Difficulties and Errors

Mendeleev knew of only this one exception to the atomic weight arrangement, but today we know there are four other pairs in which the element with the higher atomic number has the lower atomic weight. These are argon-potassium (argon was not discovered until 1894), cobalt-nickel (the difference is only 0.24 atomic mass units, which was within the limits of experimental error in Mendeleev’s time), thorium protactinium (the latter was made in 1940). Had he known of several of these isotope anomalies Mendeleev might have been deterred from using atomic weights as his first principle.

In his classic paper published in the new Russian Journal of General Chemistry (Zh. Russ. Khim. Obshch.) Mendeleev published both forms of the periodic table. The best-known version of his table, however, is the one he published in 1871.

Closer inspection of Mendeleev's periodic table of 1869 (vertical form), which is generally reproduced as the original periodic table, reveals some of the difficulties he had in putting the elements into groups. He spotted that the titanium group needed a final heavy metal member, and predicted that it would be found among titanium ores. This missing element was discovered by the Hungarian chemist George de Hevesy in 1923. It's atomic weight of 178 is very close to that of 180 predicted by Mendeleev. The element is now called hafnium, and is found in what is now called group 4.


The manganese group Mendeleev got entirely wrong, but this is not surprising since the multiple oxidation states displayed by elements at the centre of the d block were of little help in assigning elements to groups. Moreover, the element below manganese is technetium, which has no stable isotopes long-lived enough to have survived since the formation of the Earth. This element was first isolated by Emilio Segrè in 1939 from a sample of molybdenum that had been bombarded by deuterons.

Mendeleev also imagined mercury, with its two common oxidation states of I and II, to be in the same group as copper and silver, even though this meant its coming before gold which has a lower atomic weight in the table. Consequently, Mendeleev felt he had to query the atomic weight of gold, which he put in the boron group under uranium. This last element was wrongly placed because its atomic weight was erroneously thought to be 116, half of what it should be.

Similarly, Mendeleev struggled unsuccessfully to accommodate those other f-block elements which were known at the time : erbium, yttrium, cerium, thorium and didymium. This last element was in fact a mixture of praseodymium and neodymium. Most of these had atomic weights that were wrong in any case, and that was true of indium, which had been discovered in 1863 by Ferdinand Reich and Hieronymus Richter. Had more of its chemistry and had the correct atomic weight been available, then Mendeleev would been able to displace uranium from the boron group and put indium there.

Despite all these errors in his table, and maybe even because of them, we can appreciate the struggle that Mendeleev had on that winter’s day in St Petersburg 116 years ago.

It has even been suggested that Mendeleev realised that there were gaps in the list of elements between H and Li, F and Na and Cl and K. These gaps in the table are now occupied by the noble gasses, discovered by Lord Rayleigh, William Ramsay and Morris Travers in the mid-1890s. Curiously, the first to be isolated, argon, is the one whose atomic weight is out of line, so its position in the table is not immediately obvious. Even before argon was discovered by Ramsey had written to Rayleigh speculating that there should be three gaseous elements that would fit into the periodic table above the three elements of group VIII, iron, cobalt and nickel.

The discovery of argon followed soon after, but its position was clearly not next to fluorine but in the row beneath. However, Ramsay was able to deduce that there must be another inert monatomic gas above argon, and he calculated its atomic weight at about 20. This was isolated by the group in 1898 as neon, mass = 20.18.

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