Chapter 10: Atomic Structure and the Periodic Table 337

VOCABULARYatomic mass p. 337periodic table p. 338group p. 342period p. 342

BEFORE, you learned

• Atoms have a structure• Every element is made from

a different type of atom

NOW, you will learn

• How the periodic table is organized

• How properties of elementsare shown by the periodic table

KEY CONCEPT

Elements make up theperiodic table.

EXPLORE Similarities and Differences of Objects

How can different objects be organized?

PROCEDURE

With several classmates, organize the buttons into three or more groups.

Compare your team’s organizationof the buttons with another team’sorganization.

WHAT DO YOU THINK?• What characteristics did you

use to organize the buttons?• In what other ways could you

have organized the buttons?

2

1

Elements can be organized by similarities.One way of organizing elements is by the masses of their atoms.Finding the masses of atoms was a difficult task for the chemists ofthe past. They could not place an atom on a pan balance. All theycould do was find the mass of a very large number of atoms of a certain element and then infer the mass of a single one of them.

Remember that not all the atoms of an element have the sameatomic mass number. Elements have isotopes. When chemists attemptto measure the mass of an atom, therefore, they are actually findingthe average mass of all its isotopes. The of the atoms ofan element is the average mass of all the element’s isotopes. Evenbefore chemists knew how the atoms of different elements could bedifferent, they knew atoms had different atomic masses.

atomic mass

MATERIALSbuttons

Mendeleev’s Periodic TableIn the early 1800s several scientists proposed systems to organize theelements based on their properties. None of these suggested methodsworked very well until a Russian chemist named Dmitri Mendeleev(MENH-duh-LAY-uhf) decided to work on the problem.

In the 1860s, Mendeleev began thinking about how he could organ-ize the elements based on their physical and chemical properties. Hemade a set of element cards. Each card contained the atomic mass ofan atom of an element as well as any information about the element’sproperties. Mendeleev spent hours arranging the cards in variousways, looking for a relationship between properties and atomic mass.

The exercise led Mendeleev to think of listing the elements in achart. In the rows of the chart, he placed those elements showing similar chemical properties. He arranged the rows so the atomic masses increased as one moved down each vertical column. It tookMendeleev quite a bit of thinking and rethinking to get all the relation-ships correct, but in 1869 he produced the first of theelements. We call it the periodic table because it shows a periodic, orrepeating, pattern of properties of the elements. In the reproductionof Mendeleev’s first table shown below, notice how he placed carbon(C) and silicon (Si), two elements known for their similarities, in thesame row.

check your reading What organizing method did Mendeleev use?

periodic table

Dmitri Mendeleev (1834–1907)first published a periodic tableof the elements in 1869.

338 Unit 3: Chemical Interactions

Chapter 10: Atomic Structure and the Periodic Table 339

Predicting New ElementsWhen Mendeleev constructed his table, he left some empty spaceswhere no known elements fit the pattern. He predicted that new ele-ments that would complete the chart would eventually be discovered.He even described some of the properties of these unknown elements.

At the start, many chemists found it hard to accept Mendeleev’spredictions of unknown elements. Only six years after he publishedthe table, however, the first of these elements—represented by thequestion mark after aluminum (Al) on his table—was discovered.This element was given the name gallium, after the country France(Gaul) where it was discovered. In the next 20 years, two other elements Mendeleev predicted would be discovered.

The periodic table organizes the atoms of theelements by properties and atomic number.

The modern periodic table on pages 340 and 341 differs fromMendeleev’s table in several ways. For one thing, elements with similarproperties are found in columns, not rows. More important, the ele-ments are not arranged by atomic mass but by atomic number.

Reading the Periodic TableEach square of the periodic table gives particular information aboutthe atoms of an element.

The number at the top of the square is the atomic number,which is the number of protons in the nucleus of an atom of that element.

The chemical symbol is an abbreviation for the element’sname. It contains one or two letters. Some elements thathave not yet been named are designated by temporary three-letter symbols.

The name of the element is written below the symbol.

The number below the name indicates the average atomic mass of all the isotopes of the element.

The color of the element’s symbol indicates the physicalstate of the element at room temperature. White letters—suchas the H for hydrogen in the box to the right—indicate a gas.Blue letters indicate a liquid, and black letters indicate a solid. Thebackground colors of the squares indicate whether the element is ametal, nonmetal, or metalloid. These terms will be explained in thenext section.

