionic Compounds The periodic table is useful for remembering ionic charges, especially those of elements on the left and right sides of the table. The charges of these ions relate in a simple way to their positions in the table:

• The group 1A elements (alkali metals) form 1+ ions• The group 2A elements (alkaline earths) form 2+ ions• The group 6A elements form 2- ions• The group 7A elements (halogens) form 1- ions


SAMPLE EXERCISE 2.8 Predicting Ionic Charge

Predict the charge expected for the most stable ion of barium and the most stable ion of oxygen.

SOLUTIONWe will assume that these elements form ions that have the same number of electrons as thenearest noble-gas atom. From the periodic table, we see that barium has atomic number 56.The nearest noble gas is xenon, atomic number 54. Barium can attain a stable arrangement of54 electrons by losing two electrons, forming the cation.

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

PRACTICE EXERCISEPredict the charge expected for the most stable ion of (a) aluminum and (b) fluorine.

Answer: (a) , (b)

The periodic table is very useful for remembering ionic charges, especially those ofelements on the left and right sides of the table. As ! FIGURE 2.20 shows, the charges ofthese ions relate in a simple way to their positions in the table: The group 1A elements(alkali metals) form ions, the group 2A elements (alkaline earths) form ions, thegroup 7A elements (halogens) form ions, and the group 6A elements form ions.(Many of the other groups do not lend themselves to such simple rules.)

Ionic CompoundsA great deal of chemical activity involves the transfer of electrons from one substance toanother. " FIGURE 2.21 shows that when elemental sodium is allowed to react withelemental chlorine, an electron transfers from a sodium atom to a chlorine atom, form-ing a ion and a ion. Because objects of opposite charge attract, the and the

ions bind together to form the compound sodium chloride (NaCl). Sodium chlo-ride, which we know better as common table salt, is an example of an ionic compound,a compound made up of cations and anions.

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 andanions are nonmetal ions. Consequently, ionic compounds are generally combinations ofmetals and nonmetals, as in NaCl. In contrast, molecular compounds are generally com-posed of nonmetals only, as in H2O.

Cl-Na+Cl-Na+

2-1-2+1+

1-3+

O2-

Ba2+

H!

Li!

Na!Mg2!

Cs! Ba2!

Rb! Sr2!

K! Ca2!

1A

2A 3A 4A 5A 6A

7AH"

8ANOBLE

GASES

Al3!

N3" O2"

S2"

Se2"

Te2"

F"

Cl"

Br"

I"

Transition metals" FIGURE 2.20 Predictable charges of some common ions. Notice that the red stepped line that divides metals from nonmetals also separates cations from anions. Hydrogen forms both and ions.1-

1+

G O F I G U R EThe most common ions for silver, zinc, and scandium are , , and

. Locate the boxes in which you would place these ions in this table. Which of these ions have the same number of electrons as a noble-gas element?

Sc3+Zn2+Ag+

Predictable charges of some common ions. Notice that the red stepped line that divides metals from nonmetals also separates cations from anions. Hydrogen forms both 1+ and 1- ions.

Ionic Compounds Practice exercise Identifying Ionic and Molecular Compounds

• Which of these compounds would you expect to be ionic: N2O, Na2O, CaCl2, SF4?

SOLUTION We predict that Na2O and CaCl2 are ionic compounds because they are composed of

a metal combined with a nonmetal. We predict (correctly) that N2O and SF4 are molecular compounds because they are composed entirely of nonmetals.

• Which of these compounds are molecular: CBr4, FeS, P4O6, PbF2?

Ionic Compounds Chemical formulas that give only the relative number of atoms of each type in a molecule are called empirical formulas.

E.g. The empirical formula for hydrogen peroxide H2O2 is HO. The empirical formula for ethylene C2H4 is CH2.

We can write the empirical formula for an ionic compound if we know the charges of the ions. This is true because chemical compounds are always electrically neutral (if not differently specified).

E.g. There is one Na+ to one Cl- (giving NaCl), one Ba2+ to two Cl- (giving BaCl2), and so on.

