The Past and Future of the Periodic Table This stalwart symbol of the field of chemistry always faces scrutiny and debate Eric R. Scerri t graces the walls of lecture halls making up one period. The lengths elements, Döbereiner found that the Iand laboratories of all types, from of periods vary: The first has two ele- weight of the middle element—such as universities to industry. It is one of ments, the next two eight each, then 18 selenium in the triad formed by sulfur, the most powerful icons of science. It and 32, respectively, for the next pairs selenium and tellurium—had an atom- captures the essence of chemistry in of periods. Vertical columns make up ic weight that was the approximate one elegant pattern. The periodic table groups, of which there are 18, based average of the weights of the other two provides a concise way of understand- on similar chemical properties, related elements. Sulfur’s atomic weight, in ing how all known chemical elements to the number of electrons in the outer Döbereiner’s time, was 32.239, where- react with one another and enter into shell of the atoms, also called the va- as tellurium’s was 129.243, the average chemical bonding, and it helps to ex- lence shell. For instance, group 17, the of which is 80.741, or close to the then- plain the properties of each element halogens, all lack one electron to fill measured value for selenium, 79.264. that make it react in such a fashion. their valence shells, all tend to acquire The importance of this discovery lay But the periodic system is so funda- electrons during reactions, and all form in the marrying of qualitative chemical mental, pervasive and familiar in the acids with hydrogen. properties, such as degree of reactivity, study of chemistry that it is often taken with numerical data on the elements. for granted. A century after the death The Classics It suggested that there might be some of the leading discoverer of the period- There have been many changes to the underlying numerical order that could ic system, the Russian chemist Dimitri table since Mendeleev’s first, which serve to relate the elements to one an- Mendeleev, it seems time to revisit the showed eight groups, 12 rows and 66 other in a systematic way. origins and modern status of this now- elements, was published in 1869. But Döbereiner also discovered other standard chemical classification. There neither did Mendeleev’s table spring triads, such as calcium, strontium and were a number of historic precursors from a vacuum. Historians of chem- barium, and lithium, sodium and po- to Mendeleev’s periodic system. But istry have long recognized two ideas tassium. Other chemists discovered there are also current ongoing debates that contributed to the evolution of the yet more triads and began to make ta- regarding the best way to display the periodic system: the notion of triads bles that also attempted to relate triads periodic system, and whether there is of elements and Prout’s hypothesis, among one another. But some of these really a “best way” of doing so. whereby the atomic weights of the el- contributions degenerated into mere The periodic system of elements ements are integral multiples of the numerology, especially when they ne- gets its moniker because it graphs how atomic weight of hydrogen, the light- glected chemical relations between the certain properties of chemicals repeat est of all the elements. elements. For example, in his 1857 ar- after regular intervals. In the modern In 1817 the German chemist Johann ticle, German chemist Ernst Lenssen table of 117 elements, each is placed Döbereiner noticed that several groups suggested the existence of a triad con- across rows in order of increasing of three elements formed triads with sisting of silicon, boron and fluorine, atomic number—the number of pro- two interesting features. Not only was even though there was no conceivable tons in the nucleus of one atom of each the middle element of a triad of inter- chemical connection between these el- element. There are seven rows, each mediate chemical reactivity, but it also ements. Nevertheless, the lure of the had an intermediate atomic weight. search for triads encouraged chemists Differing from atomic number, a val- to determine atomic weights more ac- Eric R. Scerri is a lecturer in chemistry and the ue that had not yet been ascertained, curately, a pursuit that served chemis- history and philosophy of science at University of atomic weight had been measured try in many other ways. California, Los Angeles. He is the founder and edi- since the start of the 1800s. The idea A little earlier, in 1815, the London tor-in-chief of the journal Foundations of Chem- was to determine the weight of each based physician, William Prout, pro- istry and the author of The Periodic Table: Its indivisible unit of matter relative to posed another general principle. In a Story and Its Significance (Oxford University Press, 2007). He received his Ph.D. in the history hydrogen, whose weight was taken few papers, which he published anony- and philosophy of science from King’s College Lon- as 1.00. Because formulas for many mously, Prout wrote that the fact that don. Address: Department of Chemistry & Bio- compounds were unknown, atomic the atomic weights of many elements chemistry, UCLA, Los Angeles, CA 90095-1569. weights remained imprecisely mea- seemed to be integral multiples of the Internet: [email protected] sured for some time. But in triads of weight of hydrogen suggested that all 52 American Scientist, Volume 96 © Theodore Gray 2007 www.periodictable.com Figure 1. With its compact form displaying a wealth of information about the elements, the periodic table has long been a standard-issue reference in the field of chemistry. Theodore Gray created this museum display to improve on the appeal of the table by adding samples of each element behind its entry. (This close-up shows, at left, the triad sulfur, selenium and tellurium, studies of which were a precursor to the periodic system.) The story of the periodic table is one of gradual improvements, from early measurements of atomic weight to current-day proposals for new layouts of the elements that look nothing like a two-dimensional chart. the elements were composite multiples curate atomic-weight data began to to positive charge, or the number of of hydrogen. He also went on to claim accumulate. The notion of triads was protons in the nucleus of any atom. that this would imply the essential unity found to be too approximate and even On re-examining the notions of triads of all matter. But some elements such as then only applied to carefully selected and Prout’s hypothesis in the light of nitrogen, which then had a value of 12.6 groups of three elements. Meanwhile, atomic number, one finds a remarkable relative to hydrogen, seemed to point Prout’s hypothesis showed too many sense in which both notions have made against Prout’s hypothesis. Prout’s sup- nonintegral exceptions. In the lan- what another famous philosopher of porters regarded such facts as anoma- guage of philosopher of science Karl science, Imre Lakatos, has termed a lies that would eventually disappear Popper, both ideas had been refuted theoretical comeback. In terms of with the more accurate determination by the second half of the 19th century. atomic number, the elements have ex- of atomic weights. At the start of the 20th century, it act multiples of the number of protons As in the case of triads, attempts to was found that atomic number, rather in the hydrogen atom—as hydrogen confirm or refute Prout’s hypothesis than atomic weight, serves as the more has only one proton, everything is a contributed to renewed efforts on the correct criterion for ordering the ele- multiple of it. And perhaps in a deeper part of chemists to measure atomic ments in a linear sequence. Research- sense, modern astrophysics has shown weights. However, although these ers such as the British physicist Henry that almost all of the elements are liter- ideas were fruitful in some ways, they Moseley found that they could use x- ally formed from hydrogen and he- were also found wanting as more ac- ray diffraction to relate atomic number lium atoms, which combined together www.americanscientist.org 2008 January–February 53 during the Big Bang at the not be further simplified. start of the universe, as well This is the notion of ele- as in the interiors of stars ments as first emphasized and supernovae. by Antoine Lavoisier in the Moreover, if we consider 1700s, when he called them atomic numbers instead of ”simple substances.” atomic weights for the tri- But there is a second no- ads discovered in the 19th tion, which Mendeleev century, it turns out that sometimes called ”real ele- the atomic number of the ments,” in order to indicate middle element is exactly their more fundamental the average of the other two status. In this sense, the el- elements. Indeed, about half ements represent abstract of all the possible triads in substances that lack what the modern periodic table we normally regard as are exact in this sense. How- properties and that repre- ever, many other potential sent the form that elements triads are not even approxi- take when they occur in mately correct in that the compounds. For example, atomic number of the mid- sodium and chlorine as dle element is nowhere near simple substances—a grey the average of the other two metal and a greenish gas re- elements.