Henry Moseley

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Henry Moseley (1887–1915) was an English physicist who helped prove that the atomic number—the number of protons in an atom—is a real physical property. He studied how X-rays are produced by different elements and discovered Moseley’s law, a simple rule that links the X-ray wavelength to the element’s atomic number. This gave strong experimental support for Bohr’s ideas about atoms and showed that the periodic table should be arranged by atomic number, not just by atomic weight.

Moseley showed there were gaps in the table for certain elements. He correctly indicated the existence of technetium (atomic number 43) and promethium (61), and he predicted two more missing elements (72 and 75). He also helped confirm that there are exactly 15 lanthanide elements in the periodic table. His work made the ordering of elements clear and gave a solid physics basis for the table.

Born in Weymouth, Moseley studied at Summer Fields, Eton College, and Oxford University. He worked with Ernest Rutherford in Manchester before returning to Oxford. When World War I began, he joined the British Army as a telecommunications officer. He was killed by a sniper at Gallipoli in 1915, at age 27. Many scientists believed he would have won the Nobel Prize had he lived.

Today, Moseley is remembered for his crucial insight into atomic structure. The Henry Moseley Medal and Prize is named in his honor.