If the charges on the cation and anion are equal, the subscript on each ion is 1. If the charges are not equal, the charge on one ion 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:


SAMPLE EXERCISE 2.9 Identifying Ionic and Molecular Compounds

Which of these compounds would you expect to be ionic: N2O, Na2O, CaCl2, SF4?

SOLUTIONWe predict that Na2O and CaCl2 are ionic compounds because they are composed of a metalcombined with a nonmetal. We predict (correctly) that N2O and SF4 are molecular com-pounds because they are composed entirely of nonmetals.

PRACTICE EXERCISEWhich of these compounds are molecular: CBr4, FeS, P4O6, PbF2?

Answer: CBr4 and P4O6

The ions in ionic compounds are arranged in three-dimensional structures, asFigure 2.21(b) shows for NaCl. Because there is no discrete “molecule” of NaCl, we areable to write only an empirical formula for this substance. This is true for most otherionic compounds.

We can write the empirical formula for an ionic compound if we know the chargesof the ions. This is true because chemical compounds are always electrically neutral.Consequently, the ions in an ionic compound always occur in such a ratio that the totalpositive charge equals the total negative charge. Thus, there is one to one (giv-ing NaCl), one to two (giving BaCl2), and so forth.

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


Why don’t we write the formula for the compound formed by and asCa2O2?

O2-Ca2+

Mg Mg3N22 ! N 3 "

N3-Mg2+

Cl-Ba2+Cl-Na+

Cl# ion

17e#

11e#

e!

Na! ion

10e#11p!

11p!

Loses anelectron

Gains anelectron

17p! 17p!

Cl atom

Na atom

(b) (c)(a)

18e#

! FIGURE 2.21 Formation of an ionic compound. (a) The transfer of an electron from a Na atom to a Cl atom leads to the formation of a ion and a ion. (b) Arrangement of these ions in solid sodium chloride, NaCl. (c) A sample of sodium chloride crystals.

Cl-Na+

Ionic Compounds Practice exercise

• Write the empirical formulas for (a) glucose (molecular formula C6H12O6); (b) nitrous oxide, a substance used as an anesthetic and commonly called laughing gas (molecular formula N2O).

SOLUTION (a) The subscripts of an empirical formula are the smallest whole-number ratios. The

smallest ratios are obtained by dividing each subscript by the largest common factor, in this case 6. The resultant empirical formula for glucose is CH2O.

(b) Because the subscripts in N2O are already the lowest integral numbers, the empirical formula for nitrous oxide is the same as its molecular formula, N2O.

• Give the empirical formula for diborane, whose molecular formula is B2H6.

Names and Formulas of Ionic Compounds

a. Cations formed from metals have the same name as the metal:

Na 2+ sodium ion Zn 2+ zinc ion Al 3+ aluminum ion

b. Some metals can form cations with different charges. In these cases the positive charge is indicated by a Roman numeral in parentheses following the name of the metal:

Fe 2+ iron (II) ion Cu + copper (I) ion

Fe 3+ iron (III) ion Cu 2+ copper (II) ion

An older method still widely used for distinguishing between differently charged ions of a metal uses the endings -ous and -ic added to the root of the element’s Latin name:

c. Common polyatomic cations formed from nonmetal atoms have names that end in -ium:

Cations

Fe 2+ ferrous ion Cu + coprous ion

Fe 3+ ferric ion Cu 2+ copric ion

NH4+ ammonium ion H3O+ hydronium ion


NoteMost transition metals form cations with different charges. The metals that form only one cation are those of group 1A and group 2A, as well as Al3+ and two transition metal ions: Ag+ (group 1B) and Zn2+.

SECTION 2.8 Naming Inorganic Compounds 59

(c) Two ions are needed to balance the charge of one , yielding Mg(NO3)2. Notethat the formula for the polyatomic ion, , must be enclosed in parentheses so that it isclear that the subscript 2 applies to all the atoms of that ion.

PRACTICE EXERCISEWrite the empirical formula for the compound formed by (a) and , (b)and , (c) and .

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

2.8 | NAMING INORGANIC COMPOUNDSThe names and chemical formulas of compounds are essential vocabulary in chemistry.The system used in naming substances is called chemical nomenclature, from theLatin words nomen (name) and calare (to call).

