The INTERNET Database of Periodic Tables
There are hundreds of periodic tables in web space, but there is only one comprehensive database of periodic tables & periodic system formulations. If you know of an interesting periodic table that is missing, please contact the database curator: Dr Mark R Leach.
Johann Dobereiner (1780 - 1849) found 'triads', a sequence of three similar elements, where the middle element has a mass equal to the average of the least and most massive. A brief biography can be found on the Nature website.
The diagram below, updated from here, uses mid-nineteenth century atomic mass information rather than modern data. If atomic numbers (Z) are used (a property unknown in 1850), the triads are exact:
© Mark R. Leach 1999-
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