In previous chapters we have seen how chemistry has evolved in stages. First was the world of the ancients with their concept of the four elements — fire, earth, water and air. Then the alchemists in their laboratories developed chemical techniques and discovered various compounds. Next, modern chemistry was born when Lavoisier recognized the true elements (“simple substances”). Finally, Davy was able to prepare and/or recognize the active metals and nonmetals. However, before the Periodic Table was possible, a critical step was necessary — the development of atomic theory and atomic weights (today called “atomic masses”).
The British chemist John Dalton (1766-1844) was the first to introduce modern atomic theory with a table of atomic weights of selected elements. He assumed that each element was unique with its own atomic weight. Interestingly, this idea was prompted by his colorblindness; he had to choose some way to give each element its unique properties, and weight seemed to him to be a natural choice. (“Daltonism” is the modern term for red-green colorblindness.) In his A New System of Chemical Philosophy (1808) he proposed atomic weights for twenty of the more common elements (see Figure 1). He assumed each element was a featureless sphere; he constructed wooden models of the atoms themselves, appearing as featureless brown croquet balls. These models may be viewed today in the Science and Industry Museum in Manchester, England.
In his system, Dalton included illustrations of combinations of atoms to form compound substances (he called these atoms, also — the word “molecule” was adopted by Avogadro three years later). A water “atom”, for example, was portrayed by an atom of hydrogen and an atom of oxygen tangentially joined together, and ammonia as an atom of hydrogen and an atom of nitrogen (it was not known yet that water and ammonia were composed of multiple hydrogen atoms). Examples of his representation of compound substances can be see in "Part 2: The evolution of models" by Dr. David Stone (Chem 13 News, March 2018).
Figure 1. Dalton’s atomic weights for 20 elements.
The next step preparing the way for the Periodic Table was the work of the Swedish chemist Jöns Jacob Berzelius (1779-1848) (Figure 2). Berzelius raised the sophistication of laboratory chemistry to new heights and in 1818 was able to determine precise atomic weights for 45 of the known 49 elements. The well-known contemporary Scottish biographer Thomas Thomson (1773-1852) wrote “There is no living chemist to whom analytical chemistry lies under greater obligation than to Berzelius, whether we consider the number or exactness of the analyses which he has made.”1 The broad laboratory skill and imagination of Berzelius was legendary — indeed, he was the first to recognize the electrical character of chemistry by observing the drift of alkalis and earths to the negative pole of a voltaic cell and of oxygen, acids, and oxidized substances to the positive pole. This was three years before Humphry made his acclaimed discovery of the preparation of the active metals themselves (see previous chapter on “Davy’s Metals”). In fact, Berzelius graciously and unselfishly gave advice to Humphry on how best to construct the electrolysis experiment by using mercury amalgams.
Berzelius also had an incredible ability to assimilate all the current chemistry and recognize the significance of important discoveries of others. He launched his own journal (Treaties in Physics, Chemistry, and Mineralogy) where he single-handedly reported and evaluated all of the recent happenings in the physical sciences. Soon his fame became legendary throughout Great Britain and the Continent. A recent well-known mineralogist noted, “Berzelius always seemed to be at the center of every significant scientific discovery in chemistry.”2 The list of element discoveries by Berzelius is impressive. He discovered thorium in a new mineral (thorite, ThSiO4) obtained from an island in a Norway fjord; shiny dark crystals of this mineral had been discovered by a parson hunting ducks in a rowboat. In Mariefred, Sweden, Berzelius set up a sulfuric acid plant and analyzed the residue from pyrite, which had the odor of radishes, thereby discovering selenium (this site is now a Swedish Red Cross training center; his laboratory is now used as a tool shed). Berzelius was the first to prepare elemental silicon from quartz (SiO2) mined from a local quarry in Sweden. He also prepared elemental zirconium from zircon (zirconium silicate, ZrSiO4) from India; the mineral itself had been characterized three decades earlier by Klaproth. He and his business partner Wilhelm Hisinger discovered cerium from cerite (a mixed rare earth silicate) collected from Hisinger’s iron mine in Riddarhyttan, central Sweden (this was one of Berzelius’ original discoveries and chronologically belongs in the “Age of the Miners”). Because of the extreme precision in his laboratory, he and his student Johan Arfwedsen were quick to recognize the importance of a tiny discrepancy in careful weighings during the analysis of the mineral petalite (lithium aluminum silicate) from a Swedish iron mine on a Baltic Sea island; their meticulous research allowed them to discover lithium, the third lightest element. Previously, laboratory chemists, when analyzing lithium compounds, had sometimes observed this same discrepancy but casually assumed it was due to “experimental error”.
