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November/December 2019

How I discovered the periodic law of the elements

Dmitri Mendeleev speaks to us, through narrator Alf Larcher, about his life and work as he sought to determine how the properties of chemical elements are related.

It is February 1869 and I, Dmitri Mendeleev, have just completed an important figure for my textbook, Osnovy khimii [Principles of chemistry], which orders the 63 known elements to demonstrate their periodicity of chemical and physical properties.

I would like to tell you how I came to make this discovery. So, let us begin.I am 35 years old and have recently been appointed Professor of General Chemistry at St Petersburg University, St Petersburg, Russia. My story starts where I was born – in a small village just outside Tobolsk in Siberia, many kilometres east of where I am now. My beloved mother noticed that I was doing well in Science subjects at secondary school and, after I graduated, she endured the long and arduous journey to bring me to a teachers’ college in St Petersburg (see July/August issue, p. 14, for Maria Mendeleeva’s story). The college trained teachers and conducted basic scientific research in return for newly graduated teachers completing placements in a regional area for two years. The college was in the grounds of the University of St Petersburg, with university students and the public having limited access.

I was 16 when I arrived and I found it difficult at first – the other students were from local areas and could visit nearby family, but my mother had died shortly after bringing me there and most of my family were in far-off Siberia. But the college looked after me well as I studied and lived there. A professor from the university, Alexandr A. Voskresenskii, looked out for me and encouraged my scientific interests. For my master’s research project, under the mentorship of Professor Voskresenskii, I studied organic isomorphism, where two different organic molecules have the same crystalline structure. In this project, I developed important skills of classification and looking at the physical properties of compounds and linking them with structure. For this research I was given the Gold Medal, and in 1856 I successfully completed all my studies and graduated from the college.

Dmitri was thus a trained teacher, and clarity of presentation in an ordered manner would have been of utmost importance to him. Professor Voskresenskii continued to be a significant influence throughout the rest of Dmitri’s life. Apart from being a dedicated teacher, he was an acclaimed organic chemist, making several important contributions to the field, including the isolation and identification of theobromine in cacao (coffee) beans.

My posting after graduation was a regional school in the Crimea, but with war breaking out in the area it was a difficult place to teach, and I had to move around to avoid the conflict. I was glad when my posting was over and in 1859 I left for Heidelberg, Germany, where I had successfully applied for a government-funded visit to the laboratories of Robert Bunsen. I found the laboratories there very uncomfortable, being noisy and full of fumes – so much so that I set up my own laboratory in my apartment. There I became totally engrossed in chemical research, studying the capillary effect and the critical point of alcohol–water solutions.

While at Heidelberg I was able to attend the first Chemical Congress, held at Karlsruhe in Germany in 1860, where I heard the Italian chemist Stanislao Cannizzaro speak and re-present his countryman Amadeo Avogadro’s hypothesis, making the distinction between atoms and molecules and defining valence more accurately. I heard how this new understanding was used to recalculate and reassign many elemental atomic weights. This started me thinking – could the elements now be ordered on these corrected values? I was so excited about what had occurred at Karlsruhe that I wrote about it to Professor Voskresenskii, who published my letter in the St Petersburg local newspaper. Shortly after the Congress, it was time to leave Heidelberg and my many friends, and I was deeply moved when my German patron Emil Erlenmeyer organised a party for me before I left.

Dmitri seems to have forged his powerful personality at Heidelberg. He was away from Russia and thus his Siberian origins may have emancipated him from being an outsider – he was just another visiting academic from Russia. At any gathering, it appears that Dmitri was at the centre of the conversation.

I returned to St Petersburg in 1861, having to restart my career in Russia. I was in debt after borrowing money to set up the laboratory in Heidelberg and had to find an apartment to live in. It was the middle of the academic year and teaching positions were scarce, so I decided to approach a publisher about translating J.R. Wagner’s German text on chemical technology, and publishing my own organic chemistry textbook. I was keen to bring the new chemistry I had learnt in Germany to Russia. Luckily for me, the publisher agreed, and I began busily writing. I completed my book Organicheskaia khimiia [Organic chemistry] a few months after and it was well received by students. I was also awarded the Demidov prize of the Imperial Saint Petersburg Academy of Sciences for outstanding scholarly work – the judges were impressed that the book was the first Russian-language chemistry textbook written. I used the prize money to clear my debts, after which I approached my mentor Professor Voskresenskii, who was able to give me some teaching work at the University of St Petersburg. This was not enough to support me and my wife Feozva Leshcheva, whom I married in 1862, but I was able to pick up more teaching at the Technological Institute in St Petersburg.

