Thursday 11 February 2016

Technetium unmissing

If you did a bit of Googling on Monday, you will have noticed, from the Google-Doodle that it was the birthday of Дми́трий Ива́нович Менделе́ев Dimitri Mendeleev [L auditioning for the part of Fagin in Dickens' Oliver Twist]. 182 this week! He doesn't look a day older than Rasputin . Hilarious in-joke cartoon. Mendeleev was a Russian chemist who has some claim to inventing the periodic table of chemical elements which is such a powerful metaphor for t-shirts and graphical display of pretty much any sort of extensive information: periodic table of . . . farm animals . . . Disney characters  . . . Harry Potter. Like Harry Potter competitors, other earlier and contemporary publications of charts to show chemical regularity by Julius Meyer, and Alexandre-Émile Béguyer de Chancourtois have been left in the dustbin of history, along with Johann Döbereiner's theory of triads and John Newland's theory of Octaves. As with Priestley and Lavoisier with Oxygen, acceptance of these chem-explanatory schemes was often along nationalist lines.

Apparently, a key factor in helping Mendeleev to his solution for regularity in the world of chemistry was the fact that he was a regular player of cards especially solitaire/patience.  When he wrote out cards containing the names, weights and properties of each of  the elements then known., it was natural - to him - shuffle them about the table looking for patterns.  Another idea which helped to gain his schema universal acceptance was the humility to accept that we didn't know everything; that there were elements which chemistry had not yet encountered . . . and to leave gaps/blanks in the growing table of affinities. Because he was a scientist, Mendeleev then went off to form hypotheses about the properties that these missing data might possess. This was also a key part of  Döbereiner's theory of triads: the middle of three elements "in line" would be a sort of average of its two flankers. To one of these missing elements, found between Molybdenum and Ruthenium according to the list of atomic weights, Menedeleev gave the name EkaManganese to acknowledge its similarity to the lighter element.  We now know it as Tc Technetium to acknowledge the fact that it is vanishingly rare in the natural world and almost all our stocks have been created by nuclear transmutation in laboratories and atomic reactors.

It is vanishingly rare in nature because it is the lightest element all of whose isotopes are radioactive; every atom ever created in the broiling chunk of star-matter that became Earth has long since changed into something else.  It was thus impossible for Mendeleev and his peers to verify the hypothesised properties of Tc because they couldn't find enough of the stuff to fill the smallest chemical crucible in the lab.  It was found later, initially by Carlo Perrier and Emilio Segrè in 1937, and later in some abundance among the waste of nuclear piles as one of the breakdown products of Uranium and Plutonium. After nuclear physicists had enough scrabbled together to do the chemistry, they were able to measure the wavelengths of light given off when Technetium was heated to incandescence. When this atomic spectrum was shared with the Astronomy Department, the star-gazers were able to report that many red giant stars, in retirement at the end of their long and active lives, showed evidence of Technetium beneath their skins. This proved that Technetium must have been created anew in those stars because any original aliquot would have long since decayed as it has on Earth. That was interesting and informative because it helped cement the idea that elements heavier than Iron must have been created in stars.  What boggles the mind is that these heavy elements are then supposed to have been redistributed round the universe when the creating star went all supernova at the end of its billion year life.

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