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The ever-changing periodic table


Because of evolution, we expect biology to be an open-ended subject, but it seems that chemistry should be more constrained.  So how is it that, eleven years into the 21st century, we still don't have a complete list of the elements?  The most recent formal additions to the Periodic Table of Elements came about in June of this year.  The newest jewels of the realm are unofficially named ununquadium (114) and ununhexium (116).

As everyone learns in Chem 101, the periodic table was first drawn up by Russian chemist Dmitri Mendeleev in 1869.  There were, of course, previous attempts to categorize the elements; Lavoisier published a list of 33 as early as 1789, dividing them into gasses, metals, non-metals and earths, distinctions that are still maintained.  Even the periodicity of the elements had been recognized previously; chemist John Newlands came up with his Law of Octaves, with reference to the musical scale, and was profoundly ridiculed for his insight.  Mendeleev gets the priority for the table because he was clever enough to fill gaps in his version with predictions of elements yet to be discovered.  He also placed elements in order according to their properties and valencies rather than just adhering to their reported atomic weights, which he distrusted.  For instance, in putting tellurium ahead of iodine he correctly ordered them with respect to atomic number, even though the tellurium is actually heavier, using the modern method of averaging the isotopes.  Unlike modern tables, Mendeleev's did not actually list the atomic number; it was not until 1911 that Antonius van den Broek realized that the elements were organized by their nuclear charge. 

Mendeleev's table completely lacked the not-yet-discovered noble gasses, which were tacked on later without disturbing the basic pattern. In the 1940s/50s, nuclear chemist Glenn Seaborg added ten elements to the table -- plutonium (94), americium (95), curium (96), berkelium (97), californium (98) and einsteinium (99) to name a few. He created them through nuclear bombardment.  The current periodic table looks very neat, with element 118 (ununoctium) filling out the column of noble gasses, and the heavy actinides, 89-103, completing a row below the lanthanides.  Is this the end of it then?  Probably not.  Seaborg predicted another 50 elements to come.

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