150 years ago, Mendeleev perceived the relationships of the chemical elements
Every scientific discipline has a favorite anniversary.
Newton’s Principia in 1687 introduced the rules of motion and gravity to the field of physics. Darwin’s birthday and On the Origin of Species (1859) are commemorated in biology (1809). Astronomy enthusiasts celebrate the year 1543, when Copernicus positioned the sun at the center of the solar system.
And for chemistry, the creation of the periodic table of elements by the Russian chemist Dmitrii Ivanovich Mendeleev 150 years ago this March is the greatest occasion for celebration.
Students in chemistry are as familiar with Mendeleev’s table as accountants are with spreadsheets. It summarizes a whole scientific discipline in approximately 100 squares containing symbols and numbers. It enumerates the components that make up all terrestrial substances and arranges them so as to reveal patterns in their properties, so guiding the theoretical and practical pursuit of chemical research.
According to chemist Peter Atkins, the periodic table is possibly the most essential notion in chemistry.
Mendeleev’s table appeared to be a haphazard chart, but he wanted it to convey a profound scientific truth he had discovered: the periodic law. Mendeleev was able to forecast the presence of elements that had not yet been discovered since his law revealed fundamental familial similarities among the known chemical elements – they exhibited comparable qualities at regular intervals (or periods) when organized by atomic weight.
Mendeleev stated, “Before the publication of this law, the chemical components were merely fragmentary, incidental facts in Nature.” “The law of periodicity allowed us for the first time to perceive undiscovered elements at a distance that was previously inaccessible to chemical eyesight.”
Mendeleev’s periodic chart did more than simply predict the existence of new elements. It supported the then-controversial notion that atoms are real. It alluded to the presence of subatomic structure and foreshadowed the mathematical mechanism underlying the principles controlling matter that quantum theory would later uncover. His table completed the transition of chemistry from the medieval magical mysticism of alchemy to the contemporary scientific rigor. The periodic chart represents not only the elements of matter, but also the logical consistency and principled rationality of all of science.
Developing the foundation
According to legend, Mendeleev conceptualized and constructed his periodic table in a single day: February 17, 1869, according to the Russian calendar (March 1 in most of the rest of the world). However, that is likely an exaggeration. Mendeleev had contemplated grouping the elements for years, and other chemists had mulled over the concept of elemental connections multiple times in the preceding decades.
Johann Wolfgang Dobereiner, a German scientist, observed irregularities in elemental groupings as early as 1817. In those days, chemists lacked a complete understanding of the nature of atoms, as detailed in John Dalton’s 1808 atomic theory. In his New System of Chemical Philosophy, Dalton assumed that each elementary substance was composed of a specific type of atom to explain chemical reactions.
Dalton hypothesized that chemical reactions formed new compounds when atoms were separated or united. He reasoned that each element was composed solely of a single type of atom, differentiated from others by weight. Dalton felt that oxygen atoms were eight times as heavy as hydrogen atoms and carbon atoms were six times as heavy as hydrogen. With the knowledge of these atomic weights, it was possible to determine the amounts of elements that reacted to form new compounds.
Dalton was incorrect about several of the weights; oxygen is actually sixteen times heavier than hydrogen, while carbon is twelve times heavier. However, his hypothesis made the concept of atoms practical, resulting in a revolution in chemistry. Accurately measuring atomic weights became a leading concern for chemists in the decades that followed.
Dobereiner observed, while considering these weights, that certain sets of three components (which he termed triads) had an unusual relationship. The atomic weight of bromine, for instance, was midway between those of chlorine and iodine, and all three elements exhibited comparable chemical behavior. Sodium, potassium, and lithium were also a triumvirate.
Other chemists saw connections between atomic weights and chemical properties, but it wasn’t until the 1860s that atomic weights were sufficiently known and measured for more profound insights to emerge. In a study published in 1865, the English scientist John Newlands observed that organizing the known elements in ascending atomic weight order resulted in a repetition of chemical properties every eighth element, a pattern he termed the “law of octaves.” A reviewer suggested that Newlands should attempt arranging the elements in alphabetical order instead, as his pattern did not hold up well after the first few octaves. Mendeleev quickly understood that the relationship between element characteristics and atomic weights was a bit more difficult.
