Kelter P., Mosher M., Scott A. Chemistry. The Practical Science

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68 Chapter 2 Atoms: A Quest for Understanding

PRACTICE 2.4

Fill in the information missing from the following table.

See Problems 25, 26, 65, and 66.

Some substances, known as ionic compounds, are composed entirely of ions.

Calcium oxide (CaO) is one example. How do we know if the compound we are

examining is an ionic compound? Although the distinction is clear with respect

to the presence of ions in the structure, this is often not apparent at ﬁrst glance

because the formula for the substance doesn’t include charges. However, the

composition of the elements that make up the compound can provide a hint.

Ionic compounds are often formed from the combination of metals and

nonmetals.

Chemists use a

chemical formula, containing element symbols and numbers,

to represent the new combination of atoms or ions known as a chemical com-

pound. A chemical formula is a kind of very precise chemist’s shorthand. It shows

the symbols for the elements found in the compound and the quantity of each of

those atoms or ions. The ratios of the atoms in a chemical formula arise from the

law of multiple proportions. For example, the formula NO tells us that each mol-

ecule of this compound contains one atom of nitrogen and one atom of oxygen.

The formula NO

indicates one atom of nitrogen for every two atoms of oxygen

in the molecule. The formula Sb

tells us that two atoms of antimony (Sb) and

three atoms of sulfur (S) make up the formula unit of the beautiful mineral stib-

nite (see Figure 2.29) used as a lubricant in antifriction alloys. Could we have de-

termined this formula without knowing more than the identity of the ions in the

compound?

When we are considering ionic compounds, the great clue to the ratio is that

the ions must combine in a ratio that makes the compound electrically neutral over-

all. In sodium chloride, for example, each sodium cation has a +1 charge, and

each chloride anion has a –1 charge, so these ions will combine in a 1:1 ratio to

form the compound. Written as individual ions, they are Na

and Cl

−

. When we

write chemical formulas, we generally place any positive ion ﬁrst, so the formula

for sodium chloride is NaCl. Note that the charges are not shown. If no number

follows the symbol of an element in a formula, it is assumed to contain just one

atom of the element, as with CaO, discussed earlier. The formula CaF

, for the

chemical calcium ﬂuoride, indicates that there is one calcium ion for every two

ﬂuoride ions.

Can we predict what the charge of an ion is likely to be in an ionic compound?

As it so often does, the periodic table gives us insight. The main-group elements

gain or lose electrons in predictable and consistent ways when they form ionic

compounds. For example, the metals in Group IIA tend to lose two electrons

when combining with nonmetals. This results in these atoms becoming cations

with a charge of +2. The metals in Group IIIA tend to lose three electrons when

forming ionic compounds, forming cations with a charge of +3. The nonmetals

of Group VA tend to gain three electrons when forming ionic compounds, to

form anions with a –3 charge. The nonmetals of Group VIA tend to gain two

electrons to acquire a charge of –2. Keep in mind that these are tendencies—

strong ones, in fact. However, as with many things in life and the chemistry that

Symbol Protons Neutrons Electrons Charge

___

19 20 18

35 44 1

FIGURE 2.29

Stibnite is a mineral with the chemical

formula Sb

Visualization: Determining

Formulas for Ionic Compounds

Video Lesson: Describing

Chemical Formulas

Tutorial: Determining Formulas

for Ionic Compounds

2.7 Ionic Compounds 69

is a part of it, there are exceptions, especially with main-group metals in Periods 5

and 6. For example, lead commonly exists as Pb

or Pb

, depending on condi-

tions. Still, it is a reasonable strategy to decide how many electrons a main-group

element is likely to gain or lose on the basis of the group to which it belongs. On

the other hand, many of the transition elements of the B groups can form ions

with different charges. A list of some common ions is given in Figure 2.30.A sum-

mary of the charges found on the typical main-group ions is shown in Table 2.6.

EXERCISE 2.5 Determining Formulas

1. The ionic compound used in some sidewalk de-icers is made from calcium

and chlorine. What formula would represent that compound?

2. Magnesium combines with oxygen to give a bright ﬂash of light used in some

ﬂares and ﬁreworks. What formula would represent the resulting compound?

Solution

1. Because Ca is in Group IIA, it will form ions with a charge of +2. Chlorine is

in Group VIIA and so will tend to form ions with a charge of –1. To form an

electrically neutral ionic compound, there must be two chloride ions for every

calcium ion. Therefore, the formula will be CaCl

2. Magnesium is found in Group IIA, so it will form cations with a charge of +2.

Oxygen is found in Group VIA and so will form anions with a charge of –2.

