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10 Mixtures and Chemical Compounds; Molecules and Covalent Bonds

10 Mixtures and Chemical Compounds; Molecules and Covalent Bonds

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2.10 MIXTURES AND CHEMICAL COMPOUNDS; MOLECULES AND COVALENT BONDS



mixed in any ratio without changing them (as long as there is no flame nearby to initiate reaction), just as a spoonful of sugar and a spoonful of salt can be mixed.

A chemical compound, in contrast to a mixture, is a pure substance that is formed

when atoms of different elements combine in a specific way to create a new material

with properties completely unlike those of its constituent elements. A chemical compound has a constant composition throughout, and its constituent units are all

identical. For example, when atoms of sodium (a soft, silvery metal) combine with

atoms of chlorine (a toxic, yellow-green gas), the familiar white solid called sodium

chloride (table salt) is formed. Similarly, when two atoms of hydrogen combine with

one atom of oxygen, water is formed.

To see how a chemical compound is formed, imagine what must happen when

two atoms approach each other at the beginning of a chemical reaction. Because the

electrons of an atom occupy a much greater volume than the nucleus, it’s the electrons that actually make the contact when atoms collide. Thus, it’s the electrons that

form the connections, or chemical bonds, that join atoms together in compounds.

Chemical bonds between atoms are usually classified as either covalent or ionic. As a

general rule, covalent bonds occur primarily between nonmetal atoms, while ionic

bonds occur primarily between metal and nonmetal atoms. Let’s look briefly at both

kinds, beginning with covalent bonds.

A covalent bond, the most common kind of chemical bond, results when two

atoms share several (usually two) electrons. A simple way to think about a covalent

bond is to imagine it as a tug-of-war. If two people pull on the same rope, they are

effectively joined together. Neither person can escape from the other as long as both

hold on. Similarly with atoms: when two atoms both hold on to some shared electrons, the atoms are bonded together (Figure 2.11).



᭡ The crystalline quartz sand on this

beach is a pure compound (SiO2), but the

seawater is a liquid mixture of many

compounds dissolved in water.



+



The two teams are joined together because both are tugging on the same rope.



Figure 2.11



A covalent bond between atoms is analogous to a tug-of-war.



The unit of matter that results when two or more atoms are joined by covalent

bonds is called a molecule. A hydrogen chloride molecule (HCl) results when a

hydrogen atom and a chlorine atom share two electrons. A water molecule (H2O)

results when each of two hydrogen atoms shares two electrons with a single oxygen

atom. An ammonia molecule (NH3) results when each of three hydrogen atoms

shares two electrons with a nitrogen atom, and so on. To visualize these and other

molecules, it helps to imagine the individual atoms as spheres joined together to

form molecules with specific three-dimensional shapes, as shown in Figure 2.12. Balland-stick models specifically indicate the covalent bonds between atoms, while

space-filling models accurately portray overall molecular shape but don’t explicitly

show covalent bonds.



55



+



Similarly, two atoms are

joined together when both

nuclei (+) tug on the same

electrons (dots).



56



Chapter 2 ATOMS, MOLECULES, AND IONS



Figure 2.12



Molecular models. Drawings such as

these help in visualizing molecules.

Ball-and-stick

models show atoms

(spheres) joined

together by covalent

bonds (sticks).



Space-filling models

portray the overall

molecular shape but

don’t explicitly show

covalent bonds.

Hydrogen chloride

(HCl)



Water

(H2O)



Ammonia

(NH3)



Methane

(CH4)



Chemists normally represent a molecule by giving its structural formula, which

shows the specific connections between atoms and therefore gives much more information than the chemical formula alone. Ethyl alcohol, for example, has the chemical

formula C2H6O and the following structural formula:



H



C2H6O



H



H



C



C



H



H



Chemical

formula



O



H



Structural

formula



Molecular

model



Ethyl alcohol



A structural formula uses lines between atoms to indicate the covalent bonds.