Arranging the constituents
Mendeleev, the 17th child of his parents, was born in Tobolsk, Siberia, in 1834. He pursued various hobbies and traveled a circuitous route to recognition. During his graduate studies at a St. Petersburg teaching institute, he nearly died from a terrible illness. After receiving his bachelor’s degree, he taught math and science in middle schools (as required by his teaching institute scholarship) while conducting research for his master’s degree.
Afterwards, he worked as a tutor and lecturer (along with some popular scientific writing) until he was awarded a fellowship for an extended tour of research in Europe’s most prestigious university chemistry facilities.
When he returned to St. Petersburg, he was unemployed, so he composed a comprehensive guide on organic chemistry in the hopes of obtaining a substantial cash reward. The valuable Demidov Prize was awarded in 1862 as a result of a risky wager. In addition, he got employment as an editor, translator, and consultant for numerous chemical firms. He eventually returned to studies, getting his Ph.D. in 1865 and subsequently becoming a professor at the University of St. Petersburg.
Mendeleev was thereafter preparing to teach inorganic chemistry. As he prepared to learn this new-to-him field, he found the existing texts unimpressive. Therefore, he chose to write his own. Organizing the text meant organizing the elements, so he pondered the optimal way to arrange them.
Mendeleev had made sufficient progress by the beginning of 1869 to recognize that some groupings of identical elements exhibited a systematic increase in atomic weights, while other elements with about equal atomic weights shared comparable features. Ordering the elements by their atomic weight seems to be the key to classifying them.
According to Mendeleev, he organized his thoughts by noting the properties of each of the 63 known elements on a separate notecard. Then, through a game of chemical solitaire, he discovered the desired pattern. By arranging the cards in vertical columns from lowest to highest atomic weight, identical attributes were assigned to each horizontal row. The birth of Mendeleev’s periodic table. On March 1, he sketched his table, sent it to the printer, and included it in his soon-to-be-published textbook. He hastily drafted a paper for the Russian Chemical Society.
Mendeleev stated in his work, “Elements grouped according to the size of their atomic weights exhibit clear periodic features.” “All the comparisons I’ve conducted… have led me to the conclusion that the atomic weight size dictates the composition of the elements.”
During this time, the German chemist Lothar Meyer was also organizing the elements. He developed a table comparable to Mendeleev’s, probably even before Mendeleev did. Mendeleev published first, however.
Mendeleev’s use of his table to make bold predictions about unknown elements was, however, more significant than beating Meyer to the publication punch. Mendeleev had observed that some note cards were missing while preparing his table. He was required to leave blank spaces in order for the known elements to align appropriately. In his lifetime, the previously unknown elements gallium, scandium, and germanium filled three of these gaps.
Mendeleev not only anticipated the existence of these elements, but also accurately defined their properties in depth. Gallium, which was discovered in 1875, had a measured atomic weight of 69.9 and a density six times that of water. Mendeleev had predicted the existence of an element, which he named eka-aluminum, with the exact density and atomic weight of 68. In terms of atomic weight (72 anticipated, 72.3 seen) and density, his projections for eka-silicon closely matched germanium (found in 1886) (5.5 versus 5.469). Additionally, he accurately anticipated the density of germanium compounds with oxygen and chlorine.
The table of Mendeleev had become an oracle. It was as if the Scrabble tiles at the conclusion of the game revealed the mysteries of the cosmos. Others had recognized the power of the periodic law, but Mendeleev was the master at harnessing it.
Mendeleev’s accurate forecasts granted him legendary reputation as a chemical magician. Today, however, historians disagree as to whether the finding of the predicted elements solidified acceptance of his periodic rule. It is possible that the law’s approval was influenced by its ability to explain recognized chemical correlations. In any event, Mendeleev’s predictive accuracy drew considerable attention to the value of his table. In the 1890s, scientists acknowledged his law as a significant contribution to chemical understanding. In 1900, the future Nobel laureate in chemistry William Ramsay dubbed it “the finest generalization in chemistry to date.” And Mendeleev did it without a profound grasp of why it worked at all.
