|The development of the Periodic Table (pre-16)|
Alexandre-Emile Béguyer de Chancourtois
Can France claim the first Periodic Table? Probably not, but a French Geology Professor made a significant advance towards it even though at the time few people were aware of it. How did this happen?
All Periodic Tables list the elements in order of a particular property. A property which can be expressed by a number (such as relative atomic mass) is better than a property which cannot (such as colour). In 1860 a conference was held in Karlsruhe (Germany) which produced a much more accurate list of atomic weights than previously available. (Not only were some earlier values slightly inaccurate, faulty reasoning had led to some being a half or a third of the correct value). See Problems with relative atomic masses. Alexandre Béguyer de Chancourtois was the first person to list the known elements in order of increasing weight of their atoms.
In 1862, before Newlands announced his Law of Octaves and Mendeleev described his Periodic System, de Chancourtois presented a paper to the French Academy of Sciences which was then published in Comptes Rendus, their in-house journal. Even for French speakers it was difficult to understand and the diagram (see right) which would have made his ideas much clearer was omitted although it did later appear in a less widely-read geological pamphlet. It is not surprising then, that chemists in other countries were unaware of his ideas. Indeed they were unrecognised until after Mendeleev's more detailed ideas of a Periodic Table had become accepted and de Chancourtois belatedly pointed out his contribution.
de Chancourtois called his idea vis tellurique or telluric spiral because the element tellurium came in the middle. It was also somewhat appropriate coming from a geologist as the element tellurium is named after the Earth. He plotted the atomic weights on the outside of a cylinder such that one complete turn corresponded to an atomic weight increase of 16. The redrawn image shows only the first two turns which have been unwound.
Note that similar elements appear in the same vertical line. Elements in the same vertical line have atomic weights which differ by 16. For example, lithium's atomic weight is 7, sodium’s is 7 + (16 x 1) = 23 and potassium’s 7 + (16 x 2) = 39. Mathematically these obey the equation:
atomic weight = 7 + 16n where n is a whole number.
In general if m is the atomic weight of the element in the first spiral, then the atomic weights of similar elements are given by
atomic weight = m + 16n
The graph used this table of values.
The graph is effectively atomic weight against atomic weight at an angle of 45º, so no wonder it is a perfect straight line despite any inaccuracy in the data.
Use graph paper, or a graphics programme to draw a complete telluric spiral as far as iron. You could then roll this into a cylinder to form a replica of what de Chancourtois described.