Thus, the two carbon atoms in ethyl alcohol are covalently bonded to each other, the

oxygen atom is bonded to one of the carbon atoms, and the six hydrogen atoms are

distributed three to one carbon, two to the other carbon, and one to the oxygen.

Structural formulas are particularly important in organic chemistry—the chemistry of carbon compounds—where the behavior of large, complex molecules is

almost entirely governed by their structure. Take even a relatively simple substance

like glucose, for instance. The molecular formula of glucose, C6H12O6, tells nothing

about how the atoms are connected. In fact, you could probably imagine a great

many different ways in which the 24 atoms might be connected. The structural formula for glucose, however, shows that 5 carbons and 1 oxygen form a ring of atoms,

with the remaining 5 oxygens each bonded to 1 hydrogen and distributed on different carbons.



C

O



H



O



C



H

O



O



C



H

H



H



O



H H H



C



C



H



O



Glucose—C6H12O6



H



C

H

H

[Red ϭ O, gray ϭ C, ivory ϭ H]



2.10 MIXTURES AND CHEMICAL COMPOUNDS; MOLECULES AND COVALENT BONDS



Even some elements exist as molecules rather than as individual atoms. Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine all exist as diatomic

(two-atom) molecules whose two atoms are held together by covalent bonds. We

therefore have to write them as such—H2, N2, O2, F2, Cl2, Br2, and I2—when using

any of these elements in a chemical equation. Notice that all these diatomic elements

except hydrogen cluster toward the far right side of the periodic table.

1A



8A



H2 2A



3A 4A 5A 6A 7A

N2 O2 F2

3B 4B 5B 6B 7B



8B



Cl2



1B 2B



Br2

I2



WORKED EXAMPLE 2.8



DRAWING A STRUCTURAL FORMULA

Propane, C3H8, has a structure in which the three carbon atoms are bonded in a row, each

end carbon is bonded to three hydrogens, and the middle carbon is bonded to two hydrogens. Draw the structural formula, using lines between atoms to represent covalent bonds.

SOLUTION



H



H



H



H



C



C



C



H



H



H



H

Propane



WORKED CONCEPTUAL EXAMPLE 2.9



VISUAL REPRESENTATIONS OF MIXTURES AND COMPOUNDS

Which of the following drawings represents a mixture, which a pure compound, and

which an element?

(a)



(b)



(c)



STRATEGY



Most people (professional chemists included) find chemistry easier to grasp when they

can visualize the behavior of atoms, thereby turning symbols into pictures. The Conceptual Problems in this text are intended to help you do that, frequently representing

atoms and molecules as collections of spheres. Don’t take the pictures literally; focus

instead on interpreting what they represent.

continued on the next page



57



58



Chapter 2 ATOMS, MOLECULES, AND IONS

SOLUTION



Drawing (a) represents a mixture of two diatomic elements, one composed of two red

atoms and one composed of two blue atoms. Drawing (b) represents molecules of a

pure diatomic element because all atoms are identical. Drawing (c) represents molecules of a pure compound composed of one red and one blue atom.

Ī PROBLEM 2.15 Draw the structural formula of methylamine, CH5N, a substance

responsible for the odor of rotting fish. The carbon atom is bonded to the nitrogen atom

and to three hydrogens. The nitrogen atom is bonded to the carbon and two hydrogens.

Ī PROBLEM 2.16 Methionine, one of the 20 amino acid building blocks from which

proteins are made, has the following structure. What is the chemical formula of methionine? In writing the formula, list the element symbols in alphabetical order and give the

number of each element as a subscript.



H

H

H



C



S



C



H H



H

C



C



H H



O

C

N



O



H



Methionine

(an amino acid)



H



H

CONCEPTUAL PROBLEM 2.17 Which of the following drawings represents a collection of hydrogen peroxide (H2O2) molecules? The red spheres represent oxygen atoms

and the ivory spheres represent hydrogen.

