Harry Moseley
On August 13th, 1913, Harry Moseley sent a letter to his mother explaining how his results on x-ray diffraction could help prove Bohr's hypothesis that atomic number was more than just a placeholder. A few days later he completed the work, concluding that every time there was an increase in atomic number, there was a change in x-ray diffraction patterns. This was the push that Bohr's model of the atom needed in order to be taken seriously by the scientific community—and it radically changed our understanding of the atom. Two years later (almost to the day), Harry Moseley died in World War I.
Source: Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon & Schuster, 1986.
Public Domain
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Full disclosure, I may occasionally borrow a sentence from Will Durant's Story of Civilization. I absolutely love that collection!