I continued teaching at both institutions, also defending my Doctor of Science dissertation entitled ‘On the Combinations of Water with Alcohol’.

In 1867, I was appointed Professor of Chemistry at the University of St Petersburg, the chair becoming available on the retirement of Professor Voskresenskii.I was now responsible for the teaching of General Chemistry, with all students in the Science faculty required to attend my lectures. I set about my work, using a new broom to sweep away the dated textbooks, including my own, which we based on translations of German texts. Karlsruhe had changed the chemical landscape and many more elements had been discovered in recent years. I was determined not to present my students with an undefined array of the chemical elements – I would find order in the system!

I began writing Osnovy khimii in early 1868, completing the first volume in January of this year (1869). This details the practices of chemistry and how chemical knowledge is acquired, which was important to my students as their course contained a major practical component. It also allowed me to review how the critical properties of chemical compounds and elements are determined and how they could be prone to error. My mind was ticking.

It was time to address the issue of the organisation of the elements. I wrote each of the 63 known elements on a separate card with their newly assigned atomic weights and their physical and chemical properties, e.g. density, valency, oxide composition. How should I order them? I began by using their atomic weight, concentrating firstly on the lighter elements.

The process by which Dmitri ordered the elements has been the subject of constant conjecture over the decades. The idea that it came to him in a dream was favoured for some time, but has been discounted by recent sources who favour a process using cards, concentrating initially on the light elements. Dmitri’s discovery process described in this article is imagined and not based on fact. While Dmitri’s original manuscript corrections of his first ordering exist, the workings that led to his original discovery do not.

I began by placing my first card, that of the element with the lowest atomic weight (hydrogen – atomic weight 1), on the left side of the bench that I was working on. I then placed the next card of the element of the next lowest atomic weight (lithium – atomic weight 7) below it and so on to sodium (atomic weight 23) in a single column. I had begun volume 2 of my chemistry textbook discussing the composition and properties of common salt and knew the alkali metals and halogens well. I noticed that lithium and sodium were in my single column, one near the top, the other at the bottom. Why were these similar alkali elements so far apart? Perhaps my list should be ordered in two columns instead of one. I moved my hydrogen card further to the left, to head another column, then moved lithium underneath hydrogen, not immediately below but next to its chemical cousin sodium in the second (original) column. I continued this trend by placing the other alkali earths next to sodium and lithium in a row across the columns in order of increasing atomic weight. Similarly, I added the halogens next to fluorine in another row across the table. The arrangement so far looked very neat  (see below).

I continued with the next highest atomic weight element after sodium, being magnesium (atomic weight 24). Once again, instead of extending the second column, I started a third column, placing it next to beryllium, which had similar chemical properties, e.g. both had a valency of +2, forming oxides of composition RO, R being a generic symbol for an element. The next elements in increasing atomic weight in this third column were aluminium, silicon, phosphorus and sulphur. To my amazement, after these elements, the third column was completed by the halogen and alkali earth elements (Cl and K), which I had placed as rows previously. Order in the elements was emerging!

The next elements after potassium were calcium of valency 2 and titanium of valency 4. I thus reasoned that a new yet undiscovered element must exist after calcium, having valency 3. By averaging the atomic weights of its neighbours, I estimated its atomic weight to be 45. I thus placed a card with a question mark below calcium in the third column.

Dmitri named the proposed element eka-boron, the prefix from the Sanskrit word for ‘one’ and ‘boron’ for the lowest atomic weight member of the valency 3 elements (group 3). From the chemical and physical properties of its neighbouring elements and other members of its group, Dmitri boldly predicted a long list of his new element’s properties. He also cheekily predicted how this element would be discovered!

Note that below eka-boron, erbium  and yttrium (Yt, symbol now Y) and indium are listed in the figure with question marks and are not classified in the system. This is understandable because the first is a lanthanoid whose position in the periodic table would not be determined for decades, while the atomic weights for the other two elements are incorrect. Dmitri correctly ignored these as he ordered the rest of the elements.

Let’s continue with Dmitri’s imagined sorting of his elemental pack of cards.

I knew the valency of the element after eka-boron must be four and I still had titanium in my stack of cards, which was definitely of that valency. I thus began the fourth column with titanium, starting well above bromine, whose position I had determined previously. Progressing down the group, I listed the remaining elements with increasing atomic number, starting with vanadium, until I reached zinc and arsenic, the latter having similar chemical properties to phosphorus but which was two places after zinc. To effect this alignment, I had to leave two spaces after zinc to account for two more yet-to-be discovered elements. I temporarily named them eka-aluminium and eka-silicon, according to their adjacent elements, and whose atomic weights I averaged to calculate the atomic weights of the new elements to be 68 and 70 respectively. The elements that I placed after these eka-elements were the previously placed arsenic then selenium, with their chemical and physical properties fitting well with the elements in each of their rows. After bromine and rubidium, I placed strontium, which also fitted well with the calcium row. The next card after strontium was zirconium, which had the same valency (4) and similar properties to titanium, and I thus decided to place it next to this element on top of a fifth column.

