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Organizing the Elements
Dmitri Mendeleev discovered a set of patterns that applied
to all the elements. He found that some elements have
similar chemical and physical properties.
For example:
fluorine and chlorine are gases form similar compounds
and are very corrosive
Silver and copper are shiny metals that tarnish if exposed
to the air.
Mendeleev looked each element’s melting point, density,
color, atomic mass, and number of chemical bonds
it could form.
Mendeleev noticed that a pattern of properties appeared
when he arranged the elements in order of increasing
atomic mass.
In 1869 he published the first periodic table. He arranged
the 63 known elements according to their atomic mass
and similar properties.
In the modern periodic table, the properties of the elements
repeat in each period or row of the table.
Periodic means “in a regular, repeated pattern.”
New elements were added as they were discovered.
In the early 1900’s the table was rearranged in the
order of atomic number (number of protons).
Atomic number – number of protons
Average Atomic Mass – result of an average of
all the isotopes of an element
Isotope are atoms with the same number of
protons, but a different number of neutrons
found in the nucleus of the atom.
Organization of the Periodic Table
The properties of an element can be predicted from its
location in the periodic table.
Periods are the horizontal rows. As you move across
a period from left to right, properties of the elements
change according to a pattern. There are 7 periods.
Groups are the elements that are in a column. Each
column is called a group or family. There are 18 groups.
Most groups are named for the first element in the
The elements in each group have similar characteristics.