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Chapter 1: Atomic Structure and the Periodic Table 17
VOCABULARY
atomic mass p. 17
periodic table p. 18
group p. 22
period p. 22
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 elements
are shown by the periodic table
KEY CONCEPT
Elements make up the
periodic 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 organization
of the buttons with another team’s
organization.
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 of
the past. They could not place an atom on a pan balance. All they
could 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 same
atomic mass number. Elements have isotopes. When chemists attempt
to measure the mass of an atom, therefore, they are actually finding
the average mass of all its isotopes. The of the atoms of
an element is the average mass of all the element’s isotopes. Even
before chemists knew how the atoms of different elements could be
different, they knew atoms had different atomic masses.
atomic mass
MATERIALS
buttons
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Mendeleev’s Periodic Table
In the early 1800s several scientists proposed systems to organize the
elements based on their properties. None of these suggested methods
worked 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. He
made a set of element cards. Each card contained the atomic mass of
an atom of an element as well as any information about the element’s
properties. Mendeleev spent hours arranging the cards in various
ways, looking for a relationship between properties and atomic mass.
The exercise led Mendeleev to think of listing the elements in a
chart. 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 took
Mendeleev quite a bit of thinking and rethinking to get all the relation-
ships correct, but in 1869 he produced the first of the
elements. We call it the periodic table because it shows a periodic, or
repeating, pattern of properties of the elements. In the reproduction
of Mendeleev’s first table shown below, notice how he placed carbon
(C) and silicon (Si), two elements known for their similarities, in the
same row.
check your reading What organizing method did Mendeleev use?
periodic table
Dmitri Mendeleev (1834–1907)
first published a periodic table
of the elements in 1869.
18 Unit: Chemical Interactions
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Chapter 1: Atomic Structure and the Periodic Table 19
Predicting New Elements
When Mendeleev constructed his table, he left some empty spaces
where 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’s
predictions of unknown elements. Only six years after he published
the table, however, the first of these elements—represented by the
question 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 the
elements by properties and atomic number.
The modern periodic table on pages 20 and 21 differs from Mendeleev’s
table in several ways. For one thing, elements with similar properties
are found in columns, not rows. More important, the elements are not
arranged by atomic mass but by atomic number.
Reading the Periodic Table
Each square of the periodic table gives particular information about
the 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’s
name. It contains one or two letters. Some elements that
have 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 physical
state of the element at room temperature. White letters—such
as the H for hydrogen in the box to the right—indicate a gas.
Blue letters indicate a liquid, and black letters indicate a solid. The
background colors of the squares indicate whether the element is a
metal, nonmetal, or metalloid. These terms will be explained in the
next section.
4
3
2
1
Hydrogen
1.008
1
H
chemical
symbol
atomic
mass
atomic
number
1 2
4
MAIN IDEA WEB
Make a main idea web
to summarize the infor-
mation you can learn
from the periodic table.
name
3
Page 3 of 7
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 Gas
Fe Hg
O
2
3 4 5 6 7 8 9
2
Beryllium
9.012
4
Sodium
22.990
11
3
Magnesium
24.305
12
Potassium
39.098
19
4
Calcium
40.078
20
Scandium
44.956
21
Titanium
47.87
22
Vanadium
50.942
23
Chromium
51.996
24
Manganese
54.938
25
Iron
55.845
26
Cobalt
58.933
27
Rubidium
85.468
37
5
Strontium
87.62
38
Yttrium
88.906
39
Zirconium
91.224
40
Niobium
92.906
41
Molybdenum
95.94
42
Technetium
(98)
43
Ruthenium
101.07
44
Rhodium
102.906
45
Cesium
132.905
55
6
Barium
137.327
56
Lanthanum
138.906
57
Hafnium
178.49
72
Tantalum
180.95
73
Tungsten
183.84
74
Rhenium
186.207
75
Osmium
190.23
76
Iridium
192.217
77
Francium
(223)
87
7
Radium
(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
Cerium
140.116
58
Praseodymium
140.908
59
Neodymium
144.24
60
Promethium
(145)
61
Samarium
150.36
62
Thorium
232.038
90
Protactinium
231.036
91
Uranium
238.029
92
Neptunium
(237)
93
Plutonium
(244)
94
Ce Pr Nd Pm Sm
Th Pa U Np Pu
Lithium
6.941
3
Li
Hydrogen
1.008
1
1
1
H
The Periodic Table of the Elements
20 Unit: Chemical Interactions
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Page 4 of 7
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.
