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By 1750 there were only 17 elements identified
In 1860 Dmitri Mendeleev, Russian chemist, organized the elements in rows of increasing mass number so that elements with similar properties were in the same column
Mendeleev noticed holes in his periodic table and predicted the holes would be filled as more elements were discovered.
Mendeleev could also predict the properties of these missing elements because his periodic table arranged elements with the same properties together.
The Periodic table is arranged with increasing atomic number (number of protons).
Periods – a row on the periodic table
Groups or Families – a column on the periodic table
Periods The periodic table has _______ periods.
Families Elements in similar groups or families have
similar properties This pattern of the periodic properties is called
the periodic law
Atomic Mass – the average of all the isotopes found in nature.
Atomic Mass Problem Chlorine 35 = 75.78% Chlorine 37 = 24.22%
What is their average? 35 x .7578 = 26.523 37 x .2422 = + 8.9614 35.4844
Metals Left side of the periodic table Good conductors Mostly solid at room temp Most are malleable Most are ductile
Nonmetals Right side of the periodic table Poor conductors Most are gases and liquids at room temperature Can be very reactive or very unreactive
Metalloids Elements with properties that are in between
metals and nonmetals
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