Students of Berzelius went on to make further impressive discoveries. Carl Gustaf Mosander analyzed cerite in greater detail to recognize the presence of additional rare earths — lanthanum, didymium, erbium and terbium (didymium was later shown by Auer von Welsbach to be a mixture of two rare earths, which he named praseodymium and neodymium). Berzelius’ postdoctoral student Friedrich Wöhler returned to Germany where he gained fame as the destroyer of “vitalism” — the belief that organic compounds had a special “vital force” — by showing he could prepare organic urea OC(NH2)2 from inorganic ammonium cyanate (NH4CNO).
Berzelius realized that in order to promote modern atomic theory, it was necessary to streamline the symbol formalism. He introduced simple alphabet characters to symbolize each element. Thus, for Dalton’s various circles he simply wrote H, N, C, O, Si, Fe, etc. Furthermore, Berzelius’ proposed system meant much more than simplification of characters — he meant them to designate precisely the stoichiometry of each compound, i.e., how many atoms were present of each element in the final compound. He originally designated the number of oxygen atoms by dots surrounding a center atom, but later used superscripted numbers, e.g., H2SO4 for sulfuric acid. A publisher changed this to subscripted numbers (H2SO4), the protocol used ever since.
Figure 2. Berzelius' towering statue in Berzelii Park, Stockholm, Sweden
Simultaneous with the work of Berzelius and his students were several discoveries by others on the main European Continent. Cadmium was discovered by Friedrich Stromeyer, who isolated the brown impurity in commercial zinc oxide (a topical ointment) and showed it to be another element; he named it cadmium after the Latin word for zinc oxide, calamine.
Aluminum was isolated by Hans Christian Ørsted in Copenhagen; it had been known to be a common element in clay, but had stubbornly resisted the electrolytic apparatus of Davy.
Ruthenium was discovered by Carl Claus in Kazan, Russia, as an impurity in platinum procured from the Ural Mountains; ruthenium was the last element of the platinum group to be discovered.
Bromine was isolated by Antoine Balard in Montpelier, France, from the étangs (ancient Mediterranean lagoons) which had a flourishing business in salt production. Balard noticed that sea algae, when treated with chlorine, developed a brown color. When he passed a stream of chlorine through a mother liquor of brine, he was able to distill the brown liquid bromine into a retort.
A final preparation of metallic beryllium was made by Wöhler in Berlin; the actual discovery of beryllium had been made by Nicolas-Louis Vauquelin in Paris, France, the same person who discovered chromium, three decades earlier. Vauquelin had recognized that beryllium (originally called glucinium because it tasted sweet — danger! Poisonous!) had been misidentified as aluminum in previous analyses of emeralds and beryl (beryllium aluminum silicate), but he could not isolate metallic beryllium itself.
One more step was needed before the construction of the Periodic Table: elements discovered by spectroscopy. This story will be told in the next chapter.
References and notes
- T. Thomson, The History of Chemistry, Vol 2., 1830, Henry Colburn and Richard Bentley, London, pages 221-223.
- P. B. Moore, The Mineralogical Record, 1988, 19, pages 293-295. Paul Moore (1940-2019), a distinguished mineralogist, discovered and described 35 new mineral species and was a superb crystallographer who solved many crystal structures of minerals.