Dmitri placed four lanthanoid/actinoid elements at the end of column 4 that were not classified by his system and whose atomic weights were once again later shown to be incorrect or, in the case of didymium, mis-assigned (it was eventually shown to consist of two elements – praseodymium and neodymium). The lanthanoid/actinoid elements were to perplex him and many other scientists until American chemist Glenn Seaborg found their correct place in 1945. Once again, Dmitri extensively listed the properties of his proposed new elements eka-aluminium and eka-silicon, including in German publications, making his proposals available to an international audience. The publications did not make much of an impact, some people commenting that every gap in his proposed system did not require a new element. But Dmitri was not finished – as he progressed through the elements, he would make more controversial predictions.

I continued to add elements below zirconium in increasing atomic weight: niobium, molybdenum, rhenium … until I reached the place before the previously placed iodine (atomic weight 127). My next element was tellurium (atomic weight 128) which should rightly be placed after iodine to take the place of caesium. This was, impossible, I thought – iodine and caesium are in their correct places among the alkalis and halogens respectively. I concluded the atomic weight of tellurium must be incorrect and placed it before iodine in the same row as oxygen, with which it had similar properties.

I continued down to barium with all the elements seeming to fall into place until I reached tantalum (atomic weight 182), which I knew was an analogue of niobium. I thus began the sixth row with tantalum, but one place down, so that tantalum lined up with niobium, calculating the atomic weight of the new yet-to-be discovered element to be 180. I completed column 6 from tantalum to lead – the elements did not align as well as in other columns. I suspected that more atomic weights were incorrect, but for now this was all I could do.

So that is how I came to my first attempt at ordering the elements and the figure that I have completed for my textbook. My system has aligned the elements so that each column adds another member to the horizontal group of elements with which it has similar chemical properties; I have discovered the periodic law of the elements! It is not perfect, and I have much work to do, but I believe it will help my students enjoy and understand chemistry. I also think that my first effort is worthy enough to let my chemical colleagues know about, especially since after the Karlsruhe Congress I know many like me have been trying to order the elements. To let them know, this month I will print single-sheet drafts of my ordered elements (150 in Russian and 50 in French) and distribute them to various chemists. The title will be ‘An Attempt at a System of Elements, Based on Their Atomic Weight and Chemical Affinity’. I will also prepare a paper on my discovery (‘The correlation of the Properties and Atomic Weights of the Elements’), which I will read at the newly established Russian Chemical Society in March. Shortly after that, I will have also finished the second volume of my textbook, which will contain my ordering of the elements at its centre.

I am not sure where this work will take me; will my work be remembered in 50, 100, 150 years? I know not, but I hope I have contributed to chemistry is some small way at least! 

D. Mendeleev

Apart from his brilliance and perseverance as a chemist, Mendeleev was a great writer, able to quickly put pen to paper and publish his results. This is a great example for any scientist of how to obtain primacy of any idea, which many of his contemporaries, who were working on similar ideas, did not do. In his writings, he never used the term ‘periodic table’ for his system, instead using the terminology ‘periodicity of the elements’ or the ‘periodic law of the elements’. As his, and subsequent investigators’, orderings were presented as ‘tables of ordered elements’ and the like, the term ‘periodic table’ came into use after Dmitri’s time. Dmitri’s original ordering would undergo many refinements, with him constantly publishing the new versions. 

In 1875, the rather remarkable French chemist Lecoq de Boisbaudran published the properties of a new element that he found in rocks in a mine in the Pyrenees, calling it gallium. It was six years after Dmitri had published his original work and he had been scouring the newly published literature about new elements that would validate his predictions. He must have been very pleased when he found Lecoq’s publication; gallium’s properties matched his eka-aluminium very closely.

The predictions are based on the proposed valency of eka-aluminium and chemical/physical trends across the table (easy to say now, but you must remember Dmitri was working in 1869). And as he predicted, gallium was identified by spectroscopy, a technique developed by Bunsen and Kirchhoff in whose laboratory Dmitri had studied and perhaps gained insights on how important the technique would be. He would, however, continue to push his luck, not ‘being happy’ with Lecoq’s initial determination of gallium’s density of 4.7 g/cm3, writing to him insisting that the density should be around 5.9 as determined from his desktop calculations. Lecoq must have been surprised but rechecked his density – sure enough, he had been mistaken and revised his figure to 5.935, very close to the one Dmitri predicted (the currently accepted value is 5.904 g/cm3, which is even closer to Dmitri’s prediction). The scientific world began casting its gaze to St Petersburg.

