Analysis: It's 150 years since Dmitri Mendeleev came up with the Periodic Table of chemical elements and transformed chemistry

By Sylvia Draper, TCD

2019 marks the 150th anniversary of the Periodic Table of Chemical Elements and has been proclaimed the International Year of the Periodic Table of Chemical Elements (IYPT2019) by the United Nations General Assembly and UNESCO. 

What's all the fuss about?

Dmitri Mendeleev's discovery in St Petersburg in 1869 was a landmark event that was to transform chemistry from a tangle of disorganised facts into a disciplined science. It was to formulate the thinking of generations of scientists to come and to be a feature of every future chemistry textbook from Junior Cycle to undergraduate. His table would arrange 63 elements, (an element is a substance that is made up of identical atoms), into a recognisable and repeating pattern that all the other 58+ elements, yet to be discovered, would follow.

Was Mendeleev really the first to cop on to this?

Several scientists before Mendeleev had realised that the properties of the elements (the fundamental building blocks from which all matter is made) follow a pattern that seemed somehow to be connected to their valency (the number of hydrogen atoms that they react with) and their atomic weight. In the early 19th century, atomic weight was a difficult concept. Having agreed at this time that all elements are made up of tiny particles or atoms, the problem remained that individual atoms were too small to weigh. Thanks to Avogadro’s Law (that the same volume of different gases contains the same number of atoms), the solution emerged to compare weights.

Russian chemist Dmitri Mendeleev. Photo: Fine Art Images/Heritage Images/Getty Images

In 1858, Italian chemist Stanislao Cannizzaro compared the weight of equal volumes of oxygen and hydrogen and concluded that oxygen was 16 times heavier than hydrogen. This meant that if hydrogen had an atomic weight of one, oxygen must have an atomic weight of 16. Cannizzaro presented his idea at the First International Chemical Congress in Karlsruhe in 1860 which was attended by a young Mendeleev who was finishing his postgraduate degree at the time.

So what did Mendeleev do nine years later?

On that fateful morning in 1869, Mendeleev wrote the name of each of the known elements on separate pieces of card, added the atomic weight, a few physical properties (gas, liquid etc) and the formulae of its products when reacted with hydrogen or oxygen (essentially its preferred valency).

He began to play with these cards, eventually arranging them in rows from left to right of elements having the same valency but in order of increasing atomic weight. He was pleased with this arrangement and wrote it down on the back of an envelope which can still be seen today in St Petersburg in Russia. After a short nap after lunch, he decided to arrange the cards in eight vertical columns rather than horizontal rows and this is the format of the Periodic Table that exists today.

Where it all began: Mendeleev's first periodic system of elements. Photo: Science & Society Picture Library/SSPL/Getty Images

What he had done was come up with an arrangement in which the elements with similar properties appeared in the same vertical group. When the elements were considered as a continuum (each one increasing in atomic weight), there was a cyclical change in their properties which repeated as you moved from one row of elements to the next.

So confident was Mendeleev of the pattern he was seeing that he was able to recognise that some elements were missing. His first tables included gaps where he predicted that future discoveries of new elements would occur. For example, the table in St. Petersburg University has a gap between calcium (Ca, 40) and titanium (Ti, 48). It was filled by scandium (Sc, 44) in 1879, three years later.

Three other gaps were under boron, aluminium and silicon. Mendeleev called these unknown elements eka-boron, eka-aluminium and eka-silicon. Each one was later discovered and named after the place where it was found. Eka-aluminium (found in 1875, in Paris, France) became gallium, eka-boron (found in 1879, in Uppsala, Sweden) became scandium and eka-silicon (found in 1886, in Freiburg, Germany) became germanium. The properties of these elements, their atomic weights and their densities were as Mendeleev had predicted.

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From RTÉ Radio 1's Today With Sean O'Rourke, Dr. Shane Bergin from UCD discusses the Periodic Table

Mendeleev had difficultly placing some of the elements, but for good reasons at the time. He has trouble with tellurium and iodine, because of their atomic weights, and mercury, because it appeared to have a valency of 1 when its preferred valency is actually 2. His table also had one group missing. This was what would later become the last group in each row, known as the noble gases because of their reluctance to react! These elements were found in the years to follow from 1894 to 1900.

What happened next?

Realising the importance of his discovery, Mendeleev published his work in a paper in the first edition of a new journal, the Russian Journal of General Chemistry calling it "The relationship of properties and the atomic weights of the elements". It was translated into German in the same year ensuring that news of the discovery spread across the major Chemistry centres in Europe.

The early 20th century saw three major works (1904 J.J. Thomson, 1909 Ernest Rutherford and 1913 Henry G.J. Moseley) that revealed subatomic particles called electrons, neutrons and protons exist and that together, they give rise to the atomic weights of the atoms and their valences. The number and arrangement of these subatomic particles actually underpin the periodicity of the elements in the Periodic Table so that instead of arranging the elements in terms of their atomic weight, they are best arranged in order of their atomic number (which is the number of protons each element contains in its nucleus). Rather than undermine Mendeleev’s table this served only to emphasise just how innovative and future-thinking a discovery it had been.

The Periodic Table has predictive and instructive powers and is both the point of convergence and the point of departure for future-thinking our life on this planet

The modern Periodic Table

The change from Mendeleev’s original eight column table to the current long form of 18 groups occurred over the course of the next decades. It required the intervention of the International Union of Pure and Applied Chemists (IUPAC) so that all chemists, no matter what part of the world they are in or what language they speak, label the 18 groups numerically from 1 to 18 without any extra As or Bs.

The elements all have their own unique symbol or "initials" giving the Periodic Table a universal appeal that transcends spoken language. It has a numerical, logical and practical significance to everyone trained in using it, so that all Chemists can understand why Hydrogen and Oxygen react in the ratio of 2:1. It also means that elements can be expected to react in a similar but graduated way in comparison with their periodic neighbours (above and below, or left and right). So if in the future, the world runs out of cobalt (essential to the performance of batteries in electric cars) or iridium (used in OLEDs), chemists will be looking at their elemental neighbours in the Periodic Table and working out how to use these as useful alternatives.

The discovery of the Periodic Table is worth celebrating. As a tool, it has predictive and instructive powers and is both the point of convergence and the point of departure for future-thinking our life on this planet.

Professor Sylvia Draper is Professor in Inorganic Chemistry at the School of Chemistry at TCD. She a former Irish Research Council awardee.

The views expressed here are those of the author and do not represent or reflect the views of RTÉ