Mendeleev was incredibly detailed in his arrangement of the then known elements of his original periodic table. Using the known masses of the elements his table was arranged in rows and columns using common characteristics but primarily based upon the atomic mass.
However Mendeleev was astute enough to realize that the patterns of this arrangement were not complete and left gaps in his periodic table in order to compensate for the missing elements. The amazing part of this is how accurate Mendeleev's forecasting came to be as technology improved and new elements eventually filled the gaps that he so astutely left open in his table.
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