Dmitri had to wait another four years (1879) before Swedish chemist Lars Fredrik Nilson and his team discovered scandium and until 1886 for German chemist Clemens Winkler to discover germanium. Both these elements had properties close to those predicted by Dmitri for eka-boron and eka-silicon. Some of his predictions Dmitri would not bear witness to: hafnium, the analogue of titanium and zirconium of predicted atomic weight 180, was discovered in 1923 (atomic weight 178.5), 16 years after his death from influenza.

It was not until the work of English physicist Henry Moseley in 1913 that it was understood that the periodic table was ordered by atomic number (the number of atomic neutrons), which follows, in most instances, atomic weight. There are so-called inversions where (rarely) the order of the elements given by atomic number does not follow the order by atomic weight, due to an unusual distribution of isotopes in one of the pair of elements, boosting its (average) atomic weight. One such inversion is tellurium/iodine where iodine is of higher atomic number but lower atomic weight than its atomic number predecessor tellurium. Dmitri incorrectly questioned the atomic weight of tellurium, but impressively ‘stuck to his guns’ and was correct to swap the order between it and iodine to keep each in their correct ‘chemical families’. Another inversion occurs with nickel and cobalt, with their mixture of isotopes also resulting in their atomic weights being very similar, but inverted. Both display variable valencies, with 2 being common, making them difficult to differentiate. It seems that Dmitri in this case wisely did not want to take a risk on differentiating the two, making them equivalent in his first table.

His original table is arranged differently from the present-day format, where his rows are arranged in columns and his column as rows. The table shown transposes Dmitri’s element assignment and notation to the current periodic table format, with Dmitri’s original rows labelled alphabetically in both figures so they can be cross-referenced. The success of each of Dmitri’s elemental orderings is colour-coded so he can be given an overall ‘scorecard’.

From the number of red highlighted elements, it is evident that Dmitri’s table was far from perfect. He seems to have had difficulty in correctly identifying the members of the Be group IIA and Zn group IIB elements, placing zinc and cadmium in group IIA and calcium and strontium in a separate group. It is for this reason that some sources claim that Dmitri’s prediction of scandium is a lucky guess, but Dmitri’s strategy of requiring a valency 3 element before the adjacent valency 4 element titanium was correct. The grouped elements nickel and cobalt  are in the correct region of the periodic table, as is the following element Pl (the original symbol for palladium).

Many of the higher atomic weight elements are mis-assigned, perhaps because they display multiple valency states, which makes them difficult to categorise. In his 1871 table, Dmitri adopted a configuration similar to today’s, where he backed away (mistakenly) from his hafnium prediction but made several other predictions about the existence of new elements. Some of these were correct, e.g. technetium not officially discovered until 1937, once again after Dmitri’s death, by Italian scientists Emilio Segrè (physicist) and Carlo Perrier (mineralogist), while other predictions of the lanthanoid/actinoid family were, perhaps understandably, not correct.

His first ‘attempt (opyt)’ at ordering is, however, remarkable for 1869, as can be seen from the large areas of green in the periodic table scorecard shown. The helium group noble gases, some of which were discovered in Dmitri’s lifetime, fitted perfectly into his system. The discovery of periodic properties of groups of elements across rows and down columns of his ordered elements, the four correctly predicted elements and the correct ordering of tellurium and iodine makes Dmitri’s first ordering attempt a monumental piece of work.

Alf Larcher FRACI CChem is a petroleum, environmental and industrial chemist with an occasional urge to write science articles. The author has used mainstream references (full list available upon request) from recognised sources.

Photo: Serge Lachinov.

Dmitri Mendeleev at the Working Desk. 1886. Oil on canvas. Museum archive of Dmitri Mendeleev at Saint Petersburg State University.


Dmitri Mendeleev’s first published ordering of the elements, entitled ‘An Attempt at a System of Elements, Based on Their Atomic Masses and Chemical Affinity’. Row labels (in red) have been added by author.

It was time to address the issue of the organisation of the elements. I wrote each of the 63 known elements on a separate card ...
... Mendeleev was a great writer, able to quickly put pen to paper and publish his results.

A scorecard for Mendeleev’s element assignment in which the 1869 ordering is transposed to the present-day format. The last row of the periodic table (atomic numbers 87–118) and the lanthanoids/actinoids are not shown because they were largely unknown at the time.

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