At the beginning of the 20th century, scientists still weren’t sure what to make of the periodic table. Older classifications of the chemical elements ran in order of increasing atomic weight. Dmitri Mendeleev’s table aimed to capture periodic trends in their properties, the so-called Periodic Law, forcing him to relegate atomic weights to a secondary consideration [1]. For example, cobalt has a greater atomic weight than nickel, yet the Periodic Law dictates that cobalt comes before nickel based on its chemical properties. These variations in atomic weight also left the possibility of unknown elements that, if discovered, might not fit into the table’s periodic structure [2]. It was clear that atomic weight was not the defining characteristic of the elements, but nobody could confirm what else that might be.

Ernest Rutherford believed the solution might involve a new phenomenon called radioactivity, thanks to his experiments with radioactive decay. He assigned Henry Moseley, his new graduate student from Oxford, to study this phenomenon in 1910 [3]. But Moseley had other ideas. He has been following the developing field of X-rays closely ever since their discovery by Wilhelm Roentgen a decade earlier [4]. “Characteristic X-rays” of varying energy would be emitted when an element was struck by a stream of electrons. Moreover, the X-rays would be scattered through slightly different angles for each element used, and a technique to determine their wavelengths from this information had recently been developed by William and Lawrence Bragg [2]. This brought the intriguing problem of X-rays back within sight of the periodic table.

Moseley decided to extend this line of research and systematically measure the wavelengths of each element’s characteristic X-rays. Returning to Oxford in 1913, he did this with a remarkably simple setup. By running a little train carrying samples of each element through a vacuum tube and then passing the line of fire of an electron beam, he was able to capture the positions of the emitted X-rays on photographic plates [1]. Knowing the angle by which they had been scattered, he could then calculate the wavelengths of the characteristic X-rays for each element. Moseley found that as the elements progressed up the periodic table, the scattered X-rays decreased in wavelength – and by taking the inverse square root of the wavelength, this relationship became a straight line [5].

This became known as Moseley’s law, and Moseley could also explain how it came about. Two years earlier, Rutherford devised a model of the atomic structure: negatively charged electrons orbit a dense positively charged nucleus within the atom, with these charges cancelling each other out [5]. Moseley argued that the increasing size of this positive charge as atoms progressed up the periodic table would halt his electron beam more effectively, triggering a greater release of energy in the form of higher-frequency and lower-wavelength X-rays [2]. Because these positive charges could not be altered by chemical means, and were clearly a basic property of the atom, he suggested referring to them as “atomic numbers”.

Thanks to this breakthrough, Moseley could now “call the roll” of the elements. If the X-ray wavelengths of two elements differed by a known minimum, there could be no other elements between them. From hydrogen to uranium there were exactly 92 elements, and it soon became obvious where the missing elements had to go. As Mendeleev had done, Moseley and others identified gaps at atomic numbers 43, 61, 72, 75, 85, 87 and 91, all of which were filled in the following 30 years [4]. The reversal of cobalt and nickel was also completely justified by cobalt’s lower nuclear charge, the proper basis for its order in the table [4].

Henry Moseley had brought meaning to the order of the elements and set the periodic table on a firm foundation in the process, grounding it in a reality far deeper than the chemical and physical properties Mendeleev saw [4]. If anyone can be said to have “proved” the periodic table, it can only have been him.

References:

[1] Sacks, O. W. (2001). Uncle Tungsten: Memories of a Chemical Boyhood. New York, NY: Alfred A. Knopf, Inc.

[2] Asimov, I. (1970, April). The Nobel Prize that Wasn’t. The Magazine of Fantasy and Science Fiction.

[3] Roberts, J. (2020, April 7). The Dual Legacies of Henry Moseley. Distillations. Retrieved from https://www.sciencehistory.org/distillations/the-dual-legacies-of-henry-moseley.

[4] Scerri, E. (2014). Master of Missing Elements. American Scientist, 102(5), 358. doi:10.1511/2014.110.358

[5] Kean, S. (2010). The Disappearing Spoon: And Other True Tales of Madness, Love, and the History of the World from the Periodic Table of the Elements. United States: Little, Brown and Co.