November 28, 2021

I, Science

The science magazine of Imperial College

Mendeleev’s periodic table, a rectangle of colourful framed squares hanging on the walls of all chemistry departments in the world, is considered the “Gospel” of the chemist or, better, its portable version. Nowadays the periodic table is printed everywhere, from calendars to mouse-pads and bookmarks. We can say that it represents the essential order of matter according to human rationality.

The first scientist who attempted to give an order to matter as we know it was the French chemist Antoine Lavoisier just at the dawn of the French Revolution in 1789. He published a list of 33 chemical elements, selected as gas, metals, non-metals and earth. Other chemists tried in the following 19th century to give other classifications. The German chemist Johann Wolfgang Doebereiner made a table of the elements in groups of three (for instance lithium, sodium and potassium are all soft, so: all together) while the English chemist John Newlands in 1865 classified the elements according to their increasing atomic weight and similar chemical physical properties. He found that repetitions occurred every eight elements like notes in a musical scale.

In 1869 German chemist Julius Lothar Meyer and the Russian Dmitri Ivanovich Mendeleev published their classifications independently. Both listed the elements according to their increasing atomic weight and started a new column whenever the chemical and physical characteristics of an element appeared to be similar to a previous one.

The most famous table became Mendeleev’s one, not just because he published it first, but because he did two things that nobody had considered: first, he left gaps in the table corresponding to an element that he predicted to exist but it was not yet discovered. Secondly he privileged chemical-physical properties, ignoring the order suggested by the atomic weights. In this way he classified cobalt and nickel in a more appropriate way than his European “colleagues”.

Mendeleev’s original periodic table published in 1869

Legend has it that the periodic table appeared in his dreams and for him to be a genius, creating something that allows chemists to predict everything of an element just by looking at his position on the table. The truth is very different, though. Mendeleev came up with the periodic table during his attempts to rationalise the elements while teaching chemistry. Apparently, it all started using cards and he came to the final table within a year. In the updated periodic table, Mendeleev’s rows and columns are swapped, but only for graphical reasons and to allow space for the heavier elements, which have been discovered since.

Mendeleev did a theoretical classification and, at the time, it was seen more as a speculative theory and not greatly considered. In 1875, things changed. The chemist Paul-Emile Lecoq de Boisbaudran, unaware of Mendeleev’s work, discovered a new element, which he called “gallium”. This element had all properties of aluminium, but was heavier. When coming across the news, Mendeleev realised that it corresponded to one of the missing spaces of his table and even advised the French chemist to prepare a purer sample with the correct density.

The world picked up the story and turned its attention on Mendeleev’s work. History repeated in 1879 when Lars Nilson from Sweden discovered scandium (Sc), which filled the empty space left by Mendeleev between calcium and titanium and later in 1886, when German chemist Clemens Winkler isolated in ore–laden mountains of Saxony the semi-metal germanium (Ge), positioned in the periodic table between silicon and tin.

Nevertheless, Mendeleev had to face some problems. In 1895 the Scottish chemist William Ramsay first reported the discovery of argon. Mendeleev already noticed that there were gaps between the halogens group (the column of fluorine, chlorine, bromine and iodine) and the family of alkaline metals (lithium, sodium, calcium), but these seemed too many and he believed it was an artefact. This explains his initial concerns about Ramsay’s observations, and he claimed that, instead of having discovered argon, Ramsay was actually observing a heavy form of nitrogen (similar to ozone for the oxygen).

Ramsay’s later discoveries of other elements of similar character, such as helium, neon, krypton and xenon, soon suggested he was wrong. Mendeleev revised his opinion and added a row of elements corresponding to the family of those discovered by Ramsay. Inert gases were now welcomed into the table, but allegedly, the Nobel Committee did not like Mendeleev’s initial failure, which prevented him from receiving the Nobel Prize in 1906, when he was among the candidates.

Later the development of quantum theories of the atom showed that Mendeleev’s classification was based on the increase of the atomic number. This corresponds to the number of protons and electrons present in each element. Isotopes, which are the same elements with different numbers of neutrons in the atomic nucleus (i.e. 14C, a carbon with two more neutrons) are not distinguished. The weight of an element is reported according to a weighted average of the mass of its isotopes.

Although he was never awarded the Nobel Prize, Mendeleev received some post-mortem honours. In 1955, while exploring the stability of so called trans-uranium elements (elements heavier than uranium), American scientists at Berkeley laboratory fabricated a new atom with 101 protons-electrons using a particle accelerator, which they named “Mendelevium”.

In the middle of the cold war, this gesture, criticised by few Americans, did not pass unnoticed by Russians.  The element, with symbol Md, is radioactive, unstable and it rapidly transforms through nuclear reactions into lighter exotic elements such as fermium (Fm), and einsteinium (Es), next in the periodic table. For this reason chemists could not study its chemical properties up to now. I am sure Mendeleev has one more reason to get frustrated.