A one-to-one combination produces an electrically neutral compound.

Therefore, the formula will be MgO.

PRACTICE 2.5

What is likely to be the formula of a compound resulting from the atomic combi-

nation of cesium and chlorine?

See Problems 65–70.

3–

2–

–

IIA

IIIB

IVB

VIB

VIIB

VIIIB

IIB

IIIA

IVA

VIA

VIIA

VIIIA

FIGURE 2.30

Some common ions and their locations in the periodic table.

Charges on Typical

Main-Group Ions

Group Most Likely

Number Ionic Charge

IA +1

IIA +2

IIIA +3

IVA +4/−4

VA −3

VIA −2

VIIA −1

VIIIA 0

TABLE 2.6

70 Chapter 2 Atoms: A Quest for Understanding

Ionic compounds most often exist as part of large units such as the three-

dimensional crystals of NaCl that you shake on your french fries. The crystal is

composed of large numbers of sodium ions and chloride ions in equal amounts.

These ionic compounds are typically brittle solids that are difﬁcult to melt. For

example, cesium ﬂuoride (CsF) is an ionic compound with a melting point of

682°C. Ionic compounds vary in their solubility in water. Some, such as table salt

(NaCl), are quite soluble. Others, such as strontium ﬂuoride (SrF

), are fairly in-

soluble. When soluble compounds such as NaCl are placed in water, their ions

can separate and move around relatively independently. These mobile and elec-

trically charged ions allow electricity to be conducted through the resulting

solution.

Molecular formula

FIGURE 2.31

The molecular formula for water

indicates that two hydrogen atoms are

combined with one oxygen atom.

Because water is a molecule, none of

these atoms is an ion.

Sodium chloride.

2.8 Molecules

How is carbon dioxide (CO

) different from sodium chloride (NaCl)—table salt?

Sodium chloride has properties that we associate with ionic compounds, such as

a high melting point and the ability to conduct electricity. Carbon dioxide is a gas

that doesn’t conduct electricity. These two substances are fundamentally different

in their physical properties, and this is an indication of an essential difference in

chemical structure. Sodium chloride is an ionic compound. Carbon dioxide is a

molecule.

Molecules are distinct substances made up of two or more atoms linked to-

gether by sharing electrons between their nuclei, rather than by the transfer of

electrons from one atom to another. We call bonds that are formed by sharing

electrons between atoms

covalent bonds. Many molecules, such as CO

, water

O), and methane (CH

), are composed of only nonmetals that interact via co-

valent bonds. They differ from ionic compounds in that they are not made up of

ions. When we write H

O as the molecular formula for water, we mean that each

molecule contains just two H atoms and one O atom (Figure 2.31). This is in con-

trast to the formulas for ionic compounds, which state the ratio in which ions are

present without saying anything about the exact number of ions in any particu-

lar sample of the compound, because that number is an extensive property that

varies depending on the size of the sample.

Molecules can range in size from diatomic molecules such as N

and O

, the

main components of air, to giant molecules containing many thousands, or even

millions, of atoms. For example, the DNA that makes up the human genome and

the protein molecules that control most of the chemistry of life are giant molec-

ular compounds.

Iso-octane, a molecular compound, is important in the deﬁnition of the oc-

tane rating used to classify different grades of automobile gas. Iso-octane consists

of molecules containing 8 carbon atoms and 18 hydrogen atoms and has the

Visualization: Comparison of a

Molecular Compound and

an Ionic Compound

Video Lesson: CIA

Demonstration: The

Conductivity of Molten Salts

Video Lesson: CIA

Demonstration Conductivity

Apparatus—Ionic versus

Covalent Bonds

Side view of DNA.

2.9 Naming Compounds 71

molecular formula C

. In some cases, we may be interested

only in the smallest whole-number ratio that indicates the molec-

ular formula. The simplest way to use whole numbers to express

the ratio in which carbon and hydrogen atoms are present in iso-

octane is C

. This is known as the empirical formula for octane.

Empirical means “derived from experiment,” and an

empirical for-

mula

is the ratio in which elements are found by experiment to be

present in compounds, regardless of the molecular structure of

the compound. This distinction is shown in Figure 2.32.

