This tile identifies the data on the other tiles.  In case it's not obvious, here's a key to the key: 
 Sym
  | Atomic Symbol
  |  AN
  | Atomic Number
  |  Name
  | Name (American spellings)
  |  AW
  | Atomic Weight
  |  G/CC
  | Density in grams per cubic centimeter (I don't have small lowercase letters....) Elements that are gases have their density in grams per liter, which is marked on the individual tile.
  |  MP C
  | Melting point in degrees Centigrade
  |  BP C
  | Boiling point in degrees Centigrade
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