4

3

2

1

Hydrogen1.008

1

HH

chemicalsymbol

atomicmass

atomicnumber

1 2

4

MAIN IDEA WEBMake a main idea web to summarize the infor-mation you can learnfrom the periodic table.

name3

Period

Each row of the periodic table is called a period. As read from left to right, one proton and one electron are added from one element to the next.

Group

Each column of the table is called a group. Elements in a group share similar properties. Groups are read from top to bottom.

Metal Metalloid Nonmetal Solid Liquid GasFeFe HgHg O

2

3 4 5 6 7 8 9

2Beryllium

9.012

4

Sodium22.990

11

3Magnesium

24.305

12

Potassium39.098

19

4Calcium40.078

20

Scandium44.956

21

Titanium47.87

22

Vanadium50.942

23

Chromium51.996

24

Manganese54.938

25

Iron55.845

26

Cobalt58.933

27

Rubidium85.468

37

5Strontium

87.62

38

Yttrium88.906

39

Zirconium91.224

40

Niobium92.906

41

Molybdenum95.94

42

Technetium(98)

43

Ruthenium101.07

44

Rhodium102.906

45

Cesium132.905

55

6Barium137.327

56

Lanthanum138.906

57

Hafnium178.49

72

Tantalum180.95

73

Tungsten183.84

74

Rhenium186.207

75

Osmium190.23

76

Iridium192.217

77

Francium(223)

87

7Radium(226)

88

Actinium(227)

89

Rutherfordium(261)

104

Dubnium(262)

105

Seaborgium(266)

106

Bohrium(264)

107

Hassium(269)

108

Meitnerium(268)

109

Be

Na Mg

K Ca Sc Ti V Cr Mn Fe Co

Rb Sr Y Zr Nb Mo Tc Ru Rh

Cs Ba La Hf Ta W Re Os Ir

Fr Ra Ac Rf Db Sg Bh Hs Mt

Cerium140.116

58

Praseodymium140.908

59

Neodymium144.24

60

Promethium(145)

61

Samarium150.36

62

Thorium232.038

90

Protactinium231.036

91

Uranium238.029

92

Neptunium(237)

93

Plutonium(244)

94

Ce Pr Nd Pm Sm

Th Pa U Np Pu

Lithium6.941

3

Li

Hydrogen1.008

1

1

1 HH

The Periodic Table of the Elements

340 Unit 3: Chemical Interactions

Lanthanides & Actinides

The lanthanide series (elements 58–71) and actinide series (elements 90–103) are usually set apart from the rest of the periodic table.

Metals and Nonmetals

This zigzag line separates metals from nonmetals.

Hydrogen1.008

1

HHSymbolEach element has a symbol. The symbol's color represents the element's state at room temperature.

Atomic Mass average mass of isotopes of this element

Atomic Number number of protons in the nucleus of the element

Name

10 11 12

13 14 15 16 17Helium4.003

2

18

Boron10.811

5

Carbon12.011

6

Nitrogen14.007

7

Oxygen15.999

8

Fluorine18.998

9

Neon20.180

10

Aluminum26.982

13

Silicon28.086

14

Phosphorus30.974

15

Sulfur32.066

16

Chlorine35.453

17

Argon39.948

18

Nickel58.69

28

Copper63.546

29

Zinc65.39

30

Gallium69.723

31

Germanium72.61

32

Arsenic74.922

33

Selenium78.96

34

Bromine79.904

35

Krypton83.80

36

Palladium106.42

46

Silver107.868

47

Cadmium112.4

48

Indium114.818

49

Tin118.710

50

Antimony121.760

51

Tellurium127.60

52

Iodine126.904

53

Xenon131.29

54

Platinum195.078

78

Gold196.967

79

Mercury200.59

80

Thallium204.383

81

Lead207.2

82

Bismuth208.980

83

Polonium(209)

84

Astatine(210)

85

Radon(222)

86

Darmstadtium(269)

110

Unununium(272)

111

Ununbium(277)

112

HeHe

B C N O F NeNe

Al Si P S ClCl ArAr

Ni Cu Zn Ga Ge As Se BrBr KrKr

Pd Ag Cd In Sn Sb Te I XeXe

Pt Au HgHg Tl Pb Bi Po At RnRn

Ds Uuu Uub

Europium151.964

63

Gadolinium157.25

64

Terbium158.925

65

Dysprosium162.50

66

Holmium164.930

67

Erbium167.26

68

Thulium168.934

69

Ytterbium173.04

70

Lutetium174.967

71

Americium(243)

