6 Molecules, Compounds, and Formulas

c h a p t er 2 Atoms, Molecules, and Ions



© Cengage Learning/Charles D. Winters



66



Lanthanides. If you use a Bunsen

burner in the lab, you may light it

with a “flint” lighter. The flints are

composed of iron and “mischmetal,”

a mixture of lanthanide elements,

chiefly Ce, La, Pr, and Nd with traces

of other lanthanides. (The word

mischmetal comes from the German

for “mixed metals.”) It is produced by

the electrolysis of a mixture of lanthanide oxides.



Stretching between Groups 2A and 3A is a series of elements called the transition elements. These fill the B-groups (1B through 8B) in the fourth through the

seventh periods in the center of the periodic table. All are metals (Figure 2.4), and

13 of them are in the top 30 elements in terms of abundance in the earth’s crust

(Table 2.3). Most occur naturally in combination with other elements, but a few—

copper (Cu), silver (Ag), gold (Au), and platinum (Pt)—are much less reactive and

so can be found in nature as pure elements.

Virtually all of the transition elements have commercial uses. They are used as

structural materials (iron, titanium, chromium, copper); in paints (titanium, chromium); in the catalytic converters in automobile exhaust systems (platinum and

rhodium); in coins (copper, nickel, zinc); and in batteries (manganese, nickel, zinc,

cadmium, mercury).

A number of the transition elements play important biological roles. For example, iron, a relatively abundant element (Table 2.3), is the central element in the

chemistry of hemoglobin, the oxygen-carrying component of blood.

Two rows at the bottom of the table accommodate the lanthanides [the series of

elements between the elements lanthanum (Z = 57) and hafnium (Z = 72)] and

the actinides [the series of elements between actinium (Z = 89) and rutherfordium

(Z = 104)]. Some lanthanide compounds are used in color television picture tubes,

uranium (Z = 92) is the fuel for atomic power plants, and americium (Z = 95) is

used in smoke detectors.

rEvIEW & cHEcK FOr SEctIOn 2.5

1.



Which of the following elements is a metalloid?

(a)



2.



(c)



Be



(d) Al



Si



(b) Sc



(c)



V



(d) N



What is the most abundant element in the Earth’s crust?

(a)



4.



(b) S



What is the symbol for the element in the third period and the fourth group?

(a)



3.



Ge



Fe



(b) C



(c)



O



(d) N



What is the name given to elements that exist in different forms, such as graphite, diamond,

and buckyballs?

(a)



isotopes



(b) isomers



(c)



allotropes



(d) nonmetals



2.6 Molecules,Compounds,andFormulas

A molecule is the smallest identifiable unit into which some pure substances like

sugar and water can be divided and still retain the composition and chemical properties of the substance. Such substances are composed of identical molecules consisting of two or more atoms bound firmly together. For example, atoms of aluminum, Al, combine with molecules of bromine, Br2, to produce a molecule of the

compound aluminum bromide, Al2Br6 (Figure 2.12).

2 Al(s) + 3 Br2(ℓ) → Al2Br6(s)

aluminum + bromine → aluminum bromide



To describe this chemical change (or chemical reaction) on paper, the composition

of each element and compound is represented by a symbol or formula. Here one

molecule of Al2Br6 is composed of two Al atoms and six Br atoms.



Formulas

For molecules more complicated than water, there is often more than one way to

write the formula. For example, the formula of ethanol (also called ethyl alcohol)

can be represented as C2H6O (Figure 2.13). This molecular formula describes the



kotz_48288_02_0050-0109.indd 66



11/18/10 2:05 PM



2.6  Molecules, Compounds, and Formulas



67



Photos © Cengage Learning/

Charles D. Winters







(a)

(a) Solid aluminum and (in the beaker) liquid



(b)

(b) When the aluminum is added to the



(c)

(c) The reaction produces white, solid alu-



bromine.



bromine, a vigorous chemical reaction

occurs.



minum bromide, Al2Br6.



FIGURE 2.12   Reaction of the elements aluminum and bromine.

composition of ethanol molecules—two carbon atoms, six hydrogen atoms, and

one atom of oxygen occur per molecule—but it gives us no structural information.

Structural information—how the atoms are connected and how the molecule fills

space—is important, however, because it helps us understand how a molecule can

interact with other molecules, which is the essence of chemistry.

To provide some structural information, it is useful to write a condensed formula, which indicates how certain atoms are grouped together. For example, the

condensed formula of ethanol, CH3CH2OH (Figure 2.13), informs us that the molecule consists of three “groups”: a CH3 group, a CH2 group, and an OH group. Writing the formula as CH3CH2OH also shows that the compound is not dimethyl ether,

CH3OCH3, a compound with the same molecular formula but with a different structure and distinctly different properties.

That ethanol and dimethyl ether are different molecules is further apparent

from their structural formulas (Figure 2.13). This type of formula gives us an even

higher level of structural detail, showing how all of the atoms are attached within a

molecule. The lines between atoms represent the chemical bonds that hold atoms

together in this molecule [▶ Chapter 8].



NAME



Ethanol



MOLECULAR

FORMULA

C2H6O



CONDENSED

FORMULA



STRUCTURAL

FORMULA



MOLECULAR MODEL



H H



CH3CH2OH



H



C



C



O



C2H6O



CH3OCH3



carbon atoms



H



C

H



hydrogen atoms



H



H

O



C



H



H



FIGURE 2.13   Four approaches to showing molecular formulas.  Here the two molecules have

the same molecular formula. However, once they are written as condensed or structural formulas, or

illustrated with a molecular model, it is clear that these molecules are different.



kotz_48288_02_0050-0109.indd 67



• Standard Colors for Atoms in

Molecular Models  The colors listed

here are used for molecular models

in this book and are generally used

by chemists.