(a)



(b)



(c)



(d)



CONCEPTUAL PROBLEM 2.18 Adrenaline, the so-called “flight or fight” hormone,

can be represented by the following ball-and-stick model. What is the chemical formula

of adrenaline? (Gray = C, ivory = H, red = O, blue = N)



2.11 IONS AND IONIC BONDS

In contrast to a covalent bond, an ionic bond results not from a sharing of electrons

but from a transfer of one or more electrons from one atom to another. As noted previously, ionic bonds generally form between a metal and a nonmetal. Metals, such as

sodium, magnesium, and zinc, tend to give up electrons, whereas nonmetals, such as

oxygen, nitrogen, and chlorine, tend to accept electrons.



2.11 IONS AND IONIC BONDS



59



For example, when sodium metal comes in contact with chlorine gas, a sodium

atom gives an electron to a chlorine atom, resulting in the formation of two charged

particles, called ions. Because a sodium atom loses one electron, it loses one negative

charge and becomes an Na+ ion with a charge of +1. Such positive ions are called

cations (pronounced cat-ions). Conversely, because a chlorine atom gains an electron, it gains a negative charge and becomes a Cl- ion with a charge of -1. Such

negative ions are called anions (an-ions).

A sodium atom



A sodium cation



Na +

A chlorine molecule



1

Cl

2 2



Na+ + Cl–

A chloride anion



A similar reaction takes place when magnesium and chlorine molecules (Cl2)

come in contact to form MgCl2. A magnesium atom transfers an electron to each of

two chlorine atoms, yielding the doubly charged Mg2+ cation and two Cl- anions.

Mg + Cl2 : Mg2+ + Cl- + Cl- (MgCl2)

Because opposite charges attract, positively charged cations like Na+ and Mg2+

experience a strong electrical attraction to negatively charged anions like Cl-, an

attraction that we call an ionic bond. Unlike what happens when covalent bonds are

formed, though, we can’t really talk about discrete Na+Cl- molecules under normal

conditions. We can speak only of an ionic solid, in which equal numbers of Na+ and

Cl- ions are packed together in a regular way (Figure 2.13). In a crystal of table salt, for

instance, each Na+ ion is surrounded by six nearby Cl- ions, and each Cl- ion is surrounded by six nearby Na+ ions, but we can’t specify what pairs of ions “belong” to

each other as we can with atoms in covalent molecules.



Na

Cl



᭡ Chlorine is a toxic green gas, sodium is

a reactive metal, and sodium chloride is a

harmless white solid.



Na+

Cl–



In the sodium chloride

crystal, each Na+ ion is

surrounded by six nearestneighbor Cl– ions …

… and each Cl– ion

is surrounded by six

nearest-neighbor

Na+ ions.



Figure 2.13



The arrangement of Na؉ ions and Cl؊

ions in a crystal of sodium chloride.

There is no discrete “molecule” of NaCl.

Instead, the entire crystal is an ionic solid.



Charged, covalently bonded groups of atoms, called polyatomic ions, are also

common—ammonium ion (NH4+), hydroxide ion (OH-), nitrate ion (NO3-), and the

doubly charged sulfate ion (SO42-) are examples. You can think of these polyatomic

ions as charged molecules because they consist of specific numbers and kinds of

atoms joined together by covalent bonds, with the overall unit having a positive or

negative charge. When writing the formulas of substances that contain more than

one of these ions, parentheses are placed around the entire polyatomic unit. The formula Ba(NO3)2, for instance, indicates a substance made of Ba2+ cations and NO3polyatomic anions in a 1 : 2 ratio. We’ll say more about these ions in Section 2.12.



60



Chapter 2 ATOMS, MOLECULES, AND IONS

WORKED EXAMPLE 2.10



IDENTIFYING IONIC AND MOLECULAR COMPOUNDS

Which of the following compounds would you expect to be ionic and which molecular

(covalent)?

(b) SF4



(a) BaF2



(c) PH3



(d) CH3OH



STRATEGY



Remember that covalent bonds generally form between nonmetal atoms, while ionic

bonds form between metal and nonmetal atoms.