Examination of the molecular and empirical formulas for iso-

octane reveals something else about molecules. Even though mol-

ecules are electrically neutral like their ionic counterparts, they

cannot be constructed using the rules for ionic compounds. Re-

member that these are two distinctly different types of com-

pounds. The formula for many ionic compounds can be predicted from the typ-

ical charges found on the ions, especially in the A, or main-group, elements. For

molecules, on the other hand, experimentation is necessary to determine the

empirical and molecular formulas.

One of the themes of our study of chemistry is that small differences in struc-

ture can lead to big differences in chemical behavior. An important example of this

is given by two forms of oxygen, one very common, the other less so. Oxygen is

one of the

diatomic elements, in which the normal state is in the form of mole-

cules composed of two atoms bonded together. Other examples include H

,Cl

,Br

and I

. The diatomic form of oxygen, O

, accounts for approximately

20% of the volume of air and is the form of oxygen we need to stay alive. A dif-

ferent and much less common form of oxygen, O

, contains three oxygen atoms

bonded together. This form of oxygen is called ozone and accounts for just a few

parts per billion of the composition of air. The extra atom of oxygen in ozone re-

sults in an

allotrope, another form of an element with different chemical and physi-

cal properties. Ozone, when it occurs in the upper atmosphere, ﬁlters out haz-

ardous ultraviolet radiation. Yet, when present at the earth’s surface, O

is an

irritant and poison that can cause serious damage to the tissues of the body, in-

cluding the sensitive lining of our lungs, right where O

is the very molecule that

is needed most!

2.9 Naming Compounds

Communication between chemists is vital to effective application of the scientiﬁc

method we discussed in Chapter 1. This communication relies on being able to

convey information in writing, over the phone, and on the Internet. Complicat-

ing matters is that, according to the United Nations, over 6000 languages are

spoken worldwide. How, then, does a chemist in Minden, Nebraska, tell a col-

league in Mexico City about a particular compound? Luckily, some compounds

have “common names” that are in widespread use across a country and around

the world. For instance, benzene (C

, a compound found in petroleum distil-

lates) and taxol (the anticancer compound from Chapter 1) are known by their

common names in many different countries. But this isn’t always true.As Table 2.7

illustrates for water, some compounds are so common that there is a different

name for them in every language. For other compounds, there simply isn’t a

name that is commonly known. And moreover, without a system to follow, it

would be difﬁcult for those naming compounds to devise names that were mean-

ingful to anyone else.

A simple solution to this problem was provided by the International Union of

Pure and Applied Chemists (IUPAC), an organization formed in 1919 that con-

tinues to advise the scientiﬁc community on issues related to chemistry. IUPAC

Empirical formula

Molecular formula

FIGURE 2.32

The empirical formula for hydrogen peroxide only lists the

atoms in their simplest whole-number ratios. The molecular

formula lists all of the atoms that make up the molecule.

Allotropes of oxygen.

Water in Some

Other Languages

Language Name of H

English water

Spanish agua

Hawaiian wai

Urdu pani

Swahili maji

Cebuano pagtubig

German wasser

French eau

Slovac voda

Tagalog tubig

Frisian wetter

Japanese mizu

TABLE 2.7

72 Chapter 2 Atoms: A Quest for Understanding

has a set of rules that all scientists try to follow when naming compounds. Known

as the rules of

chemical nomenclature, they establish patterns to follow in naming

existing compounds as well as compounds that have yet to be determined. The

rules for naming

binary covalent compounds (compounds composed of only two

nonmetal elements, which have atoms that interact via covalent bonds) are listed

in Table 2.8.

Consider the formula for NCl

. This compound is a covalent compound (it

has only covalent bonds) and is a binary compound (it has only two elements).

Using the rules in Table 2.8, we write

nitrogen

as the ﬁrst step of the rules for chemical nomenclature. In the second step, we

write

nitrogen chloride

Then, to ﬁnish the name of the molecule, we add the preﬁxes. Note that we do

not add the preﬁx mono- to the ﬁrst element name. If it is not stated, mono- is

assumed:

nitrogen trichloride

The following examples give you an opportunity to see these rules in practice:

dinitrogen trioxide

HCl hydrogen chloride, not monohydrogen monochloride

CO carbon monoxide; mono- is needed here

carbon dioxide

sulfur tetraﬂuoride

sulfur hexaﬂuoride

ClO

chlorine dioxide

dichlorine heptoxide

tetraphosphorus hexoxide

disulfur dichloride

EXERCISE 2.6 Naming Binary Molecular Compounds

Provide the IUPAC name for these binary molecular compounds:

PCl

NBr

AsCl

XeO

Solution

PCl

phosphorus pentachloride XeO

xenon tetroxide

NBr

nitrogen tribromide P

tetraphosphorus trisulﬁde

AsCl

arsenic trichloride

Rules for Naming Binary Covalent Compounds

The name of the compound mentions the elements in the order given in the formula.

