Sunday, 28 September 2014

Neighbours so different

I have shown the sidebar clip from the periodic table in an earlier geeky post about the genetic code.  The point made was that the periodic table is full of information showing, among much else, that elements in the same column have similar chemical properties. Indeed it was these similaries that drove  Дми́трий Ива́нович Менделе́ев Dimitri Mendeleev to jigsaw the elements into the now familiar table. He used the consistencies of nature thus displayed to predict the existence and properties of elements that nobody had as yet discovered.  The one chemical formula that Joe Public can be guaranteed to recognise is H2O.  The next down in the series H2S is hydrogen "rotten-eggs" sulphide which is toxic and [so] smells rather rank. Chemists say the H2Se smells much worse and that H2Te is so foul that it causes even Marines on parade to ignore the staff sergeant and fan their noses in a demure fashion. Don't have anything to do with Po Polonium because you will die.  If you are normally curious you'll ask yourself why the first member of the series H2O doesn't smell at all.
These chemical trends and similarities are caused by the number of electrons available in the outer shell of each element. It is what makes chemistry an orderly and predictive science, so that methane with one carbon, has many similarities with ethane (2xC), propane (3xC) and butane (4xC) and . . . octane (8xC): they are all used as fuels for example.
Sulphur hexafluoride is inert in a rock solid way so that its residence time in the atmosphere is something like 2000 years.  But the equivalent compounds SeF6 and TeF6 are said to have a repulsive smell and an extremely unpleasant smell respectively.  And what about OF6 you may well ask.  And I may well answer that it doesn't exist because the Oxygen atom isn't big enough or have enough electrons to tolerate 6 Fluorines in close proximity.

But what I really wanted to draw you attention to is that the methane -> ethane equivalent disulfur decafluoride S2F10, far from being inert is a potent toxin and has been investigated as a potential chemical warfare agent being 4x more toxic than phosgene which was used to kill people in WWI. As I mentioned in my post about Sarin, we can be grateful that young Gefreiter A. Hitler was gassed in Flanders - it put the brakes on him authorizing the development more chemical warfare agents. Chemical Final Solution agents continued to be developed. The toxicity of disulfur decafluoride is thought to be caused by the separation of inert SF6 from SF4 which reacts with water in the lungs to form sulphurous acid and hydrofluoric acid. The latter is among the most active acids known and is capable of dissolving glass (it is used in etching), so can you imagine that it will make short and painful work on your lung epithelium.

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