SOLUTION



Compound (a) is composed of a metal (barium) and a nonmetal (fluorine) and is likely

to be ionic. Compounds (b)–(d) are composed entirely of nonmetals and therefore are

probably molecular.

Ī PROBLEM 2.19 Which of the following compounds would you expect to be ionic and

which molecular (covalent)?



(a) LiBr



(b) SiCl4



(c) BF3



(d) CaO



CONCEPTUAL PROBLEM 2.20 Which of the following drawings is most likely to represent an ionic compound and which a molecular (covalent) compound? Explain.

(a)



(b)



2.12 NAMING CHEMICAL COMPOUNDS



᭡ Morphine, a pain-killing agent found in

the opium poppy, was named after

Morpheus, the Greek god of dreams.



In the early days of chemistry, when few pure substances were known, newly discovered compounds were often given fanciful names—morphine, quicklime, potash,

and barbituric acid (said to be named by its discoverer in honor of his friend Barbara)

to cite a few. Today, with more than 40 million pure compounds known, there would

be chaos unless a systematic method for naming compounds were used. Every

chemical compound must be given a name that not only defines it uniquely but also

allows chemists (and computers) to know its chemical structure.

Different kinds of compounds are named by different rules. Ordinary table salt,

for instance, is named sodium chloride because of its formula NaCl, but common table

sugar (C12H22O11) is named b -D-fructofuranosyl-a-D-glucopyranoside because of special rules for carbohydrates. (Organic compounds often have quite complex

structures and correspondingly complex names, though we’ll not discuss them in

this text.) We’ll begin by seeing how to name simple ionic compounds and then

introduce additional rules in later chapters as the need arises.



Naming Binary Ionic Compounds

Binary ionic compounds—those made of only two elements—are named by identifying first the positive ion and then the negative ion. The positive ion takes the same

name as the element, while the negative ion takes the first part of its name from the

element and then adds the ending -ide. For example, KBr is named potassium bromide: potassium for the K+ ion, and bromide for the negative Br- ion derived from the

element bromine. Figure 2.14 shows some common main-group ions, and Figure 2.15

shows some common transition-metal ions.

LiF

Lithium fluoride



CaBr2

Calcium bromide



AlCl3

Aluminum chloride



2.12 NAMING CHEMICAL COMPOUNDS



18

8A



1

1A

H+

H−

Hydride



2

2A



Li+



Be 2+



Na+



Mg 2+



Al 3+



S 2−

Cl−

Sulfide Chloride



K+



Ca 2+



Ga3+



Se 2−

Br−

Selenide Bromide



Rb+



Sr 2+



In3+



Sn 2+

Sn 4+



Cs+



Ba 2+



Tl+

Tl3+



Pb 2+

Pb 4+



13

3A



14

4A



15

5A



16

6A



O 2−

F−

Oxide Fluoride



N3−

Nitride



Te 2−

I−

Telluride Iodide



Figure 2.14



Main-group cations (blue) and anions (purple). A cation bears the same name as the

element it is derived from; an anion name has an -ide ending.



Figure 2.14 illustrates several interesting points. Note, for instance, that metals

tend to form cations and nonmetals tend to form anions, as mentioned previously in

Section 2.11. Note also that elements within a given group of the periodic table form

similar kinds of ions and that the charge on the ion is related to the group number.

Main-group metals usually form cations whose charge is equal to the group number.

Group 1A elements form singly positive ions (M+, where M is a metal), group 2A elements form doubly positive ions (M2+), and group 3A elements form triply positive

ions (M3+). Main-group nonmetals usually form anions whose charge is equal to the

group number in the U.S. system minus eight. Thus, group 6A elements form doubly

negative ions (6 - 8 = -2), group 7A elements form singly negative ions

(7 - 8 = -1), and group 8A elements form no ions at all (8 - 8 = 0). We’ll see the

reason for this behavior in Chapter 6.

