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"Introduction to the Periodic Table"
In 1869, a Russian scientist named Dmitri
Mendeleev made a chart of the known
elements.
The chart is called the periodic table
of elements. The periodic table shows
elements arranged according to atomic
number. Atomic number is the total number
of protons in the nucleus. Each row
across the table is called a period.
There are seven periods. Each element in
the period has the same number of
electron shells.
Each column in the table is called a
group or family of elements.
The two rows of elements shown below the
original table were discovered after
Mendeleev developed his original
organizational table. This table shows
you where they would fit in if we would
add them to the original table. We don't
add them to the table because they would
shift the original organization
Mendeleev created, putting the elements
out of line. Therefore, they are not
highlighted when showing the groups.
Elements in the same group have similar
physical and chemical properties.
They also have the same number of
electrons in their outermost shell.
For example, Group 1A elements are known
as the alkali metals. They are soft, shiny
metals that are good conductors of heat
and electricity. They also have low
melting points. 
All metals are on the left side of the
table.
Nonmetals are on the right side of the
table.
A step-like line separates them.
Each box in the table gives you
important information about the element:
The symbol for the element. The name of
the element. The atomic number. The atomic
weight.
You have completed this learning object
"Introduction to the Periodic Table".
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