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12/19/05
4:41 PM
MAZER
Page 209
Predicting New Elements
When Mendeleev constructed his table, he left some empty spaces
where no known elements fit the pattern. He predicted that new elements that would complete the chart would 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 was discovered. This
element was given the name gallium, after the country France (whose
ancient name was Gallia) where it was discovered. In the next 20
years, two other elements Mendeleev predicted were discovered.
The periodic table organizes the atoms of the
elements by properties and atomic number.
MAIN IDEA WEB
Make a main idea web
to summarize the information you can learn
from the periodic table.
The modern periodic table on pages 210–211 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.
1
The number at the top of the square is the atomic number.
This is the number of protons in the nucleus of an atom of
that element.
2
The chemical symbol is an abbreviation for the element’s
name. It contains one or two letters. A few elements that
have not yet been named are designated by temporary
three-letter symbols.
3
The name of the element is written below the symbol.
4
The number below the name indicates the average
atomic mass of all the isotopes of the element.
1
The color of the element’s symbol indicates the physical
3
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 show whether the element is a
metal, a nonmetal, or a metalloid. These terms will be explained in
the next section.
2
atomic
number
chemical
symbol
1
H
Hydrogen
1.008
name
4
atomic
mass
Chapter 7: Atomic Structure and the Periodic Table 209
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