These rules apply to binary molecular compounds—those containing two elements that

are both nonmetals.

1. Name the ﬁrst element using the exact element name.

2. Name the second element by writing the stem name of the element with the sufﬁx -ide.

3. Add preﬁxes as shown in the list at the right, derived from Greek, to each element

name to denote the subscript of the element in the formula. Generally, the preﬁx

mono- is used only on the second element in the binary compound to distinguish it

from other examples containing multiple atoms.

TABLE 2.8

Number Preﬁx

1mono

2di

3tri

4tetra

5 penta

6hexa

7hepta

8octa

9nona

10 deca

Video Lesson: Naming Chemical

Compounds

PRACTICE 2.6

Provide either the name or the formula for each of these binary molecular com-

pounds:

sulfur tetraﬂuoride carbon tetrachloride diphosphorus pentoxide

PCl

OOF

See Problems 77 and 78.

2.9 Naming Compounds 73

Rules for Naming Binary Ionic Compounds

The name of the compound has the elements in the order given in the formula.

1. Name the ﬁrst element using the exact element name.

2. Name the second element by writing the stem name of the element with the sufﬁx

-ide.

3. For metals that can have more than one stable charge, report the charge as part of

the name. Do this by writing the charge, in roman numeral form, in parentheses

immediately after the metal’s name.

TABLE 2.9

The rules for naming binary ionic compounds (compounds typically composed

of a metal and a nonmetal element, which have ions that interact via electrostatic

attractions) are similar to those for the binary molecular compounds. To sum-

marize the rules in Table 2.9, a binary ionic compound can be named by stating

the name of the atom that has become a cation (positive ion) and then adding the

name of the anion (negative ion). The anion derives its name from the parent

atom but ends in -ide. The name for a binary ionic compound does not tell us

how many of each type of ion are present. How do we tell, from the name, how

many of each type of ion are in the formula unit? Remember that ionic com-

pounds contain ions whose charges must balance to result in an electrically

neutral compound. Just use the charges to determine the number of each ion.

Sodium chloride (NaCl) offers a good example. We can tell that it is likely to

be an ionic compound because it is composed of a main-group metal (Na) and a

nonmetal (Cl). The procedure outlined in Table 2.9 indicates that we ﬁrst give the

name of the metal, the element listed ﬁrst in the chemical formula. Therefore, we

begin with

sodium

Then we add the name of the other element, which yields

sodium chloride

Because we know that sodium exists only as an ion with a +1 charge, we do not

need to do step 3 of the rules. According to the IUPAC rules of chemical nomen-

clature, the name for NaCl, sodium chloride, is exactly as we expected.

EXERCISE 2.7 Naming Binary Ionic Compounds

Use the rules outlined in Table 2.9 to

supply each name or formula missing

from the list on the right.

Solution

Name Formula

sodium ﬂuoride NaF

calcium oxide CaO

aluminum sulﬁde Al

barium chloride BaCl

Name Formula

sodium ﬂuoride

calcium oxide

BaCl

74 Chapter 2 Atoms: A Quest for Understanding

Iron(II) chloride

FeCl

Iron(III) chloride

FeCl

PRACTICE 2.7

Supply each name or formula missing from the following list.

See Problems 79 and 80.

Let’s examine some cases where we need to use step 3 in the rules for naming

ionic compounds. One such case involves the metals found between Groups IIA

and IIIA in the periodic table. Most of these transition metals commonly exhibit

more than one positively charged state. It is therefore possible for two bottles of

iron chloride to contain two different ionic compounds. You might ﬁnd FeCl

one bottle and FeCl

in the other. If we considered only the ﬁrst two steps in

Table 2.9, we would conclude that they should both be named iron chloride.

However, the ﬁrst bottle contains an iron ion with a +2 charge. (Remember, it

needs to be Fe

in order to balance the charges of the two Cl

−

atoms.) The sec-

ond bottle contains Fe

for similar reasons. To name these two compounds in

such a way as to distinguish between them, we follow step 3 in Table 2.9 and add

the charge on the iron to the name of the compound. Thus FeCl

becomes

iron(II) chloride, and the other bottle contains iron(III) chloride.

