New Chemical Elements and their Periodicity

On Thursday 22 March 2007 the Society for the History of Alchemy and Chemistry held a joint meeting with the Royal Society of Chemistry Historical Group, at the Royal Society of Chemistry, Burlington House, Piccadilly. The meeting entitled New Chemical Elements and their Periodicity attracted over 50 attendees and was timed to coincide with the one hundredth anniversary of Mendeleev's death, which took place on 2 February 1907. It also celebrated the bicentenary of the isolation of potassium and sodium by Humphrey Davy. It focussed on two underlying themes and the interplay between them. Firstly, the techniques used to effect the characterisation of the elements, for example, electrochemistry, atomic spectroscopy, and radiochemistry, and secondly, the classification of the elements, notably the Periodic Table in its different manifestations.

The first paper was given by Professor David Knight of Durham University and was entitled Davy and the placing of potassium amongst the elements. In 1802, Davy in a lecture declared that metals 'are possessed of specific gravities superior to those of all other simple substances'. He had taken this term from Lavoisier's Elements, where potash and soda were not included: in defiance of his criteria, the great man was sure they must be compounds. In 1806 Davy established that electricity decomposes water into oxygen and hydrogen only, and concluded that electricity was identical with chemical affinity, and 'an essential property of matter'. Water was not in that case the only compound that could be decomposed by electricity. The next autumn, in his research time before the London Season began, he tried to decompose potash. Eventually on 19th October 1807 he succeeded by using the electric current from an enormous battery 'in a state of intense activity' to fuse his slightly damp potash. Amid explosions, flames and bright coruscations, globules of 'alkaligen' collected around the pole, and could be preserved under naphtha. They looked like mercury, but were so light that they floated on water, decomposing it violently. The other substance given off was oxygen, the supposed generator of acids but evidently a major component of the strongest alkalis. Consulting with friends, Davy concluded that 'the analogy between the greater number of properties must always be the foundation of arrangement', and that despite its lightness this was a metal: he named it 'potasium', soon amending that to 'potassium'. Clearly, sodium was a congener; and he went on to assault the alkaline earths, and then the halogens, bringing to light these important 'families' of elements.

Professor Frank James of the Royal Institution then spoke on Spectroscopy and the chemical elements in the nineteenth century. In his paper, Professor James discussed the development of spectrochemical analysis in England during the 1860s following the invention of the method by Robert Bunsen and Gustav Kirchhoff in Heidelberg in 1859. He drew attention to the roles played by earlier spectroscopic researchers, such as William Crookes and John Hall Gladstone, in developing the science after 1859, as well as the role of the London scientific instrument trade. Crookes, in particular, was able to establish his reputation as a serious scientific researcher with his spectroscopic discovery of the chemical element thallium. This, it was suggested, showed contemporaries how credit within the scientific community could be gained and explained the significant number of attempts by other chemists (including A. H. Church, H. C. Sorby, the Dupré brothers and H. C. G. Williams) to discover new chemical elements spectroscopically. However all of these individuals failed, with the exception of the special case of helium (by Edward Frankland and Norman Lockyer), since the spectral observations were found to be spurious for one reason or another. Professor James then suggested that this story should be interpreted within an analytical structure of scientific researchers wishing to gain credit in the scientific community for their work. The new science perhaps received the supreme social accolade in 1865 when August Hofmann lectured on spectrochemistry to Queen Victoria at Windsor.

The third lecture was by Professor Eric Scerri of UCLA, Berkeley, and was entitled Attempts to explain the periodic table from the discovery of triads to the present. Professor Scerri traced the history of the periodic table starting with the views of the ancient Greeks on the elements. He moved onto the work of Lavoisier and pointed out that this stage represented a turning away from regarding the elements as abstract bearers of properties. Two important philosophical principles that contributed to the development of the periodic table, namely triads and Prout's hypothesis, were discussed in order to show that both ideas were initially refuted but can now be said to have made a 'come-back'. Moving onto the discovery of the mature periodic system he emphasized that Mendeleev was primarily classifying the elements as bearers of properties rather than as Lavoisier's simple substances that can be isolated and observed.

The paper then turned to the impact of physics on the periodic table and Professor Scerri argued that although physics has had a profound explanatory impact, it does not explain the periodic system quite to the extent that is usually supposed. Once again the concept of abstract elements made an appearance in the work of Paneth in response to the threats to the periodic system posed by the proliferation of the elements following the discovery of isotopes. He concluded by arguing that chemistry is far more philosophical than is usually believed, such as in the conceptualization of the notion of an element.