There are more than 50 million known chemical substances. Naming them allwould be a hopelessly complicated task if each had a name independent of all others.Many important substances that have been known for a long time, such as water (H2O)and ammonia (NH3), do have traditional names (called common names). For most sub-stances, however, we rely on a set of rules that leads to an informative and unique namefor each substance, a name based on the composition of the substance.

The rules for chemical nomenclature are based on the division of substances intocategories. The major division is between organic and inorganic compounds. Organiccompounds contain carbon and hydrogen, often in combination with oxygen, nitrogen,or other elements. All others are inorganic compounds. Early chemists associated organiccompounds with plants and animals and inorganic compounds with the nonliving por-tion of our world. Although this distinction is no longer pertinent, the classification be-tween organic and inorganic compounds continues to be useful. In this section weconsider the basic rules for naming three categories of inorganic compounds: ioniccompounds, molecular compounds, and acids.

Names and Formulas of Ionic CompoundsRecall from Section 2.7 that ionic compounds usually consist of metal ions combined withnonmetal ions. The metals form the cations, and the nonmetals form the anions.

1. Cations

a. Cations formed from metal atoms have the same name as the metal:

CO32-Fe3+SO4

2-Zn2+PO4

3-Na+

NO3-

Mg2+NO -3

Na+ sodium ion Zn2+ zinc ion Al3+ aluminum ion

iron(II) ionFe2+ copper(I) ionCu+

iron(III) ionFe3+ copper(II) ionCu2+

b. If a metal can form cations with different charges, the positive charge is indicated bya Roman numeral in parentheses following the name of the metal:

Ions of the same element that have different charges have different properties,such as different colors (! FIGURE 2.23).

Most metals that form cations with different charges are transition metals,elements that occur in the middle of the periodic table, from group 3B to group2B. The metals that form only one cation (only one possible charge) are those ofgroup 1A and group 2A, as well as (group 3A) and two transition-metalions: (group 1B) and (group 2B). Charges are not expressed whennaming these ions. However, if there is any doubt in your mind whether a metalforms more than one cation, use a Roman numeral to indicate the charge. It isnever wrong to do so, even though it may be unnecessary.

Zn2+Ag+Al3+

" FIGURE 2.23 Different ions of thesame element have different properties.Both substances shown are compounds ofiron. The substance on the left is Fe3O4,which contains Fe2+ and Fe3+ ions. Thesubstance on the right is Fe2O3, whichcontains Fe3+ ions.

Ions of the same element that have different charges have different properties, such as different colors.

E.g.Both substances shown on the right are compounds of iron. The substance on the left is Fe3O4, which contains Fe2+ and Fe3+ ions. The substance on the right is Fe2O3, which contains Fe3+ ions.

Cations

Names and Formulas of Ionic Compounds List of common Cations

The most often used ions are in boldface

Cations


An older method still widely used for distinguishing between differentlycharged ions of a metal uses the endings -ous and -ic added to the root of theelement’s Latin name:

ferrous ionFe2+ cuprous ionCu+

ferric ionFe3+ cupric ionCu2+

Although we will only rarely use these older names in this text, you mightencounter them elsewhere.

c. Cations formed from nonmetal atoms have names that end in -ium:

ammonium ionNH4+ hydronium ionH3O+

These two ions are the only ions of this kind that we will encounter frequently inthe text.

The names and formulas of some common cations are shown in ! TABLE 2.4and on the back inside cover of the text. The ions on the left side in Table 2.4 arethe monatomic ions that do not have more than one possible charge. Those onthe right side are either polyatomic cations or cations with more than one possi-ble charge. The ion is unusual because, even though it is a metal ion, it isnot monatomic. It is called the mercury(I) ion because it can be thought ofas two ions bound together. The cations that you will encounter mostfrequently are shown in boldface. You should learn these cations first.


a. Why is CrO named using a Roman numeral, chromium(II) oxide, whereas CaOis named without a Roman numeral, calcium oxide?

b. What does the -ium ending on the name ammonium ion tell you about thecomposition of the ion?