Hydrogen
1.008
1
H
Symbol
Each 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 17
Helium
4.003
2
18
Boron
10.811
5
Carbon
12.011
6
Nitrogen
14.007
7
Oxygen
15.999
8
Fluorine
18.998
9
Neon
20.180
10
Aluminum
26.982
13
Silicon
28.086
14
Phosphorus
30.974
15
Sulfur
32.066
16
Chlorine
35.453
17
Argon
39.948
18
Nickel
58.69
28
Copper
63.546
29
Zinc
65.39
30
Gallium
69.723
31
Germanium
72.61
32
Arsenic
74.922
33
Selenium
78.96
34
Bromine
79.904
35
Krypton
83.80
36
Palladium
106.42
46
Silver
107.868
47
Cadmium
112.4
48
Indium
114.818
49
Tin
118.710
50
Antimony
121.760
51
Tellurium
127.60
52
Iodine
126.904
53
Xenon
131.29
54
Platinum
195.078
78
Gold
196.967
79
Mercury
200.59
80
Thallium
204.383
81
Lead
207.2
82
Bismuth
208.980
83
Polonium
(209)
84
Astatine
(210)
85
Radon
(222)
86
Darmstadtium
(269)
110
Unununium
(272)
111
Ununbium
(277)
112
He
B C N O F Ne
Al Si P S Cl Ar
Ni Cu Zn Ga Ge As Se Br Kr
Pd Ag Cd In Sn Sb Te I Xe
Pt Au Hg Tl Pb Bi Po At Rn
Ds Uuu Uub
Europium
151.964
63
Gadolinium
157.25
64
Terbium
158.925
65
Dysprosium
162.50
66
Holmium
164.930
67
Erbium
167.26
68
Thulium
168.934
69
Ytterbium
173.04
70
Lutetium
174.967
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 1: Atomic Structure and the Periodic Table 21
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22 Unit: Chemical Interactions
Groups and Periods
Elements in a vertical column of the periodic table show similarities
in their chemical and physical properties. The elements in a column
are known as a and they are labeled by a number at the top of
the column. Sometimes a group is called a family of elements, because
these 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 many
other elements and compounds, especially with the elements in
Groups 1 and 2. Although the halogens have some similarities to one
another, 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 but
not 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 the
same 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 of
a 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 are
nonmetals. The chemical properties of the elements show a progression;
similar progressions appear in the periods above and below this one.
Trends in the Periodic Table
Because 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 one
positive charge (1+). Atoms of the elements in Group 2, likewise, can
lose two electrons to form ions with a charge of 2+. At the other side
of the table, the atoms of elements in Group 18 normally do not form
ions at all. Atoms of elements in Group 17, however, often gain one
period.
group,
Chlorine
Cl
Fluorine
F
Bromine
Br
Iodine
I
Astatine
At
Group 17
Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Argon
Na Mg Al Si P S
Chlorine
Cl Ar
Period 3
The elements in Group 17,
the halogens, show many
similarities.
Period 3 contains elements
with a wide range of
properties. Aluminum (Al)
is used to make drink cans,
while argon (Ar) is a gas
used in light bulbs.
Page 6 of 7
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Chapter 1: Atomic Structure and the Periodic Table 23
electron to form a negative ion (1–). Similarly, the atoms of elements
in Group 16 can gain two electrons to form a 2– ion. The atoms of the
elements in Groups 3 to 12 all form positive ions, but the charge
can vary.
Other information about atoms can
be determined by their position in the
table. The illustration to the right
shows how the sizes of atoms vary
across periods and within groups.
An atom’s size is important because it
affects how the atom will react with
another atom.
The densities of elements also
follow a pattern. Density generally
increases from the top of a group to the
bottom. Within a period, however, the
elements at the left and right sides of
the table are the least dense, and the elements in the middle are the
most 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 of
one 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 organized
by atomic number was a victory for all of the scientists like Mendeleev
who 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 CONCEPTS
1. How is the modern periodic
table organized?
2. What information about an
atom’s properties can you read
from the periodic table?
3. How are the relationships of
elements in a group different
from the relationships of
elements in a period?
CRITICAL THINKING
4. Infer Would you expect
strontium (Sr) to be more like
potassium (K) or bromine (Br)?
Why?
5. Predict Barium (Ba) is in
Group 2. Recall that atoms
in Group 1 lose one electron
to form ions with a 1+ charge.
What type of ion does barium
form?
CHALLENGE
6. Analyze Explain how
chemists can state with
certainty that no one will
discover an element between
sulfur (S) and chlorine (Cl).
Atomic size decreases.
Li
H
BBe
Na
Mg
K
Ca
Rb
Sr
Cs
Ba
Al
Si
Ga
Ge
In
Sn
Tl
Pb
P
S
As
Se
Sb
Te
Bi
Po
Cl
Ar
Br
Kr
I Xe
At
Rn
C
N
O F Ne
He
Atomic size is one
property that changes
in a predictable way
across, up, and down
the periodic table.
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