When the first periodic table of the elements was published, the elements were arranged by atomic mass. Although several elements seemed out of order, later discoveries showed the order to be correct. Which event most likely helped the development of the periodic table?

The early periodic table arranged elements by atomic mass, but some elements did not fit the expected pattern. Later discoveries about atomic structure—especially atomic number and the number of protons—explained why the elements’ positions should be adjusted. Thus, new information changed scientists’ understanding of the atom’s structure; changing measured masses was not the key development.

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