Thursday, 5 January 2012

#12



45Rh
102.906
Rhodium

9F
18.998
Fluorine

4Be
9.012
Beryllium

57La
138.906
Lanthanum

C2H6O Example 1
  H H
  | |
H-C-C-O-H
  | |
  H H


Chemistry – A-level standard.

Education cutbacks are clearly hitting hard – this establishment can only afford a periodic table with four elements, and a hand-drawn one at that. Though not displayed in the correct order, all the information about the elements is correct, with the exception of lanthanum’s atomic weight. According to IUPAC, to three decimal places this should be 138.905 instead of 138.906 (actually 138.90547, so the rounding error is not great). Analysing the history of IUPAC’s regular redefinitions of elements’ atomic weight (as scientists become more accurate in their measurements) it can be deduced that not only is this teacher using a hand-drawn periodic table containing only four elements, but that it is at least seven years old (lanthanum's atomic weight was first published as its current value in 2005) and possibly dates back as far as 1969 (when 138.9055 replaced the previous figure of 138.91). Highlighting tiny discrepancies in atomic weight may seem like nitpicking, but it is this kind of fourth decimal place analysis that led to the discovery of deuterium.

What kind of chemistry can be taught using just these four elements is unclear - even the ancient Greeks thought there were at least five elements (according to Aristotle, air, fire, earth, water and aether - none of them actually elements). Fluorine will bond with pretty much anything, so that’s a good start, but not much useful can be made with them together. Lanthanum fluoride can be used as an ion-sensitive electrode, and beryllium fluoride is used in liquid-fluoride nuclear reactors, but you’d have a hard time making the rest of one with just rhodium and lanthanum. Indeed it is ironic that these impoverished students will be devoting a quarter of their time to studying rhodium, the most expensive element that it's actually possible to buy.

What is also ironic is that none of these elements are actually used in Example 1, which uses the much more common (though sadly unknown to this institution) elements of hydrogen, carbon and oxygen to make ethanol. It would be more useful to give the molecular formula of C2H5OH rather than the empirical one, to distinguish it from dimethyl ether, but the structure shown is correct. It’s just a shame that it will mean as much to these students as lanthanum would have done to Aristotle.

6/10 A good effort in trying economic times.

(Many thanks to Vytautas for sending this picture in.)

3 comments:

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