Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader 87 The Periodic Table
SECTION
1
Organizing the Elements
The Periodic Table
Name Class Date
CHAPTER 5
As you read this section, keep these questions in mind:
•
How did Dmitri Mendeleev organize his periodic table?
•
How are the elements arranged in a modern periodic
table?
KEY IDEAS
How Are the Elements Organized?
People organize things so that a particular item is
easier to find. For example, a person may organize her
CDs by the names of the bands. Another person may
organize his books by subject.
In the 1860s, scientists knew of about 60 elements.
A Russian schoolteacher named Dmitri Mendeleev was
trying to organize these elements. Mendeleev listed the
properties of each element on a separate piece of paper.
He then tried to arrange the pieces of paper into rows
and columns so that they formed a pattern.
MENDELEEV’S PERIODIC TABLE
In 1869, Mendeleev published the first periodic table
of the elements. In his periodic table, Mendeleev
arranged elements in rows by increasing atomic mass.
Within a row, elements with lower atomic masses were
on the left. Mendeleev started a new row every time the
chemical properties of the elements repeated. Thus, all
the elements in a column had similar properties.
Each row of Mendeleev’s periodic table represented a repeating pattern.
Because the pattern of chemical properties repeated by rows, all elements
lined up in a column had similar properties.
READING TOOLBOX
Organize Create a Spider
Map that has two legs and
several lines on each leg.
Use the map to compare
Mendeleev’s periodic table
with the modern periodic
table.
READING CHECK
1. Identify What property
organize his periodic table?
SECTION 1
Name Class Date
Organizing the Elements continued
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader 88 The Periodic Table
PREDICTING ELEMENTS
When Mendeleev arranged the elements in a table, he
left gaps, or spaces, in the table. Look again at Mendeleev’s
table on the last page. Notice that Mendeleev included
question marks in his table. The question marks repre-
sented elements with certain properties that scientists had
not discovered yet. Because he saw patterns of chemical
properties, Mendeleev predicted that scientists would
eventually find the elements that filled those gaps.
Mendeleev was not the only person to develop a
periodic table. However, he was the first to use the table
to make predictions. For example, Mendeleev left a space
in his table for an element after silicon. He predicted that
this element would be a gray metal that had a high
melting point. In 1886, the element germanium was
discovered.
Mendeleev’s
prediction
Actual
Property
Atomic mass 70 72.6
Density 5.5 g/cm
3
5.3 g/cm
3
Appearance Dark gray metal Gray metalloid
Melting point High 937 °C
Germanium has the properties similar to those that Mendeleev predicted.
PROBLEMS WITH MENDELEEV’S TABLE
Mendeleev found that some elements did not fit the
pattern in his table. For example, Mendeleev had to place
tellurium (Te) before iodine (I) in his table so that they
fit the pattern of chemical properties. However, when
he switched Te and I, they were no longer in order of
increasing atomic mass.
Mendeleev thought that the values for the atomic
masses of Te and I might be incorrect. He thought that
careful measurements would show that the atomic mass
of Te was actually less than that of I. However, measure-
ments by other scientists showed that the atomic masses
of the two elements were correct. This problem was
finally solved about 40 years later by an English chemist
named Henry Moseley.
READING CHECK
2. Explain Why did
Mendeleev leave gaps in the
periodic table?
READING CHECK
3. Explain Why did
Mendeleev have problems
arranging the elements Te
and I?
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader 89 The Periodic Table
SECTION 1
Name Class Date
Organizing the Elements continued
How Is the Periodic Table Organized Today?
Unlike Mendeleev, Mosely did not organize elements
by increasing atomic masses. Instead, he organized the
elements into a periodic table by atomic number. Recall
that an element’s atomic number is the number of
protons in an atom of the element.
The new way of organizing did not change the
locations of most elements in the periodic table.
However, a few elements, including Te and I, did move.
Although Te has a higher atomic mass than I, it has
a lower atomic number. Thus, Mosely could place Te
before I in the periodic table without disturbing the
pattern of chemical properties.
The modern periodic table has more than 100 elements,
and organizes the elements by atomic number. Because
elements are arranged by atomic number, elements
with similar properties are located in the same column.
Therefore, the properties of the elements on the periodic
table repeat at regular intervals. This principle is known as
the periodic law.
PERIODS AND GROUPS
A horizontal row on the periodic table is called a
period. There are seven periods on the periodic table. The
properties of the elements in a period are different. A
vertical column on the periodic table is called a group. All
the elements in a group have similar chemical properties.
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READING CHECK
4. Identify What property
did Mosely use to organize
the periodic table?
8g^i^XVaI]^c`^c\
5. Compare Which
information helps you
predict the properties of an
element—the period it is in,
or the group it is in? Explain
your answer.
Name Class Date
Section 1 Review
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Reader 90 The Periodic Table
SECTION VOCABULARY
group a vertical column of elements in the
periodic table; elements in a group share
chemical properties
period in chemistry, a horizontal row of
elements in the periodic table
periodic law
the law that states that the
repeating chemical and physical properties of
elements change periodically with the atomic
numbers of the elements
1. Compare Find oxygen, sulfur, and fluorine in the periodic table in the back of the
book. Are the chemical properties of oxygen more similar to those of sulfur or to
those of fluorine? Explain your answer.
2. Identify Complete the table below to describe several elements. Use the periodic
table in the back of the book to help you. Round off atomic masses to the nearest
whole number. For example, change 15.9994 to 16.
Element Symbol Atomic number Atomic mass Period Group
Gold
47
42
Fe
3. Predict If scientists found element 117, into which period and group would
they place it? Identify one element that would have properties similar to those of
element 117.
4. Infer Before 1937, scientists had not found element 43. Chemists predicted the
properties of element 43. How was it possible for chemists to predict these
properties?