An algebraic map
In numerous cases throughout the history of science, predictions based on unique equations have proven to be accurate. Math somehow discloses some of nature’s secrets before scientists discover them. Antimatter and the expansion of the universe are two examples. Mendeleev’s forecasts of new elements arose without the use of inventive mathematics. In reality, however, Mendeleev had uncovered a profound mathematical map of nature, as his table represented the implications of quantum physics, the mathematical principles regulating the atomic architecture.
Mendeleev stated in his textbook that “internal differences of the stuff that makes the atoms” could be responsible for the periodically recurring features of the elements. He did not, however, pursue this line of thought. In fact, he fluctuated over the years regarding the significance of atomic theory to his table.
However, others could read the words on the table. In 1888, the German chemist Johannes Wislicenus stated that the periodicity of the properties of the elements when grouped by weight showed that atoms are made up of regular arrangements of smaller particles. Thus, in a way, Mendeleev’s table anticipated (and provided evidence for) the intricate interior structure of atoms, at a period when no one knew what an atom actually looked like or whether it even had an internal structure.
At the time of Mendeleev’s death in 1907, scientists were aware that atoms were composed of electrons, which carried a negative electric charge, and a positively charged component that rendered atoms electrically neutral. In 1911, physicist Ernest Rutherford, working at the University of Manchester in England, discovered the atomic nucleus. This discovery provided a crucial insight as to how these pieces were organized. Henry Moseley, a physicist who had collaborated with Rutherford, established shortly thereafter that the quantity of positive charge in the nucleus (the number of protons it possessed or its “atomic number”) determined the correct order of the elements in the periodic table.
Atomic weight and Moseley’s atomic number were so closely related that ordering elements by weight varied from ordering by number in only a few places. Mendeleev maintained that the weights were incorrect and needed to be remeasured, and in some instances he was correct. A few inconsistencies remained, but Moseley’s atomic number clarified the situation.
Niels Bohr, a Danish physicist, realized at the same time that quantum theory governed the arrangement of electrons surrounding the nucleus and that the outermost electrons determined the chemical properties of an element.
Explaning the patterns that Mendeleev’s table had initially revealed, similar configurations of the outer electrons would recur frequently. In 1922, based on experimental observations of electron energies, Bohr constructed his own version of the table (along with some guidance from the periodic law).
Bohr’s table included elements discovered after 1869, although it was fundamentally the same periodic arrangement as Mendeleev’s. Without any understanding of quantum theory, Mendeleev developed a table that reflected the atomic architecture mandated by quantum physics.
The new table created by Bohr was neither the first nor the last variation on Mendeleev’s original design. There have been hundreds of variations of the periodic table created and published. The contemporary form, a horizontal design as opposed to Mendeleev’s initial vertical variant, did not gain widespread acceptance until after World War II, partly thanks to the efforts of the American chemist Glenn Seaborg (a longtime member of the board of Science Service, the original publisher of Science News).
Seaborg and his colleagues had synthesized many new elements with atomic numbers higher than uranium, the last naturally occurring element in the periodic table. Seaborg recognized that these elements, the transuranics (together with the three elements before uranium), need a new row in the table, which Mendeleev had not anticipated. Seaborg’s table added the row for these elements below a similar row for the rare earth elements, whose proper position had also never been quite obvious. In a 1997 interview, Seaborg, who passed away in 1999, remarked, “It took a lot of courage to oppose Mendeleev.”
Seaborg’s contributions to chemistry earned him the 106th element to bear his name: seaborgium. It is one of a handful of elements named to honor a notable scientist, a list that includes, of course, element 101, discovered by Seaborg and colleagues in 1955 and dubbed mendelevium — for the chemist who deserved a position on the periodic table more than anybody else.