3

3B



4

4B



5

5B



Sc 3+



Ti3+



V3+



Y3+



6

6B



7

7B



Cr2+

Mn2+

Cr3+



8



9

8B



10



11

1B



12

2B



Fe2+

Fe3+



Co2+



Ni2+



Cu+

Cu2+



Zn2+



Ru3+



Rh3+



Pd2+



Ag+



Cd2+



17

7A



Hg2+



Figure 2.15



Common transition metal ions. Only ions that exist in aqueous solution are shown.



Notice also, in both Figures 2.14 and 2.15, that some metals form more than one

kind of cation. Iron, for instance, forms both the doubly charged Fe2+ ion and the

triply charged Fe3+ ion. In naming these ions, we distinguish between them by using

a Roman numeral in parentheses to indicate the number of charges. Thus, FeCl2 is



61



62



Chapter 2 ATOMS, MOLECULES, AND IONS



named iron(II) chloride and FeCl3 is iron(III) chloride. Alternatively, an older method

distinguishes between the ions by using the Latin name of the element (ferrum in the

case of iron) together with the ending -ous for the ion with lower charge and -ic for

the ion with higher charge. Thus, FeCl2 is sometimes called ferrous chloride and

FeCl3 is called ferric chloride. Although still in use, this older naming system is being

phased out and we’ll rarely use it in this book.

Fe2+

Fe3+

Iron(II) ion

Iron(III) ion

Ferrous ion

Ferric ion

(From the Latin ferrum = iron)



᭡ Crystals of iron(II) chloride tetrahydrate

are greenish, and crystals of iron(III)

chloride hexahydrate are brownish yellow.



Sn2+

Sn4+

Tin(II) ion

Tin(IV) ion

Stannous ion

Stannic ion

(From the Latin stannum = tin)



In any neutral compound, the total number of positive charges must equal the

total number of negative charges. Thus, you can always figure out the number of

positive charges on a metal cation by counting the number of negative charges on the

associated anion(s). In FeCl2, for example, the iron ion must be Fe(II) because there

are two Cl- ions associated with it. Similarly, in TiCl3 the titanium ion is Ti(III)

because there are three Cl- anions associated with it. As a general rule, a Roman

numeral is needed for transition-metal compounds to avoid ambiguity. In addition,

the main-group metals tin (Sn), thallium (Tl), and lead (Pb) can form more than one

kind of ion and need Roman numerals for naming their compounds. Metals in group

1A and group 2A form only one cation, however, so Roman numerals are not needed.

WORKED EXAMPLE 2.11



NAMING BINARY IONIC COMPOUNDS

Give systematic names for the following compounds:

(a) BaCl2



(b) CrCl3



(c) PbS



(d) Fe2O3



STRATEGY



Try to figure out the number of positive charges on each cation by counting the number of negative charges on the associated anion(s). Refer to Figures 2.14 and 2.15 as

necessary.

SOLUTION



(a) Barium chloride



No Roman numeral is necessary because barium, a

group 2A element, forms only Ba2+.

(b) Chromium(III) chloride The Roman numeral III is necessary to specify the +3

charge on chromium (a transition metal).

(c) Lead(II) sulfide

The sulfide anion (S2-) has a double negative charge, so

the lead cation must be doubly positive.

(d) Iron(III) oxide

The three oxide anions (O2-) have a total negative charge

of -6, so the two iron cations must have a total charge of

+6. Thus, each is Fe(III).



WORKED EXAMPLE 2.12



CONVERTING NAMES INTO FORMULAS

Write formulas for the following compounds:

(a) Magnesium fluoride



(b) Tin(IV) oxide



(c) Iron(III) sulfide



STRATEGY



For transition metal compounds, the charge on the cation is indicated by the Roman

numeral in the name. Knowing the number of positive charges, you can then figure out

the number of necessary negative charges for the associated anions.

SOLUTION



(a) MgF2 Magnesium (group 2A) forms only a 2+ cation, so there must be two fluoride ions (F-) to balance the charge.