EXERCISE 2.8 Naming Additional Compounds

Use Table 2.9 to supply each name or formula missing from the following list.

First Thoughts

To supply these names, we need to be familiar with the rules outlined in Table 2.9.

More important, we will have to consult Figure 2.30 to determine the charges on

each ion. For those with more than one charge, we must follow step 3 in Table 2.9.

Solution

Formula Name

BaCl

barium chloride

TiCl

titanium(II) chloride

CuO copper(II) oxide

MnO

manganese(IV) oxide

Further Insight

Writing the names of compounds requires, ﬁrst of all, that we understand the na-

ture of the atoms involved in the formulas. Often, it is easy to forget that ionic com-

pounds and molecular compounds are named using different rules. We wouldn’t

want to accidentally name CuO as copper monoxide. This implies that the formula

Formula Name

BaCl

TiCl

copper(II) oxide

manganese(IV) oxide

Name Formula

magnesium chloride

lithium ﬂuoride

NaBr

2.9 Naming Compounds 75

represents a molecule. And, as we saw earlier, molecules and ionic compounds have

markedly different properties.

PRACTICE 2.8

Use Table 2.9 to supply each name or formula missing from the list.

See Problems 83, 84, 89, and 90.

Polyatomic Ions

Many compounds are made up of more than two elements. Some of these are still

classiﬁed as ionic compounds, because they involve the association of ions. In

some cases, the ion itself contains two or more atoms. The entire ion that results

can carry a positive or negative charge and behave just like a monatomic ion. Ions

such as this are known as

polyatomic ions or molecular ions. Figure 2.33 shows the

structure of some examples. Table 2.10 lists many others.

Polyatomic ions are everywhere. For example, the famous White Cliffs of

Dover on the southern end of England are made of chalk, the same stuff we use

to write on chalkboards. Chalk has the formula CaCO

and is made up of two

ions, Ca

and CO

2−

Using Table 2.10, we can arrive at the name calcium carbonate for this com-

pound. The CO

2−

ion in the compound is the polyatomic “carbonate” ion with

a charge of −2. The calcium and carbonate ions combine to form the electrically

neutral ionic compound calcium carbonate, but there are covalent bonds within

the structure of the carbonate ions.

One common garden fertilizer has the formula (NH

)

. The ammonium

ion, NH

, has a +1 charge, and the phosphate ion, PO

3−

, has a −3 charge.

Three ammonium ions will be present for each phosphate ion. We must use

parentheses in the formula to indicate that our subscript 3 applies to the entire

ammonium ion, not just to the hydrogen atoms within it. What is the name of

this compound? It is known as ammonium phosphate.

EXERCISE 2.9 Naming with Polyatomic Ions

Provide each name or formula missing from the table.

Solution

Formula Name

CaC

calcium oxalate

Mg(NO

)

magnesium nitrate

CuSO

copper(II) sulfate

Al(OH)

aluminum hydroxide

CaC

Mg(NO

)

copper(II) sulfate

aluminum hydroxide

Formula Name

CuCl

CrO

nickel(II) oxide

palladium(IV) sulﬁde

Sulfate

2–

FIGURE 2.33

Three polyatomic ions. These ions are

made up of chemically bonded atoms.

As a whole, they act as ions.

Application

Cyanide

–

Hydroxide

–

76 Chapter 2 Atoms: A Quest for Understanding

Selected Polyatomic Ions

Formula Name Charge for Ion

Cation with a NH

ammonium +1

+1 Charge

Anions with a CH

COO

−

acetate −1

−1 Charge

ClO

−

hypochlorite −1

ClO

−

chlorite −1

ClO

−

chlorate −1

ClO

−

perchlorate −1

−

cyanide −1

HCO

−

hydrogen carbonate (bicarbonate) −1

−

dihydrogen phosphate −1

HSO

−

hydrogen sulfate(bisulfate) −1

HSO

−

hydrogen sulﬁte (bisulﬁte) −1

MnO

−

permanganate −1

−

nitrite −1

−

nitrate −1

−

hydroxide −1

Anions with a CO

2−

carbonate −2

−2 Charge

2−

oxalate −2

CrO

2−

chromate −2

2−

dichromate −2

HPO

2−

monohydrogen phosphate −2

2−

sulﬁte −2

2−

sulfate −2

2−

peroxide −2

2−

thiosulfate −2

Anions with a PO

3−

phosphate −3

−3 Charge

TABLE 2.10

PRACTICE 2.9

What is the name of the compound with the formula KMnO

? What is the formula

for ammonium dichromate?