Hg+

Hg22+

TABLE 2.4 • Common Cations*

Charge Formula Name Formula Name

1+ H! hydrogen ion NH4! ammonium ion

Li+ lithium ion Cu+ copper(I) or cuprous ion

Na! sodium ion

K! potassium ion

Cs+ cesium ion

Ag! silver ion

2+ Mg2! magnesium ion Co2+ cobalt(II) or cobaltous ion

Ca2! calcium ion Cu2! copper(II) or cupric ion

Sr2+ strontium ion Fe2! iron(II) or ferrous ion

Ba2+ barium ion Mn2+ manganese(II) or manganous ion

Zn2! zinc ion Hg22+ mercury(I) or mercurous ion

Cd2+ cadmium ion Hg2! mercury(II) or mercuric ion

Ni2+ nickel(II) or nickelous ion

Pb2! lead(II) or plumbous ion

Sn2+ tin(II) or stannous ion

3+ Al3! aluminum ion Cr3+ chromium(III) or chromic ion

Fe3! iron(III) or ferric ion

*The ions we use most often in this course are in boldface. Learn them first.


a. The names of monatomic anions are formed by replacing the ending of the name of the element with -ide:

b. Polyatomic anions containing oxygen are called oxyanions. The -ate is used for the most common oxyanion, and -ite is used for an oxyanion that has the same charge but one O atom fewer:

A few polyatomic anions also have names ending in -ide:

Anions

H- hydride ion O2- oxide ion N3- nitride ion

OH- hydroxide ion O22- peroxide ion CN- cyanide ion

NO3- nitrate ion

NO2- nitrite ion

SO42- sulfate ion

SO32- sulfite ion


Prefixes are used when the number of oxyanions of an element is more than four, as with the halogens. The prefix per- indicates one more O atom than the oxyanion ending in -ate; hypo- indicates one O atom fewer than the oxyanion ending in -ite:


hydride ionH- oxide ionO2- nitride ionN3-

hydroxide ionOH- cyanide ionCN- peroxide ionO22-

nitrate ionNO3- sulfate ionSO4

2-

nitrite ionNO2- sulfite ionSO3

2-

perchlorate ion (one more O atom than chlorate)ClO4-

chlorate ionClO3!

chlorite ion (one O atom fewer than chlorate)ClO2-

hypochlorite ion (one O atom fewer than chlorite)ClO-

A few polyatomic anions also have names ending in -ide:

Prefixes are used when the series of oxyanions of an element extends to fourmembers, as with the halogens. The prefix per- indicates one more O atom thanthe oxyanion ending in -ate; hypo- indicates one O atom fewer than the oxyan-ion ending in -ite:

These rules are summarized in ! FIGURE 2.24.


What information is conveyed by the endings -ide, -ate, and -ite in the name ofan anion?

" FIGURE 2.25 can help you remember the charge and number of oxygen atoms inthe various oxyanions. Notice that C and N, both period 2 elements, have only three Oatoms each, whereas the period 3 elements P, S, and Cl have four O atoms each. Begin-ning at the lower right in Figure 2.25, note that ionic charge increases from right to left,from for CIO4

- to for PO43-. In the second period the charges also increase from

right to left, from for NO3- to for CO3

2-. Notice also that although each of theanions in Figure 2.25 ends in -ate, the ClO4

- ion also has a per- prefix.2-1-

3-1-

per______ate(perchlorate, ClO4

!)_________ite

(chlorite, ClO2!)

hypo____ite(hypochlorite, ClO!)

_________ide(chloride, Cl!)

_________ate(chlorate, ClO3

!)

Common orrepresentativeoxyanion

Simpleanion

Oxyanions"O atom !O atom !O atom

# FIGURE 2.24 Procedure for naming anions. The first part of the element’s name, such as “chlor” for chlorine or “sulf” for sulfur, goes inthe blank.

