In this installment on the history of atom theory, physics professor (and my dad) Dean Zollman discusses the origin of something we all remember from science class – a table listing elements by atomic weight. In the 1860s, two chemists whose work endures today created periodic tables – and they argued for years over who should get the credit. – Kim
By Dean Zollman
The Karlsruhe Conference of 1860, which we discussed last time, ended by making some progress toward decreasing the chaos in chemistry but without reaching most of its organizer’s goals. By the end of the conference, Stanislao Cannizzaro provided the participants some structure and information on which they could build. That information was particularly valuable to two young chemists – Julius Lothar Meyer and Dmitri Mendeleev.
Prior to the Karlsruhe Conference, confusion existed over the atomic weights of some elements. Cannizzaro had helped clarify some issues by reminding the participants of Amedio Avogadro’s work that equal volumes of any gases contained equal numbers of particles. Cannizzaro had used his knowledge to sort out some of the issues related to whether they were studying molecules made of atoms or just a single atom. Thus, the chemistry community now had a much more accurate list of atomic weights for the elements. This list was still not entirely correct, but it enabled people like Meyer and Mendeleev to make some real progress in understanding the relations among the elements.
With a list of elements and their atomic weights, a chemist could organize the elements in order of increasing atomic weight and see if anything interesting popped out. Today, this seems rather trivial, but in the 1860s, it was not obvious to everyone, even many accomplished chemists, that such a list would have any interesting properties. In the historical literature today, six chemists are credited with pursuing this type of research. We will take a brief look at two of them who were at the Karlsruhe Conference and whose work still endures, although only one of them gets the lion’s share of the credit.
Julius Lothar Meyer and Dmitri Mendeleev were professors, and in the 1860s were writing chemistry textbooks. A question that faced any chemistry textbook author at that time was how to organize the presentation in the book. Because chemistry did not have an agreed upon foundation, they needed to invent organizational schemes for their writing. Thus, one of the issues was the order in which to present the elements.
Meyer was the first of the two to order the elements by atomic weight. He then noted that he could see regular changes in other properties of the elements. For example, potassium, rubidium, and cesium had particularly large volumes; they also tended to combine with only one hydrogen atom when making molecules. Elements with atomic weights between two of these elements had smaller volumes and would combine with larger number of hydrogen atoms. In his table, he made rows which increased in atomic weight with columns in which the elements in any one column had similar properties. By 1862, he developed the table shown below.
There is a lot of information in the table that I won’t discuss here. The most important features are the general order of the elements, the similar properties of the elements in the vertical columns, and that Meyer left blanks (dashed lines) where no known element fit the necessary properties.
Mendeleev was somewhat slower off the starting blocks. After the Karlsruhe Conference, he did not immediately begin using Cannizzaro’s ideas about atomic weights. His lecture notes from 1865 still show the old system. By 1868 when trying to organize the second volume of his textbook, he was using atomic weights. However he was apparently unaware of Meyer’s work even though it had been published several years earlier. And Mendeleev had studied in Germany, so he knew the language.
In addition to being a professor Mendeleev worked with the Free Economic Society of St. Petersburg. He had received a letter, dated February 17, 1869, which discussed arrangements for him to make an inspection of a cheese factory. Such a letter would be insignificant today except that Mendeleev used the back of the letter to sketch out his first ideas about a periodic table. Later that same day, he wrote out in some detail his idea for a periodic table. By 1871, his table was looking a lot like the ones we see in chemistry labs today.
As with Meyer, Mendeleev left blanks where no known element seemed to fit. He them made predictions about properties of elements to be discovered.
Volumes have been written about the history of the periodic table, so I won’t repeat that information here. If you are interested, a readable book is Eric Scerri’s The Periodic Table.
Meyer and Mendeleev argued for essentially the rest of their lives about who should receive credit for the periodic table. In fact, Mendeleev continued the argument after Meyer died in 1895. So why does Mendeleev get essentially all of the credit for discovery the periodic nature of the chemical elements? Some historians claim that Mendeleev’s predictions made his work more famous. However, Scerri notes that Mendeleev made as many wrong predictions as he did correct ones. An interesting view is that the credit had more to do with international politics than science. In the early 20th century, the Soviet Union was a rising economic power; Germany was disliked because of the World Wars. Thus, the Russian/Soviet scientist received the credit. See “An Element of Order” in Chemical Heritage Magazine for more on this idea.
Today, we use atoms and particularly the way electrons arrange themselves in the atoms to explain why the periodicity exists in the properties of atoms. However, neither Meyer nor Mendeleev needed atoms to create their tables. They worked with the physical and chemical properties of the elements. Meyer apparently occasionally mentioned atoms, but he died before the discovery of the electron. Mendeleev had no room in his system for electrons. He even did not like the discovery of the noble gasses. They did not fit in his system. (Today they are tacked on the far right side of the periodic table for good reason – their electronic structure.) So, even though we see atomic structure as the underlying principle that explains the periodic table, its discoverers had little or no reason to think about atoms.
Next month, we will step back in time to follow the beginnings of a thread that was running parallel to the developments in the properties of the elements. We will see how scientists starting looking at light being emitted by atoms, even though they did not yet know the origin of that light.
Dean Zollman is university distinguished professor of physics at Kansas State University where he has been a faculty member for more than 40 years. During his career he has received four major awards — the American Association of Physics Teachers’ Oersted Medal (2014), the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and AAPT’s Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.