Henry Moseley, X-ray spectroscopy and the periodic table

Abstract

Just over 100 years ago, Henry Moseley carried out a systematic series of experiments which showed that the frequencies of the X-rays emitted from an elemental target under bombardment by cathode rays were characteristic of that element and could be used to identify the charge on its atomic nucleus. This led to a reorganization of the periodic table, with chemical elements now arranged on the basis of atomic number Z rather than atomic weight A, as had been the case in previous tables, including those developed by Mendeleev. Moseley also showed that there were four ‘missing elements’ before gold. With further measurements up to uranium Z = 92, the Swedish physicist Manne Siegbahn identified two more missing elements. This paper provides an introduction to Moseley and his experiments and then traces attempts to ‘discover’ missing elements by X-ray spectroscopy. There were two successes with hafnium (Z = 72) and rhenium (Z = 75), but many blind alleys and episodes of self-deception when dealing with elements 43, 61, 85 and 87. These all turned out to be radioactive, with extremely small natural abundances: all required synthesis by a nuclear reaction, with radiological characterization in the first instance. Finally, the paper moves on to consider the role of X-ray spectroscopy in exploring the periodic table beyond uranium. Although the discovery of artificial radioactive elements with Z > 92 again depended on nucleosynthesis and radiological characterization, measurement of the frequencies or energies of characteristic X-rays remains the ultimate goal in proving the existence of an element. This article is part of the theme issue ‘Mendeleev and the periodic table’.

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The periodic law of the chemical elements: ‘ The new system of atomic weights which renders evident the analogies which exist between bodies ' []

“…Working as a demonstrator and research fellow within Rutherford's group, Henry Moseley went on to show that the charge on the atomic nucleus could be equated with the order number of an element in the periodic table [25,26]. Moseley's conclusions were based on the measurement of the frequencies of X-rays emitted from different elements under bombardment by cathode rays, as discussed in the paper by Egdell and Bruton in this volume [27]. In parallel with Moseley's experiments, the Danish physicist Niels Bohr, who was a regular visitor to Manchester, was working on his famous trilogy of papers published in 1913 [28].…”

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“…Moseley revised the periodic tables from atomic weight to atomic number and proved the relationship between frequency (energy) and the atomic number (Moseley's Law), which is the principle of X‐ray spectrometry. [ 7 ] He observed in an X‐ray spectrum that the K line transitions moved the same amount each time the atomic number increased by one. [ 8 ] In 1925, Coster and Nishina were the first to use primary X‐rays instead of electrons to excite a sample.…”