H



H H

Dimethyl

ether



• Writing Formulas  When writing

molecular formulas of organic compounds (compounds with C, H, and

other elements) the convention is to

write C first, then H, and finally other

elements in alphabetical order. For example, acrylonitrile, a compound used

to make consumer plastics, has the

condensed formula CH2CHCN. Its molecular formula would be C3H3N.



oxygen atoms

nitrogen atoms



chlorine atoms



11/18/10 2:06 PM



68



c h a p t er 2   Atoms, Molecules, and Ions

Mehau Kulyk/Science Photo Library/Photo Researchers, Inc.; model by S.M. Young



FIGURE 2.14   Ice.  Snowflakes

are six-sided structures, reflecting the

underlying structure of ice. Ice consists of six-sided rings formed by

water molecules, in which each side

of a ring consists of two O atoms and

an H atom.



Molecular Models



© Cengage Learning/Charles D. Winters



Molecular structures are often beautiful in the same sense that art is beautiful, and

there is something intrinsically beautiful about the pattern created by water molecules assembled in ice (Figure 2.14).

More important, however, is the fact that the physical and chemical properties of a molecular compound are often closely related to its structure. For example, two well-known features of ice are related to its structure. The first is the

shape of ice crystals: The sixfold symmetry of macroscopic ice crystals also appears at the particulate level in the form of six-sided rings of hydrogen and oxygen atoms. The second is water’s unique property of being less dense when it is

solid than when it is liquid. The lower density of ice, which has enormous consequences for earth’s climate, results from the fact that molecules of water are not

packed together tightly in ice.

Because molecules are three dimensional, it is often difficult to represent their

shapes on paper. Certain conventions have been developed, however, that help represent three-dimensional structures on two-dimensional surfaces. Simple perspective drawings are often used (Figure 2.15).

Several kinds of molecular models exist. In the ball-and-stick model, spheres,

usually in different colors, represent the atoms, and sticks represent the bonds

holding them together. These models make it easy to see how atoms are attached

to one another. Molecules can also be represented using space-filling models.

These models are more realistic because they offer a better representation of

relative sizes of atoms and their proximity to each other when in a molecule. A

disadvantage of pictures of space-filling models is that atoms can often be hidden

from view.



H

H



C

H



H



Simple perspective

drawing



Plastic model



Ball-and-stick model



Space-filling model



All visualizing techniques

represent the same molecule.



FIGURE 2.15   Ways of depicting a molecule, here the methane (CH4) molecule.



kotz_48288_02_0050-0109.indd 68



11/18/10 2:06 PM



69



2.7 Ionic Compounds: Formulas, Names, and Properties







rEvIEW & cHEcK FOr SEctIOn 2.6

Cysteine, whose molecular model and structural formula are illustrated here, is an important

amino acid and a constituent of many living things. What is its molecular formula?



+



NH3 H



−



O

C

O



Molecular model



(a)



C3H6O2S



C



C



H



H



S



H



Structural

al fform

formula

ula



(b) C3H7NO2S



(c)



C3H7N2OS



(d) C3H7NO2



2.7 IonicCompounds:Formulas,Names,

andProperties

The compounds you have encountered so far in this chapter are molecular

compounds—that is, compounds that consist of discrete molecules at the particulate level. Ionic compounds constitute another major class of compounds. They

consist of ions, atoms, or groups of atoms that bear a positive or negative electric

charge. Many familiar compounds are composed of ions (Figure 2.16). Table salt, or

sodium chloride (NaCl), and lime (CaO) are just two. To be able to recognize ionic

compounds and to write formulas for these compounds, it is important to know the

formulas and charges of common ions. You also need to know the names of ions

and be able to name the compounds they form.



Modules 2: Predicting Ion Charges

and 3: Names to Formulas of Ionic

Compounds cover concepts in this

section.



Ions

Atoms of many elements can gain or lose electrons in the course of a chemical reaction. To be able to predict the outcome of chemical reactions [▶ Chapter 3], you need

to know whether an element will likely gain or lose electrons and, if so, how many.



Cations

If an atom loses an electron (which is transferred to an atom of another element in

the course of a reaction), the atom now has one fewer negative electrons than it has

positive protons in the nucleus. The result is a positively charged ion called a cation



FIGURE2.16 Some common ionic compounds.



Calcite

Fluorite

Gypsum



Hematite

Orpiment



kotz_48288_02_0050-0109.indd 69



Hematite, Fe2O3

Name



Formula



Ions Involved



Calcium

carbonate

Calcium

fluoride

Calcium

sulfate

dihydrate

Iron(III)

oxide

Arsenic

sulfide



CaCO3



Ca2+, CO32−



CaF2



Ca2+, F−



CaSO4 ∙ 2 H2O



Ca2+, SO42−



Fe2O3



Fe3+, O2−



As2S3



As3+, S2−



© Cengage Learning/Charles D. Winters



Common

Name



Gypsum, CaSO4 ⋅ 2 H2O



Calcite, CaCO3



Fluorite, CaF2



Orpiment, As2S3



11/18/10 2:06 PM