In 2019, marks the 150th anniversary of the most beloved icon in chemistry, Dmitri Mendeleev’s Periodic Table of Chemical Elements. To honor this milestone, the United Nations Educational, Scientific, and Cultural Organization (UNESCO) proclaimed 2019 “The United Nations International Year of The Periodic Table of Chemical Elements” (IYPT 2019). The American Chemical Society (ACS), the International Union of Pure and Applied Chemistry (IUPAC), and scientific societies around the world will all be celebrating with special events, contests, and more.
Antoine Lavoisier, now known as the ‘father of modern chemistry,’ publishes a list of 33 elements or “simple substances,” as he calls them. Although his list includes things such as heat and light, it is a major departure from previous thinking about elements. For Lavoisier, an element represents the final stage of chemical decomposition. This view moves away from earlier metaphysical notions about the nature of elements and emphasizes what can be observed and measured.
John Dalton, a Manchester schoolteacher and a Quaker, revives the atomic theory of ancient Greek philosophers while making it quantitative. Dalton also provides a new list of elements but his includes the relative weights of atoms of each element compared with an atom of hydrogen, which is assigned a weight of one unit. This development provides a basis from which other chemists can begin to discern relationships between different elements and is an essential step in the development of the periodic table.
Wolfgang Döbereiner, a chemist working in Jena, Germany, draws on John Dalton’s atomic weights to discover triads, which are relationships among several groups of three elements whereby one of the three elements is the average of the two others in two respects. For example, a sodium atom has about the same weight as the averaged weights of lithium and potassium. Also, sodium’s chemical reactivity is the average of lithium and potassium. Triads thus hint at mathematical relationships between different elements, representing a foreshadowing of the discovery of chemical periodicity.
Over a period of about five years, multiple scientists independently develop significant precursors to the periodic table. The first is French geologist Alexandre-Emile Béguyer De Chancourtois, who arranges the elements in a line in order of increasing atomic weight. This line is then arranged in a helical fashion around a metal cylinder so that similar elements fall along vertical lines drawn along the length of the cylinder. Soon after, John Alexander Reina Newlands and William Odling, working independently in England, publish two-dimensional periodic tables, as does Gustavus Heinrichs, a Danish exile working in the United States. None of these systems receive much credit for a variety of reasons both scientific and sociological.
Julius Lothar Meyer, a German chemist, publishes a number of periodic tables that represent the discovery of a fully mature table system. However, although he successfully accommodates most of the more than 60 then-known elements, Lothar Meyer fails to predict any new or missing elements, with one exception. He made a tentative prediction for the existence of a single element that he believed would have an atomic weight of 44.55. This element would eventually be discovered in Sweden and named scandium. Its weight when first measured was 44.6.
Dmitri Mendeleev, a Siberian by birth, working in St. Petersburg, Russia, publishes his first of many periodic tables and predicts the existence of four new elements that he provisionally names eka-aluminum, eka-silicon, eka-boron, and eka-manganese. Within fifteen years, the first three of these elements are discovered by other chemists and are called respectively gallium, scandium, and germanium, thus serving to solidify Mendeleev’s reputation as the leading discoverer of the periodic table. The fourth of his initial predictions is synthesized in 1937 and named technetium.
Chemical Reviews. March 8, 2017. Volume 117, Issue 5. Halogen chemistry plays a central role in the manufacture of various chemicals, pharmaceuticals, and polymers, and has potential applications in natural-gas upgrading. Having a closed halogen loop allows these processes to operate efficiently and sustainably. To this end, the design of suitable heterogeneous catalysts is of key importance.
In three successive years, X-rays, radioactivity, and the electron are discovered, all of which have a profound impact on the study of the elements, the periodic table, and chemistry in general. X-rays lead to an experimental method to precisely identify each element. The discoveries of radioactivity and the electron show atoms are not indivisible as Dalton had supposed, but have a sub-structure. In 1900, Max Planck introduced his quantum of action. These discoveries together would soon explain why elements fall into groups on the periodic table.
In 1913, Niels Bohr, working in Copenhagen, publishes the first explanation of why certain elements fall into particular groups in the periodic table. This feature arises because of the analogous electron arrangements in concentric shells around the nucleus of an atom. Between 1913 and 1914, Henry Moseley, in Manchester and later Oxford, establishes experimentally that elements are more accurately ordered according to an ordinal number, subsequently named “atomic number,” than if ordered according to atomic weight, as had been the custom up to this point. Moseley’s method also provides the means to uniquely identify any particular element, as well as indicating the number of elements that remained to be discovered between the naturally occurring elements from hydrogen (Z = 1) and uranium (Z = 92).
The first artificially produced element is discovered in Palermo, Sicily by Emilio Segrè and coworkers. This element had been synthesized in a particle accelerator at the University of California, Berkeley, where Segrè had worked, before being sent to Italy for analysis. This was to be the first of what are now about 30 artificially produced elements, including promethium (Z = 61) and astatine (Z = 85), in addition to 26 transuranic elements. The most recent discoveries of such elements are nihonium (Z = 103), moscovium (Z = 105), tennessine (Z = 117), and oganesson (Z = 118).
The first transuranic element, synthesized at the University of California, Berkeley by Edwin Mattison McMillan and Philip Hauge Abelson, is neptunium. This is followed by the synthesis of plutonium by Glenn T. Seaborg in 1941 in the same laboratory. Seaborg would contribute to the synthesis of a total of 10 such transuranic elements, including element 106, which is named seaborgium in his honor. He would also propose a modification to the periodic table that features the actinides as part of the f-block rather than as d-block elements. Similar arrangements were independently proposed earlier by Alfred Werner and Charles Janet.
The periodic table is by no means a closed subject. Although it now stands complete for the first time since its discovery, attempts to synthesize elements 119 and 120 are being actively pursued. If discovered, these elements would form the beginning of a new eighth period. In addition, debate continues over the placement of several elements, including thecomposition of group 3, and over whether there is an optimal form of the periodic table. A good candidate to fill this role might be Charles Janet’s left-step table, which displays greater regularity than the conventional table, as well as being more in keeping with the presumed quantum mechanical foundations of the periodic system.