2.12 NAMING CHEMICAL COMPOUNDS



(b) SnO2 Tin(IV) has a +4 charge, so there must be two oxide ions (O2-) to balance the

charge.

(c) Fe2S3 Iron(III) has a +3 charge and sulfide ion a -2 charge (S2-), so there must be

two irons and three sulfurs.

Ī PROBLEM 2.21



(a) CsF



Give systematic names for the following compounds:

(b) K2O

(c) CuO

(d) BaS

(e) BeBr2



Ī PROBLEM 2.22



Write formulas for the following compounds:

(a) Vanadium(III) chloride

(b) Manganese(IV) oxide

(c) Copper(II) sulfide

(d) Aluminum oxide



CONCEPTUAL PROBLEM 2.23 Three binary ionic compounds are represented on the

following periodic table: red with red, green with green, and blue with blue. Name each,

and tell its likely formula.



Naming Binary Molecular Compounds

Binary molecular compounds with covalent bonds are named in much the same way

as binary ionic compounds by assuming that one of the elements in the compound is

more cationlike and the other element is more anionlike. As with ionic compounds,

the cationlike element takes the name of the element itself, and the anionlike element

takes an -ide ending. The compound HF, for example, is called hydrogen fluoride.

HF



Hydrogen is more cationlike because it is farther left in the periodic

table, and fluoride is more anionlike because it is farther right. The compound is therefore named hydrogen fluoride.



We’ll see a quantitative way to decide which element is more cationlike and

which is more anionlike in Section 7.4 but you might note for now that it’s usually

possible to decide by looking at the relative positions of the elements in the periodic

table. The farther left and toward the bottom of the periodic table an element occurs,

the more likely it is to be cationlike; the farther right and toward the top an element

occurs (except for the noble gases), the more likely it is to be anionlike.

More

anionlike

More

cationlike

The following examples show how this generalization applies:

CO

CO2

PCl3

SF4

N2O4



Carbon monoxide (C is in group 4A; O is in group 6A)

Carbon dioxide

Phosphorus trichloride (P is in group 5A; Cl is in group 7A)

Sulfur tetrafluoride (S is in group 6A; F is in group 7A)

Dinitrogen tetroxide (N is in group 5A; O is in group 6A)



63



64



Chapter 2 ATOMS, MOLECULES, AND IONS



Numerical Prefixes

for Naming Compounds



TABLE 2.3

Prefix



Meaning



monoditritetrapentahexaheptaoctanona-



1

2

3

4

5

6

7

8

9



deca-



10



Because nonmetals often combine with one another in different proportions to

form different compounds, numerical prefixes are usually included in the names of

binary molecular compounds to specify the numbers of each kind of atom present.

The compound CO, for example, is called carbon monoxide, and CO2 is called carbon dioxide. Table 2.3 lists the most common numerical prefixes. Note that when

the prefix ends in a or o (but not i) and the anion name begins with a vowel (oxide,

for instance), the a or o on the prefix is dropped to avoid having two vowels

together in the name. Thus, we write carbon monoxide rather than carbon

monooxide for CO and dinitrogen tetroxide rather than dinitrogen tetraoxide for

N2O4. A mono- prefix is not used for the atom named first: CO2 is called carbon

dioxide rather than monocarbon dioxide.

WORKED EXAMPLE 2.13



NAMING BINARY MOLECULAR COMPOUNDS

Give systematic names for the following compounds:

(a) PCl3



(b) N2O3



(c) P4O7



(d) BrF3



STRATEGY



Look at a periodic table to see which element in each compound is more cationlike (farther to the left or lower) and which is more anionlike (farther to the right or higher).

Then name the compound using the appropriate numerical prefix.

