Friday, March 16, 2012

Henry Moseley

Henry Moseley is an English physicist who developed X-ray methods to assign a unique atomic number Z to each element. He appears in Problem 3 of Chapter 16 in the 4th edition of Intermediate Physics for Medicine and Biology.
Problem 3 Henry Moseley first assigned atomic numbers to elements by discovering that the square root of the frequency of the Kα photon is linearly related to Z. Solve Eq. 16.2 for Z and show that this is true. Plot Z vs the square root of the frequency and compare it to data you look up.
Asimov’s Biographical Encyclopedia of Science & Technology (Second Revised Edition) describes Moseley:
“For a time [Moseley] did research under Ernest Rutherford where he was the youngest and most brilliant of Rutherford’s brilliant young men . . .

This discovery [that each element could be assigned an atomic number] led to a major improvement of Mendeleev’s periodic table. Mendeleev had arranged his table of elements in order of atomic weight, but this order had had to be slightly modified in a couple of instances to keep the table useful. Moseley showed that if it was arranged in order of nuclear charge (that is, according to the number of protons in the nucleus, a quantity that came to be known as the atomic number) no modifications were necessary . . . Furthermore, Moseley’s X-ray technique could locate all the holes in the table representing still-undiscovered elements, and exactly seven such holes remained in 1914, the year Moseley developed the concept of the atomic number.”
Moseley died when he was only 28 years old. Asimov tells the story:
World War I had broken out at this time and Moseley enlisted at once as a lieutenant of the Royal Engineers. Nations were still naïve in their understanding of the importance of scientists to human society and there seemed no reason not to expose Moseley to the same chances of death to which millions of other soldiers were being exposed. Rutherford tried to get Moseley assigned to scientific labors but failed. On June 13, 1915, Moseley shipped to Turkey and two months later he was killed at Gallipoli as part of a thoroughly useless and badly bungled campaign, his death having brought Great Britain and the world no good . . . In view of what he might still have accomplished (he was only twenty-seven when he died), his death might well have been the most costly single death of the war to mankind generally.

Had Moseley lived it seems as certain as anything can be in the uncertain world of scientific history, that he would have received a Nobel Prize in physics . . .”
To learn more about Moseley, I recommend Chapter 4 (“The Long Grave Already Dug”) in Richard Rhodes’ classic The Making of the Atomic Bomb. Rhodes writes that “When he heard of Moseley’s death, the American physicist Robert A. Millikan wrote in public eulogy that his loss alone made the war ‘one of the most hideous and most irreparable crimes in history.’ ”

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