95

Curium(247)

96

Berkelium(247)

97

Californium(251)

98

Einsteinium(252)

99

Fermium(257)

100

Mendelevium(258)

101

Nobelium(259)

102

Lawrencium(262)

103

Eu Gd Tb Dy Ho Er Tm Yb Lu

Am Cm Bk Cf Es Fm Md No Lr

Chapter 10: Atomic Structure and the Periodic Table 341

342 Unit 3: Chemical Interactions

Groups and PeriodsElements in a vertical column of the periodic table show similaritiesin their chemical and physical properties. The elements in a columnare known as a and they are labeled by a number at the top ofthe column. Sometimes a group is called a family of elements, becausethese elements seem to be related.

The illustration at the left shows Group 17, commonly referred to as the halogen group. Halogens tend to combine easily with manyother elements and compounds, especially with the elements inGroups 1 and 2. Although the halogens have some similarities to oneanother, you can see from the periodic table that their physical prop-erties are not the same. Fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids at room temperature.Remember that the members of a family of elements are related butnot identical.

Metals like copper can be used to make containers for water.Some metals—such as lithium, sodium, and potassium—however,react violently if they come in contact with water. They are all in thesame group, the vertical column labeled 1 on the table.

Each horizontal row in the periodic table is called a Properties of elements change in a predictable way from one end ofa period to the other. In the illustration below, which shows Period 3,the elements on the far left are metals and the ones on the far right arenonmetals. The chemical properties of the elements show a progression;similar progressions appear in the periods above and below this one.

Trends in the Periodic TableBecause the periodic table organizes elements by properties, an ele-ment’s position in the table can give information about the element.Remember that atoms form ions by gaining or losing electrons.Atoms of elements on the left side of the table form positive ions easily.For example, Group 1 atoms lose an electron to form ions with onepositive charge (1+). Atoms of the elements in Group 2, likewise, canlose two electrons to form ions with a charge of 2+. At the other sideof the table, the atoms of elements in Group 18 normally do not formions at all. Atoms of elements in Group 17, however, often gain one

period.

group,

The elements in Group 17,the halogens, show manysimilarities.

Period 3 contains elementswith a wide range ofproperties. Aluminum (Al)is used to make drink cans,while argon (Ar) is a gasused in light bulbs.

Chapter 10: Atomic Structure and the Periodic Table 343

electron to form a negative ion (1–). Similarly, the atoms of elementsin Group 16 can gain two electrons to form a 2– ion. The atoms of theelements in Groups 3 to 12 all form positive ions, but the charge can vary.

Other information about atoms canbe determined by their position in thetable. The illustration to the rightshows how the sizes of atoms varyacross periods and within groups.An atom’s size is important because itaffects how the atom will react withanother atom.

The densities of elements also follow a pattern. Density generallyincreases from the top of a group to thebottom. Within a period, however, theelements at the left and right sides ofthe table are the least dense, and the elements in the middle are themost dense. The element osmium (Os) has the highest known density,and it is located at the center of the table.

Chemists cannot predict the exact size or density of an atom ofone element based on that of another. These trends, nonetheless, are a valuable tool in predicting the properties of different substances.The fact that the trends appeared after the periodic table was organizedby atomic number was a victory for all of the scientists like Mendeleevwho went looking for them all those years before.

check your reading What are some properties that can be related to position on the periodic table?

KEY CONCEPTS1. How is the modern periodic

table organized?

2. What information about anatom’s properties can you readfrom the periodic table?

3. How are the relationships ofelements in a group differentfrom the relationships of elements in a period?

CRITICAL THINKING4. Infer Would you expect

strontium (Sr) to be more likepotassium (K) or bromine (Br)?Why?

5. Predict Barium (Ba) is inGroup 2. Recall that atoms in Group 1 lose one electron to form ions with a 1+ charge.What type of ion does bariumform?

CHALLENGE6. Analyze Explain how

chemists can state with certainty that no one will discover an element betweensulfur (S) and chlorine (Cl).

Atomic size is one property that changes in a predictable wayacross, up, and down the periodic table.