Who discovered the elements

Rarely does it happen that someone makes history by not doing something. But Dmitri Mendeleev managed it. The Russian chemist, who allegedly had his hair and beard trimmed by a sheep shearer only once a year, presented a classification system in 1869 in which he had sorted all the chemical elements known at the time according to their increasing atomic weight. Some people before him had already tried that, but Mendeleev left gaps in his table where he suspected other chemical elements, which were still unknown at the time. When these substances were found a few years later, the Russian's way of sorting was confirmed. That is why the United Nations has declared this year the Jubilee of Chemistry, the "International Year of the Periodic Table of Chemical Elements" or, quite simply: # IYPT2019.

The story of Mendeleev's discovery is often told as follows: The allegedly passionate solicitor is said to have transferred the chemical symbols and atomic weights of all 63 elements known at the time to individual cards and then pushed them around until what was in front of him was what is now the first version of the Periodic Table of the Elements is celebrated in the history books. According to legend, he had the crucial idea one night in a dream.

Like many beautiful stories, this one is probably not true either. The chemist and philosopher of science Eric Scerri, who has written several books on the periodic table, has doubts about this legend, because such maps did not appear in Mendeleev's estate. But from the moment he realized that he was likely to become famous, Scerri wrote a few years ago that he kept everything meticulously. This obsession with collecting is evidenced, among other things, by an invitation card from a cheese dairy, the back of which Mendeleev had scribbled with calculations of atomic weights. The document is now in a museum in Saint Petersburg, where the discoverer of the chemical world order lived for a long time.

On March 6, 1869, Mendeleev presented his system at a congress of the Russian Chemical Society. The title: "The dependence of the chemical properties of the elements on the atomic weight". Strictly speaking, he had it presented by his pupil Nikolai Alexandrowitsch Mensutkin. Dmitri Mendeleev himself was unable to attend the meeting that evening.

Shortly afterwards his idea appeared in a Russian trade journal and also in the German one Journal of Chemistry. In it he wrote among other things: "The size of the atomic weight determines the properties of the element" and "The most common elements in nature have small atomic weights". In addition, the discovery of many new elements can be foreseen. All of that should be confirmed later.

The periodic table of the elements is undoubtedly one of the most important scientific achievements. And it's a prime example of how science works. Because Mendeleev's system was not the first attempt to create order in the world of things. He built on the work of countless researchers before him.

More than 2000 years ago, philosophers were already thinking about the composition of matter. Aristotle wondered how many times a piece of gold could be divided before it loses its special properties. Like the Greek philosopher Leukippus before him, he suspected that things are all composed of the smallest basic units. Leukippus pupil Democritus came up with the term atomoswhich can be translated as "that which cannot be cut up". The English chemist John Dalton took up this name again at the beginning of the 19th century after he had recognized that chemical elements always react with one another in exactly the same weight ratio. For example, 100 grams of carbon, or C for short, reacts with 133 grams of oxygen (O) to form carbon monoxide (CO), theoretically without leaving any of the two substances. If double the amount of oxygen is added to the reaction, carbon dioxide (CO₂) is produced.

The simplest explanation for this behavior, Dalton postulated, is that every chemical element consists of the smallest basic building blocks that always form bonds with one another in the same numerical ratio. In his atomic theory, he defined the atom as the smallest unit of matter and believed that it could not be broken down further. But he was wrong.

Between Leukippus and Dalton lay a couple of centuries of alchemy, the representatives of which pursued different ideas. The sulfur-mercury theory, for example, considered that all things are composed of the four basic elements fire, water, earth and air, which combine to form the "principles" sulfur and mercury. In this imagination, different mixtures of these hypothetical substances should become the metals that were known at the time. The Swiss medical reformer Paracelsus took up the concept in the 16th century and added "Opus Paramirum"add the salt.

Little changed about these ideas, obscure from today's point of view, up to the time of the French Revolution. In 1789, the French chemist Antoine Laurent de Lavoisier took the trouble to compile a list of all 33 chemical elements known at the time. Gold, iron and sulfur had been known to humans since ancient times, while other elements such as manganese and tungsten had only recently been discovered. Only 23 of them withstood later scrutiny, but it was the first step in unraveling the order of things.

In the years that followed, chemists discovered numerous new elements. Sodium and potassium were added to the list in 1807, followed the following year by magnesium, strontium, barium, calcium and boron, and by 1825 iodine, cadmium, selenium, lithium and silicon, aluminum and bromine.

An English chemist almost got ahead of the Russian

In the middle of this boom phase, in 1816, the trained pharmacist and Goethe friend Johann Wolfgang Döbereiner noticed that barium, strontium and calcium were similar in their properties and sorted them into a group he called the "triad". Soon he found more groups of three and packed things like chlorine, bromine and iodine together. He was the first to notice that there seems to be some kind of chemical relationship between the elements. In 1829 he published his version of the classification system of the chemical elements. Until then it was not uncommon to simply organize the elements according to color, conductivity or their thermal properties.

In 1864 the English chemist John Newlands almost anticipated Mendeleev's draft. He had recognized that the chemical properties of the elements are repeated in every eighth position if they are sorted according to their mass. Based on the musical notation, he called his discovery the law of the octave - and was laughed at by his colleagues for it.

At the same time as Mendeleev, the German Lothar Meyer developed an almost identical system. The fact that Mendeleev's work will be celebrated this year is not just an injustice of history. The Russian's table was also initially questioned by his colleagues; the gaps he had left were interpreted as a weakness in his hypothesis.

In 1871, however, Mendeleev published a revised version of his system. He left the blank spaces in it. Finally, he said the elements that should be in these places beforehand and described their properties. He called them eka-aluminum, eka-boron and eka-silicon because of the chemical relationship he suspected to known substances. The subsequent discovery of the elements now known as gallium, scandium, and germanium silenced its critics.

Information graphics

Infographic: Sara Scholz

Research: Hanno Charisius, Christian Weber