|The development of the Periodic Table (pre-16)|
What is a mark of a great scientist? Good scientists discover new information and make sense of it, linking it to other data. They may go further by giving an explanation of this linked data which, maybe not immediately, other scientists accept as a correct explanation. However the outstanding scientist goes further in predicting consequences of his ideas which can be tested. This boldness identifies the great scientist if the predictions are later found to be accurate. One such person was Russian chemist Dmitri Mendeleev. Incidentally, although he is often regarded as the father of the Periodic Table, Mendeleev himself called his table, or matrix, the Periodic System.
Formulator of the Periodic Table
Other people, like Londoner John Newlands, Frenchman Alexandre Béguyer de Chancourtois and German Julius Lothar Meyer made important contributions to the first Periodic Table but the main credit goes to Mendeleev. All of them were helped by the publication in 1860 of more accurate atomic weights, as relative atomic masses were then called. There were two main problems about establishing a pattern for the elements. First only 60 elements had been discovered (we now know of over 100) and second some of the information about the 60 was wrong. It was if Mendeleev was doing a jigsaw with one third of the pieces missing, and other pieces bent!
Mendeleev had written the properties of elements on pieces of card and tradition has it that after organising the cards while playing patience he suddenly realised that by arranging the element cards in order of increasing atomic weight that certain types of element regularly occurred. For example a reactive non-metal was directly followed by a very reactive light metal, then a less reactive light metal. The image of a stamp collectors’ miniature sheet shows a stamp commemorating the hundredth anniversary of the Periodic Table superimposed on some of Mendeleev’s original jottings.
Shortly after, his ideas were presented to the Russian Physico-chemical Society. They were read by Professor Menschutkin because Mendeleev was ill. His ideas were then published in the main German chemistry periodical of the time, Zeitschrift fϋr Chemie.
The world’s first view of Mendeleev’s Periodic Table – an extract from Zeitschrift fϋr Chemie, 1869. Click here for a translation
What were the special features of Mendeleev's Periodic Table?
Why is Mendeleev considered to be the ‘father’ of the Periodic Table whilst others, such as Newlands, Meyer and De Chantcourtois are considered to be also-rans?
First he put elements into their correct places in the table. In some cases the relative atomic mass had been wrongly calculated by others. By correcting the relative atomic mass he put the element in the correct place.
At the time, relative atomic masses (then called atomic weights) were laboriously determined using the formula
atomic weight = equivalent weight x valency
The combining (or equivalent) weights (see Problems with relative atomic masses) were generally accurate but sometimes an element was given the wrong valency. Thus beryllium, combining weight 4.6, was given the valency 3 because it was chemically similar to aluminium. This gave an atomic weight of 13.8, placing it between carbon and nitrogen where there was no space. Mendeleev said the valency was 2; the problem was solved - it fitted into the space between lithium and boron.
Secondly, Mendeleev sometimes decided that atomic weights must be wrong because the elements simply appeared in the wrong place. For example he placed tellurium before iodine although its atomic weight is greater simply because iodine’s properties are so similar to those of fluorine, chlorine and bromine and tellurium’s to those of oxygen, sulfur and selenium rather than the other way round. We now know that it is atomic number, not relative atomic mass that governs an element’s position in the Periodic Table but in most cases the two result in the same order.
The greatness of Mendeleev was that not only did he leave spaces for elements that were not yet discovered but he predicted properties of five of these elements and their compounds. How foolish he would have seemed if these predictions had been incorrect but fortunately for him three of these missing elements were discovered by others within 15 years (ie within his lifetime). The first of these Mendeleev had called eka-aluminium because it was the one after aluminium (eka = 1 in Sanskrit) and was identified in Paris (1875) by Paul Emile Lecoq de Boisbaudran who named it gallium after the Latin name for France. Mendeleev was ecstatic when he heard of its properties which nearly matched his eka-aluminium. However de Boisbaudran's value for gallium's density (4.9 g/cm3) differed from Mendeleev's prediction. Mendeleev told the Frenchman, who re-measured the density to find Mendeleev was right! It is interesting to speculate whether de Boisbaudran was pleased or irritated by this. The table compares Mendeleev's predictions with de Boisbaudran's discovery.
Within the next ten years Swede Lars Nilson (1879) identified scandium, predicted by Mendeleev as eka-boron and German Clemens Winkler (1886) discovered germanium which he realised was Mendeleev's eka-silicon. These discoveries established the acceptance of the Russian's table, although two other elements whose properties were predicted were not discovered for 50 years.
One thing that Mendeleev did not predict was the discovery of a whole new Group of elements, the noble gases, by the Scot William Ramsay and co-workers during the last decade of the 19th century (see The discovery of new elements). Mendeleev was at first dismayed by this but before he died in 1907 realised that Ramsay's discoveries were further proof of the Periodic Table, not a contradiction. Ramsay was awarded a Nobel Prize for discovering five elements. Mendeleev never received that honour However, an element, atomic number 101, has been named after Mendeleev, an even rarer distinction. This is surely deserved by the original formulator of the Periodic Table.