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