The afternoon session began with a paper by Professor William Brock of the University of Leicester and was entitled Radiant Matter Spectroscopy: The Rare Earths Crusade. In 1880 the 48-year old William Crookes moved into a large house in Notting Hill where he erected three laboratories. Together with a new assistant, James Gardiner, and stimulated by Lockyer's dissociation hypothesis, he embarked on a 25-year long investigation of the rare earth elements. He bombarded rare earth salts with radiant matter (cathode rays) to induce phosphorescence, which he then examined spectroscopically (cathodeluminescence). He was particularly struck by the beautiful phosphorescent spectrum of yttrium. In five dazzling lectures to the Royal Society (1881, 1883), the British Association (1886), the Royal Institution (1887) and the Chemical Society (1888) he argued that yttrium's spectrum showed that it was composed from various unknown elements, from which he also believed he had evidence for chemical evolution. Using a zigzag periodic system devised by Emerson Reynolds, Crookes developed a pendulum and cooling model to picture how elements had evolved from a primitive protyle. Differences from integral atomic weights and the clustering of similar elements were due, he believed, to the phenomenon of meta-elements.

Although other rare earth specialists were sceptical of Crookes' speculations, they caused world-wide interest, not least among the Theosophists - a movement that Crookes joined in 1884. Crookes crowned his research in 1898 by claiming the separation of a new element he named Victorium. In fact, as Georges Urbain showed convincingly in 1905, Crookes' research programme had been doomed from the beginning because pure elements do not produce phosphorescent spectra. His yttrium samples had simply been impure. Crushingly, too, victorium turned out to be gadolinium. The episode is a lively example of how erroneous hypotheses can lead to exciting and fruitful scientific developments.

The fifth paper was delivered by Gordon Woods, retired Head of Science, Monmouth School, and entitled Mendeleev and Periodic Tables, 1834-1907. In highlighting the centenary of Mendeleev's death, Gordon Woods focussed on aspects of the scientist's life as well as discussing various periodic tables, a collection of which was displayed at the meeting. Gordon began his paper by showing images of contributions to the Periodic Table's development by J.W. Dobereiner, L. Gmelin, A.E.B. de Chancourtois and J. Newlands. Aged 18 and recently orphaned, Mendeleev started training as a schoolteacher in St Petersburg. Despite missing out on study due to illness he was a top student and he taught briefly in the Crimea before deciding that his future lay in university work.

After attending the Karlsruhe Congress in 1860 with Alexander Borodin he rose rapidly to a chemistry chair in St Petersburg. Finding that no suitable textbooks were available he wrote Osnovy Khimii in 1868. In February 1869 he formulated his periodic system, announced orally at the Russian Chemical Society in that month and published in Zeitscrift für Chemie in March 1869. In 1872 these ideas were developed as a periodic table with groups in columns and where the properties of new elements were predicted. Mendeleev's ideas became accepted as three of his predicted elements (Gallium, Scandium and Germanium) were discovered within only 15 years. Although Mendeleev acknowledged the work of other scientists in classifying the elements before 1855, he denied any knowledge of the work of later contributors before he produced his 1869 Periodic System in a later edition of Osnovy Khimii.

As his ideas became more widely accepted Mendeleev's fame spread. He received several honorary degrees and was awarded the Faraday Medal in 1889. Several years earlier in 1882 he had married a young arts student named Anna Popova. However under her influence he became active in radical student politics and ultimately lost his university post and apartment. Mendeleev's work spread towards general science consultancy and in 1893 he was appointed Director of the Weights and Measures Bureau in St Petersburg. He died in February 1907, the timing of which meant he narrowly failed to win a Nobel Prize.

The final paper brought the story of the discovery of the elements into the twenty-first century as Simon Cotton of Uppingham School examined The 5f elements and beyond. Early 20th century Periodic Tables placed the elements with Atomic Numbers 89-92 as a 6d transition metal series. However, this was based on limited information, largely about uranium compounds, that emphasised similarity in stoichiometry with compounds of Chromium, Molybdenum and Tungsten, and was not supported in later findings of crystallographers.

With the discovery of Neptunium by Edwin M. McMillan and Philip Abelson, and Plutonium by Arthur C. Wahl, Joseph W. Kennedy, Edwin M. McMillan and Glenn T. Seaborg, both in 1940, the era of nuclear synthesis had arrived. These elements had close similarities to Uranium, and Seaborg proposed his 'Actinide' concept in a paper in Chemical Engineering News in 1945. Subsequent actinides were synthesized by multiple neutron capture in a reactor or in a thermonuclear explosion, for example Fermium in 1952, before the bombardment of actinide targets with such as ¹¹Boron and ¹²Carbon led to Nobelium and Lawrencium, the last actinides in 1961.

Decreasing half-lives led to speculation that heavier elements might not exist, rendered nugatory by the syntheses of elements 104-105 around 1970 using similar routes. The Darmstadt group used 'cold fusion' routes employing lighter targets and transition metal projectiles to make elements 107-112 in 1980-1996, whilst syntheses of elements 113-116 and 118 have used the neutron-rich ⁴⁸Calcium. Some chemical properties are known for the elements as far as 108 (Hassium), which forms a very volatile oxide perhaps analogous to osmium tetroxide. Element 104 (Rutherfordium) strongly resembles Zirconium and Hafnium; so far the elements beyond the actinides appear to be another transition metal series.

Anna Simmons