G O F I G U R EName the anion obtained by removing one oxygen atom from the perbromate ion, BrO4

-.

b. Polyatomic anions containing oxygen have names ending in either -ate or -ite andare called oxyanions. The -ate is used for the most common or representativeoxyanion of an element, and -ite is used for an oxyanion that has the samecharge but one O atom fewer:

2. Anions

a. The names of monatomic anions are formed by replacing the ending of the name ofthe element with -ide:

Procedure for naming anions. The first part of the element’s name, such as “chlor” for chlorine or “sulf” for sulfur, goes in the blank.

Anions

ClO4- perchlorate ion (one more O atom than chlorate)

ClO3- chlorate ion

ClO2- chlorite ion (one O atom fewer than chlorate)

ClO- hypochlorite ion (one O atom fewer than chlorite)


Common oxyanions. The composition and charges of common oxyanions are related to their location in the periodic table.


Period 2

Period 3

CO32–

Carbonate ion

Group 4ANO3

–

Nitrate ion

PO43–

Phosphate ion

Group 5A

SO42–

Sulfate ion

Group 6A

ClO4–

Perchlorate ion

Group 7A

Maximum of 3 O atoms in period 2.

Charges increase right to left.

Maximum of 4 O atoms in period 3.

! FIGURE 2.25 Common oxyanions.The composition and charges of commonoxyanions are related to their location in theperiodic table.


Predict the formulas for the borate ion and silicate ion, assuming they contain asingle B and Si atom, respectively, and follow the trends shown in Figure 2.25.

SAMPLE EXERCISE 2.11 Determining the Formula of an Oxyanionfrom Its Name

Based on the formula for the sulfate ion, predict the formula for (a) the selenate ion and (b) theselenite ion. (Sulfur and selenium are both in group 6A and form analogous oxyanions.)

SOLUTION(a) The sulfate ion is . The analogous selenate ion is therefore .(b) The ending -ite indicates an oxyanion with the same charge but one O atom fewer than thecorresponding oxyanion that ends in -ate. Thus, the formula for the selenite ion is .

PRACTICE EXERCISEThe formula for the bromate ion is analogous to that for the chlorate ion. Write the formulafor the hypobromite and bromite ions.Answer: and BrO -

2BrO-

SeO32-

SeO42-SO4

2-

carbonate ionCO32 - phosphate ionPO4

3 -

hydrogen carbonate ionHCO3- dihydrogen phosphate ionH2PO4

-

CaCl2 calcium chlorideAl(NO3)3 aluminum nitrateCu(ClO4)2 copper(II) perchlorate (or cupric perchlorate)

Notice that each added reduces the negative charge of the parent anion byone. An older method for naming some of these ions uses the prefix bi-. Thus,the ion is commonly called the bicarbonate ion, and is some-times called the bisulfate ion.

The names and formulas of the common anions are listed in " TABLE 2.5 and onthe back inside cover of the text. Those anions whose names end in -ide are listed on theleft portion of Table 2.5, and those whose names end in -ate are listed on the right. Themost common of these ions are shown in boldface. You should learn names and formu-las of these anions first. The formulas of the ions whose names end with -ite can be de-rived from those ending in -ate by removing an O atom. Notice the location of themonatomic ions in the periodic table. Those of group 7A always have a charge( , and ), and those of group 6A have a charge ( and ).

3. Ionic Compounds

Names of ionic compounds consist of the cation name followed by the anion name:

S2-O2-2-I-F-, Cl-, Br-1-

HSO4-HCO3

-

H+

c. Anions derived by adding to an oxyanion are named by adding as a prefix theword hydrogen or dihydrogen, as appropriate:

H+

Anions

Practice exercise

• Predict the formula for (a) the selenate ion and (b) the selenite ion. (Sulfur and selenium are both in group 6A and form analogous oxyanions.)

SOLUTION (a) The sulfate ion is SO42-. The analogous selenate ion is therefore SeO42-. (b) The ending -ite indicates an oxyanion with the same charge but one O atom fewer than the

corresponding oxyanion that ends in -ate. Thus, the formula for the selenite ion is SeO32- .

