Finding the periodic table

Take a journey through the history of the periodic table, which began long before Mendeleev published his definitive version in 1869.

By David Allen

This year we celebrate the 150^th anniversary of Mendeleev’s achievement in formulating the structure of the periodic table that we use today. He announced his structure in 1869, but the challenge of creating a system to organise the elements began in earnest some 80 years earlier.

The first recorded attempt at creating a system to organise the elements was when Antoine Lavoisier published his table of elements in 1789. In 'Traite Elementaire de Chimie', Lavoisier listed 33 substances he considered elements, including light and caloric (heat).

The next significant step came in 1829 when Johann Wolfgang Dobereiner identified a relationship between the properties of elements and their atomic weights. He grouped elements into threes, where the atomic weight of the middle element was nearly the same as the average of the atomic weights of the other two elements – he called his theory 'The Law of Triads'.

In 1843, Leopold Gmelin worked with the Dobereiner’s system and developed a table of 55 elements that contained many of the similar relationships later found in the modern periodic table.

Then in 1857, Jean-Baptiste Dumas published the results of his work in 'Comptes Rendus' describing relationships between groups of metals. Even though chemists had identified relationships between small groups of elements, the one scheme to bring them all together still proved elusive.

The following year, Stanislao Cannizzaro used a hypothesis from Amedeo Avogadro to create a standard set of atomic weights. He expounded this idea at the Karlsruhe Congress in Germany in 1860 – the first ever international chemistry conference. Among the many pioneering chemists in attendance was one Dmitri Ivanovich Mendeleev.

In 1862, French geologist Alexandre-Emile Beguyer de Chancourtois developed the 'telluric screw' – a three-dimensional arrangement of the elements plotted around a cylinder.

Two years later, Julius Lothar Meyer and William Odling independently published their own versions of a periodic table. Meyer’s table was comprised of 44 elements arranged by valency; Odling’s version arranged 57 elements by their atomic weight. Odling’s table was remarkably similar to the one that Mendeleev had devised as a first attempt and actually, Odling had been able to overcome some issues that Mendeleev had not.

Between 1863 and 1866, British chemist John Newlands wrote a series of papers that went on to explain his 'Law of Octaves' having noticed similarities between elements with atomic weights that differed by seven. When Newlands gave a lecture to the Chemical Society on 1 March 1866 to announce his discovery, he was somewhat derided when he compared the repetition of elements at intervals of eight with a musical octave.  In addition, flaws in his theory led to him having to make changes to the order of some elements himself as some were occupying the same spaces. Due to the seemingly arbitrary nature of his theory and because the Society had a rule of not publishing purely theoretical papers, they declined to publish his work.

The records show that Mendeleev discovered the periodic system on 17 February 1869. In fact, he was ill at the time so his colleague Nikolai Menschutkin made the announcement on his behalf – on 6 March at a meeting of the Russian Chemical Society.

Mendeleev’s table listed elements in rows or columns in order of atomic weight, starting a new row or column when the characteristics of the elements began to repeat. What made his version successful was the fact that he left gaps where there appeared to be an element missing that had not yet been discovered. He also thought to occasionally ignore the order suggested by the atomic weights and to switch adjacent elements where they could be better classified into chemical families. Atomic numbers were not yet known, but the atomic weights worked well enough for ordering most of the elements, and Mendeleev was able to accurately predict the properties of missing elements.

John Newlands took issue with the credit that Mendeleev was given for the periodic table in his book ‘On the Discovery of the Periodic Law’ in 1884, going so far as to quote Mendeleev as supporting him: "It is possible that Newlands has prior to me enunciated something similar to the periodic law...". 

Mendeleev countered this somewhat in his book The Principles of Chemistry in 1891:

"I consider it well to observe that no law of nature, however general, has been established at once; its establishment is always preceded by many presentiments, but the acknowledgment of a law does not take place when it is recognised in all its significance, but only when it has been confirmed by experiment, which the scientific man must look to as the only proof of the correctness of his conjectures and opinions. I therefore, for my part, consider Roscoe, De Boisbaudran, Nillson, Winkler, Brauner, Carnelley, Thorpe, and others who verified the adaptability of the periodic law to chemical reality, as the true founders of the periodic law, the further development of which still awaits fresh workers."

The final piece in the puzzle wasn’t found until 1913, when Henry Moseley carried out work into the characteristic X-rays emitted by atoms – he was able to show that their frequencies were proportional to the square of the atomic number. Up until this point the atomic number simply referred to an element’s positioning in the periodic table, and was not known to correspond to a measurable physical quantity. Thanks to Moseley’s work it now became possible to place elements in the periodic table according to their atomic number rather than their atomic weight.

Today the periodic table is the starting point for understanding chemistry. On the wall of every laboratory or school chemistry department you are likely to find hung a copy where chemists and student can easily refer to it. It has entered into popular culture as a recognisable motif, spawning a variety of 'spin-off' versions. Thanks to the tireless efforts of a multitude of chemists, the periodic table is here to stay.