Up until Moseley's time, chemical elements in the iconic Periodic Table were arranged according to weight. There was other rhyme and reason to the arrangement of elements in the Table, but no true understanding of their masses beyond: things get heavier. There was hope that atomic mass would reveal something fundamental about physics, and the 1914 Nobel Prize went to Harvard's T. W. Richards for his careful and methodical measurements of atomic weights.
Moseley showed that by shining X-rays onto atomic samples, he got a distinct integer value for each element which he called Z. Others before Moseley -- namely Bunsen and Kirchoff -- had shown how unseen atoms could be "seen" and identified by burning them in flames, but Moseley's experiments were beautifully simple and related all elements together with their Z-values instead of getting a unique "fingerprint" for each. Moseley's law is still used to identify elements in deep space.
Exactly what Z was had only been postulated a few years earlier. Niels Bohr had shown that Z was the nuclear charge (1 for the hydrogen atom) and Ernest Rutherford had suggested that Z for heavy atoms might be about half an element's atomic weight. A Dutchman, Antonius van den Broek had suggested--without proof-- that Z was an element's "atomic number." Moseley proved it.
Good ideas need good proof to become good science.
The Periodic Table was never the same after Moseley.
Henry Moseley probably should have gotten the 1915 or 1916 Nobel Prize in Physics, but he was killed by a Turkish bullet at Gallipoli at the age of 27.
|Henry Moseley (1887-1915)|