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Organizing the Elements History of the Periodic Table 1860’s a Russian Scientist named Dmitri Mendeleev discovered a system for organizing all of the known elements. To help him find a pattern he put all of the known information on individual cards. He listed the elements known properties. Individual Properties Listed Known properties were: melting point, density, color, atomic mass, # of chemical bonds an element can form. Atomic mass is the average mass of one atom of that element. Mendeleev’s Periodic Table Patterns appeared when elements were arranged in order of increasing atomic mass. However, this did not always produce similar groups. So he moved the cards into the group it best fit. This left blank spaces on the table. Mendeleev’s Periodic Table Mendeleev’s Periodic Table Mendeleev predicted that that the blank spaces would be filled by elements that had not yet been discovered. He even predicted the properties of new elements. Published his table in 1869 the new elements were discovered within 16 years. Modern Periodic Table Periodic – means regular, repeated pattern. Modern Periodic Table Properties are repeated in each period or row of the table. Elements are arranged by increasing atomic number – not mass as Mendeleev had previously thought. Reading the Periodic Table 26 - atomic number Fe - elements symbol Iron 55.847 -elements name Atomic mass Organization of the Table An elements properties can be predicted from its location on the table. Across the row or down a column the elements’ properties change in a predictable way. Groups or Families Elements in a column. Numbered 1 on left to 18 on right. Elements in a group have similar characteristics. They have the same number of valence electrons. Periods or Rows Contains a series of elements from different families. The elements have very different properties. 7 periods of elements. 7 electron shells Why it works The table works because it is based on the structure of atoms, especially the valence electrons. Atomic # increases by 1, so it has 1 more valence electron than the previous family. and it has one more proton than the previous atom.