See Problems 81, 82, and 85–88.

2.10 Naming Acids

Some compounds, known as acids, are often written with a hydrogen at the start

of their formula. These compounds have special names when they are in aqueous

solutions. For those that contain only hydrogen and a halogen, the name is

hydro____ic acid, where the blank is replaced by the root name of the halogen.

For instance, aqueous HCl is known as hydrochloric acid. The names of other

acids come from the name of the polyatomic ion, by replacing the -ate ending of

the ion with -ic acid or the -ite ending with -ous acid. For example, HNO

is nitric

acid and HClO is known as hypochlorous acid.

Key Words 77

The Bottom Line

■

All matter is composed of atoms. (Section 2.1)

■

The law of conservation of mass states that the mass

of the chemicals at the start of a reaction is equal to

the mass of the chemicals at the end of the reaction.

(Section 2.1)

■

The law of deﬁnite composition states that any par-

ticular chemical is always composed of its compo-

nents in a ﬁxed ratio, by mass. (Section 2.1)

■

The law of multiple proportions states that when

the same elements can produce more than one com-

pound, the ratio of the masses of the element that

combine with a ﬁxed mass of another element corre-

sponds to a small whole number. (Section 2.2)

■

Dalton’s atomic theory stated that every substance is

made of atoms; atoms are indestructible; atoms of

any one element are identical; atoms of different ele-

ments differ in their masses; and chemical changes

involve rearranging the attachments between atoms.

(Section 2.2)

■

The law of combining volumes states that when gases

combine, they do so in small whole-number ratios,

provided that all the gases are at the same tempera-

ture and pressure. (Section 2.2)

■

Atoms are composed of electrons, protons, and neu-

trons. The most common isotope of the hydrogen

atom is the only exception. It contains only electrons

and protons. (Section 2.3)

■

The modern model of the atom indicates a tiny, but

dense, positively charged nucleus surrounded by a

diffuse electron cloud. (Section 2.3)

■

Nuclei can contain both positively charged protons

and neutral neutrons. Isotopes of the same element

differ in their number of neutrons. All atoms of

the same element have the same atomic number

(the same number of protons). (Section 2.4)

■

Atoms are electrically neutral and contain equal

numbers of protons and electrons. Ions are charged

particles and are formed from atoms or groups of

atoms transferring electrons to other atoms or

groups of atoms. Cations are positively charged ions,

and anions are negatively charged ions. (Section 2.4)

■

The atomic mass of an element is the weighted aver-

age of all the isotopes of that element. (Section 2.5)

■

The periodic table of the elements lists all the known

elements, arranged into periods and groups in a

manner that reﬂects the chemical characteristics that

particular elements share. (Section 2.6)

■

A chemical formula makes use of the atomic sym-

bols and subscripts, as needed, to represent the

atoms in a chemical compound. (Section 2.7)

■

Molecules are distinct substances made up of two

or more atoms linked together by sharing electrons

between their nuclei, rather than by the transfer of

electrons from one atom to another. We call bonds

that are formed by the sharing of electrons between

atoms covalent bonds. (Section 2.8)

■

There are systematic rules for naming compounds

and listing their formulas. (Section 2.9)

Key Words

alkali metals The highly reactive metals found in Group

IA of the periodic table, which form alkalis upon re-

action with water. (p. 66)

alkaline earth metals The metals found in Group IIA of

the periodic table. (p. 66)

allotropes Forms of an element that have very different

chemical and physical properties, such as the al-

lotropes O

and O

.(p. 71)

alpha particle A fast-moving nucleus of helium (two

protons and two neutrons) emitted during the decay

of a radioactive element. (p. 54)

anion Ions with a negative charge. (p. 56)

atomic number (Z) The number of protons in the nucleus

of an atom. (p. 56)

atomic mass unit The arbitrary unit of mass for ele-

ments based on the isotope carbon-12. One atom of

carbon-12 is deﬁned to have a mass of exactly

12.0000 amu. (p. 60)

beta particle A fast-moving electron emitted during the

decay of a radioactive element. The beta particle orig-

inates from the nucleus of the decaying element.

(p. 54)

binary covalent compounds Compounds composed of

only two nonmetal elements. (p. 72)

binary ionic compounds Compounds typically composed

of a metal and nonmetal element, which have ions

that interact via electrostatic attractions. (p. 73)

cations Ions with a positive charge. (p. 56)

chalcogens The elements found in Group VIA of the

periodic table of elements. (p. 66)