• The formula for the bromate ion is analogous to that for the chlorate ion. Write the formula for the hypobromite and bromite ions.


c. Anions derived by adding H+ to an oxyanion are named by adding as a prefix the word hydrogen or di-hydrogen, as appropriate:

Anions

Note Each H+ added reduces the negative charge of the parent anion by one. An older method for naming some of these ions uses the prefix bi-. Thus, the HCO3- ion is commonly called the bicarbonate ion, and HSO4- is sometimes called the bisulfate ion.

CO32- carbonate ion

HCO3- hydrogen carbonate ion

PO43- phosphate ion

H2PO4- dihydrogen phosphate ion


Names of ionic compounds consist of the cation name followed by the anion name:

Ionic Compounds

List of common Cations

MgBr2 magnesium bromideCaCl2 calcium chlorideNaF sodium fluoride


TABLE 2.5 • Common Anions*

Charge Formula Name Formula Name

1- H- hydride ion CH3COO! acetate ion(or )C2H3O2

-

F! fluoride ion ClO3- chlorate ion

Cl! chloride ion ClO4! perchlorate ion

Br! bromide ion NO3! nitrate ion

I! iodide ion MnO4- permanganate ion

CN- cyanide ionOH! hydroxide ion

2- O2! oxide ion CO32! carbonate ion

O22- peroxide ion CrO4

2- chromate ionS2! sulfide ion Cr2O7

2- dichromate ion

SO42! sulfate ion

3- N3- nitride ion PO43! phosphate ion

*The ions we use most often are in boldface. Learn them first.

In the chemical formulas for aluminum nitrate and copper(II) perchlorate,parentheses followed by the appropriate subscript are used because the com-pounds contain two or more polyatomic ions.

SAMPLE EXERCISE 2.12 Determining the Names of Ionic Compoundsfrom Their Formulas

Name the ionic compounds (a) K2SO4, (b) Ba(OH)2, (c) FeCl3.

SOLUTIONIn naming ionic compounds, it is important to recognize polyatomic ions and to determinethe charge of cations with variable charge.

(a) The cation is , the potassium ion, and the anion is , the sulfate ion, making thename potassium sulfate. (If you thought the compound contained and ions, you failedto recognize the polyatomic sulfate ion.)

(b) The cation is , the barium ion, and the anion is , the hydroxide ion: bariumhydroxide.

(c) You must determine the charge of Fe in this compound because an iron atom can formmore than one cation. Because the compound contains three chloride ions, , the cationmust be , the iron(III), or ferric, ion. Thus, the compound is iron(III) chloride or ferricchloride.

PRACTICE EXERCISEName the ionic compounds (a) NH4Br, (b) Cr2O3, (c) Co(NO3)2.

Answers: (a) ammonium bromide, (b) chromium(III) oxide, (c) cobalt(II) nitrate

Fe3+Cl-

OH-Ba2+

O2-S2-SO4

2-K+

SAMPLE EXERCISE 2.13 Determining the Formulas of Ionic Compoundsfrom Their Names

Write the chemical formulas for (a) potassium sulfide, (b) calcium hydrogen carbonate,(c) nickel(II) perchlorate.

SOLUTIONIn going from the name of an ionic compound to its chemical formula, you must know thecharges of the ions to determine the subscripts.

(a) The potassium ion is , and the sulfide ion is . Because ionic compounds are electri-cally neutral, two ions are required to balance the charge of one ion, giving K2S for theempirical formula.

S2-K+S2-K+


Practice exercise • Name the ionic compounds (a) K2SO4, (b) Ba(OH)2, (c) FeCl3.

SOLUTION

In naming ionic compounds, it is important to recognize polyatomic ions and to determine the charge of cations with variable charge.

(a) The cation is K+, and the anion is SO42-, the sulfate ion, making the name potassium sulfate.

(b) The cation is Ba2+, and the anion is OH-, the hydroxide ion: barium hydroxide.

(c) You must determine the charge of Fe because an iron atom can form more than one cation. Because the compound contains three chloride ions, Cl-, the cation must be Fe3+, the iron(III), or ferric, ion. Thus, the compound is iron(III) chloride or ferric chloride.

• Name the ionic compounds (a) NH4Br, (b) Cr2O3, (c) Co(NO3)2.

Ionic Compounds

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