By: Monish Singhal
2019 is an extremely special year for people around the world, chemists in particular. The reason? It is the International Year of the Periodic Table. Announced by the United Nations Educational, Scientific and Cultural Organisation (UNESCO), the year is being marked by various activities conducted throughout the year, ranging from exhibitions to talks. Meanwhile, the governing body of chemistry, the International Union of Pure and Applied Chemistry (IUPAC) will also be celebrating it’s 100th year of existence, calling for celebrations by scientists all over the world.
The history of the periodic table, despite being irrelevant, plays an important role in understanding how the classification of elements allowed scientists to pour their energies into more constructive purposes.
The First Classifications
Johann Wolfgang Döbereiner, a German chemist, tried his hand in arranging the then-known elements by grouping them into groups of three, known as triads in 1829. So did another scientist, by the name of John Newlands, when he attempted at arranging them in groups of eight, or octaves, extremely similar to musical octaves in 1856. Both these methods of classification aimed at explaining the periodicity in the properties of elements but failed to work for heavier elements, like Iron (Fe).
Julius Lother Meyer, another German chemist, began preparing Periodic tables, and by 1869, he had prepared a rudimentary form of the periodic table, with elements arranged as per the valencies. However, it was his bad luck that caused the publication of his paper to be pushed to 1870, knocking him off from the distinction of being the first person to systematically classify elements for lighter as well as heavier elements.
Dmitri Mendeleev and his role to Chemistry
Dmitri Mendeleev’s name would have might as well have been lost in the annals of history if not for his contribution to classifications of elements (and the bad luck of Meyer).
A picture of Mendeleev’s original periodic table
Supposedly a vision in his dream, Mendeleev’s periodic table was based upon the atomic ratios in which elements formed oxides, binary compounds with oxygen in February 1869. He realised that, by putting them in order of increasing atomic weight, certain types of element regularly occurred.
This prompted him to overlook the atomic weights of some elements while arranging them in the periodic table, an interesting example is that of Tellurium and Iodine. Tellurium (127) has a higher atomic mass as compared to Iodine(126), yet it is placed before it. The reason for this anomaly was that the properties of Tellurium matched with the sixth group properties, and those of Iodine with elements in group 7.
Mendeleev’s work was also profound as it took into consideration undiscovered elements. To name such elements, he used Sanskrit numerals eka [one], dvi [two] and tri [three], showed his appreciation and debt to the Sanskrit grammarians of ancient India, who had created sophisticated theories of language based on their discovery of the two-dimensional patterns in basic sounds. Mendeleev also predicted the existence of another row to the table, which were the actinides, some of the heaviest in atomic weight.
Mendeleev’s ‘Eka-Aluminium’, or Gallium, as we know it today, is one of the finest examples of how Mendeleev predicted new elements merely by his periodic table. Comparing the properties of the predicted ‘Eka-Aluminium’ and Gallium in Table 1, we find striking similarities in them.
Mendeleev’s address to the Russian Chemistry Society
It was March 1869, when Mendeleev presented his findings to the Russian Chemistry Society. Quoting him from a paper he published,
The elements, if arranged according to their atomic weights, exhibit an evident periodicity of properties.
Elements which are similar as regards their chemical properties have atomic weights which are either of nearly the same value (e.g., platinum, iridium, osmium) or which increase regularly (e.g., potassium, rubidium, caesium).
The arrangement of the elements, or of groups of elements in the order of their atomic weights correspond to their so-called valences as well as, to some extent, to their distinctive chemical properties, as is apparent among other series in that of lithium, beryllium, barium, carbon, nitrogen, oxygen and iron
The elements which are the most widely diffused have small atomic weights.
The magnitude of the atomic weight determines the character of the element just as the magnitude of the molecule determines the character of a compound body.
We must expect the discovery of many yet unknown elements, for example, elements analogous to aluminium and silicon, whose atomic weight would be between 65 and 75.
The atomic weight of an element may sometimes be amended by a knowledge of those of the contiguous elements. Thus, the atomic weight of tellurium must lie between 123 and 126, and cannot be 128.
Certain characteristic properties of the elements can be foretold from their atomic weights.
Additions to the periodic table
When Mendeleev first published the periodic table in 1869, there were about 56 known elements, with new elements being discovered at about one every year.
Slowly, the periodic table began growing, filling up empty spaces and occupying new ones by adding periods to the table, leading to the now-familiar shape, complete with 7 periods and 18 groups. With the official addition of Nihonium [Nh, At. No.: 113], Moscovium [Mc, At. No.: 115], Tennessine [Ts, At. No.: 117] and Oganesson [Og, At. No.: 118], the seventh period is finally complete. However, this does not put a full stop to the efforts for synthesis for newer, heavier elements. Across the world, scientists are attempting to synthesise ununennium and unbinilium, elements that will open up the vista to the eighth period and the possibility of the existence of the g-orbital and the necessity of rearrangement of the periodic table.
References
Mendeleev’s Periodic Table, 2005, accessed 4th October 2019, <https://corrosion-doctors.org/Periodic/Periodic-Mendeleev.htm>
Mendeleev’s Periodic Properties, 2005, accessed 4th October 2019, <https://corrosion-doctors.org/Periodic/Periodic-Mendeleev-Principles.htm>
Development of the Periodic Table, Royal Society of Chemistry, accessed 5th October 2019, <http://www.rsc.org/periodic-table/history/about>
IUPAC 100 | 100th Anniversary of IUPAC, International Union of Pure and Applied Chemistry, accessed 4th October 2019, <https://iupac.org/100>