SOLUTION



(a) Phosphorus trichloride

(c) Tetraphosphorus heptoxide

Ī PROBLEM 2.24



(a) NCl3

Ī PROBLEM 2.25



(b) Dinitrogen trioxide

(d) Bromine trifluoride



Give systematic names for the following compounds:

(b) P4O6



(c) S2F2



(d) SeO2



Write formulas for compounds with the following names:



(a) Disulfur dichloride



(b) Iodine monochloride



(c) Nitrogen triiodide

CONCEPTUAL PROBLEM 2.26



Give systematic names for the following compounds:



(a)



(b)



Purple = P, green = Cl



Blue = N, red = O



Naming Compounds with Polyatomic Ions

Ionic compounds that contain polyatomic ions (Section 2.11) are named in the same

way as binary ionic compounds: First the cation is identified and then the anion. For

example, Ba(NO3)2 is called barium nitrate because Ba2+ is the cation and the NO3polyatomic anion has the name nitrate. Unfortunately, there is no simple systematic

way of naming the polyatomic ions themselves, so it’s necessary to memorize the

names, formulas, and charges of the most common ones, listed in Table 2.4. The

ammonium ion (NH4+) is the only cation on the list; all the others are anions.



2.12 NAMING CHEMICAL COMPOUNDS



TABLE 2.4



Some Common Polyatomic Ions



Formula



Name



Formula



Name



Cation

NH4+



Ammonium



Singly charged anions (continued)

NO2Nitrite

NO3Nitrate



Singly charged anions

CH3CO2-



CN

ClOClO2ClO3ClO4H2PO4HCO3-



Acetate

Cyanide

Hypochlorite

Chlorite

Chlorate

Perchlorate

Dihydrogen phosphate

Hydrogen carbonate

(or bicarbonate)



HSO4-



Hydrogen sulfate

(or bisulfate)



OH-



Hydroxide

Permanganate



MnO4-



Doubly charged anions

CO32Carbonate

2CrO4

Chromate

Cr2O72Dichromate

2O2

Peroxide

HPO42Hydrogen phosphate

SO32Sulfite

2SO4

Sulfate

S2O32Thiosulfate

Triply charged anion

PO43Phosphate



Several points about the ions in Table 2.4 need special mention. First, note that

the names of most polyatomic anions end in -ite or -ate. Only hydroxide (OH-),

cyanide (CN-), and peroxide (O22-) have the -ide ending. Second, note that several of

the ions form a series of oxoanions, binary polyatomic anions in which an atom of a

given element is combined with different numbers of oxygen atoms—hypochlorite

(ClO-), chlorite (ClO2-), chlorate (ClO3-), and perchlorate (ClO4-), for example.

When there are only two oxoanions in a series, as with sulfite (SO32-) and sulfate

(SO42-), the ion with fewer oxygens takes the -ite ending and the ion with more oxygens takes the -ate ending.

SO32NO2-



Sulfite ion (fewer oxygens)

Nitrite ion (fewer oxygens)



SO42NO3-



Sulfate ion (more oxygens)

Nitrate ion (more oxygens)



When there are more than two oxoanions in a series, the prefix hypo- (meaning

“less than”) is used for the ion with the fewest oxygens, and the prefix per- (meaning

“more than”) is used for the ion with the most oxygens.

ClOClO2ClO3ClO4-



Hypochlorite ion (less oxygen than chlorite)

Chlorite ion

Chlorate ion

Perchlorate ion (more oxygen than chlorate)



Third, note that several pairs of ions are related by the presence or absence of a

hydrogen. The hydrogen carbonate anion (HCO3-) differs from the carbonate anion

(CO32-) by the presence of H+, and the hydrogen sulfate anion (HSO4-) differs from

the sulfate anion (SO42-) by the presence of H+. The ion that has the additional

hydrogen is sometimes referred to using the prefix bi-, although this usage is now

discouraged; for example, NaHCO3 is sometimes called sodium bicarbonate.

HCO3- Hydrogen carbonate (bicarbonate) ion

HSO4- Hydrogen sulfate (bisulfate) ion



CO32SO42-



Carbonate ion

Sulfate ion



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10 Mixtures and Chemical Compounds; Molecules and Covalent Bonds

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