Dmitri Mendeleev and Henry Moseley developed their periodic tables about forty years apart. During that time, many discoveries in chemistry were made.

Mendeleev arranged elements mainly by atomic mass, while Moseley showed that the periodic table’s order is based on atomic number—the number of protons. This clarified that an atom’s proton count is not the same as its total mass, which is determined mostly by protons and neutrons. So the discovery described by choice C helped explain the difference between the two tables.

AI-written and checked against the verified answer.