Henry Moseley - English Physicist
(1887-1915)
1913
Using the newly discovered field of X-ray spectroscopy, Moseley discovers a regular, step like increase in the frequency of the Ka line in the spectra of elements. The progression of the Ka line could be expressed as a function of an integer, Z, which Moseley named the atomic number. He surmised that it defined the net positive charge, or number of protons, in an atom. By ordering the elements by increasing atomic number and not by increasing atomic mass as Mendeleev had done, Moseley was able to find agreement with Mendeleev's table without having to switch the positions of any elements. The real importance of Moseley's periodic table is that by ordering the elements in terms of atomic number, which increases by integer values, there is no possibility of finding new elements in between other successive elements because we can not have half a proton. So Moseley's table limited the number of known elements to 92 (H [Z=1] to U [Z=92]). This allowed Moseley to predict the existence of seven elements which had not yet been discovered and, along with Soddy's idea of isotopes, allowed scientists to sort out all of the radioactive decay fragments which had been misinterpreted as new elements.
"Moseley's step ladder" arranged with frequency decreasing from left to right. The darker of the two lines is Ka, the other Kb. The regular decrease in frequency of both lines can be seen. It is obvious that something is missing between Ca and Ti. If we move Ca up, we find another spectra can be placed in the sequence. This is Scandium, which was known to Moseley but which he had not yet examined when he drew up the ladder.
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