Final answer:
Henry Moseley discovered the importance of atomic number, which corresponds to the number of protons in an atom's nucleus, leading to the proper ordering of elements in the periodic table based on atomic number rather than atomic mass.
Step-by-step explanation:
Henry Moseley discovered that the atomic number of an element is far more important than its atomic mass for the periodic classification of the elements. Through his experiments with X-ray spectroscopy in 1913, he observed that the X-ray frequencies emitted by elements correlated with their atomic number, which is the number of protons in the nucleus. This led to a pivotal modification of the periodic table, where elements were arranged by increasing atomic number instead of atomic mass, rectifying previous inconsistencies, and providing a clearer understanding of the Periodic Law.
The discovery by Moseley resolved doubts and exceptions that did not fit with the arrangement based on atomic masses, such as the placement of some transition metals and the noble gases. His findings underscored the atomic number as a fundamental property defining the identity of each element and shaping their chemical behavior. Consequently, the modern periodic table reflects this understanding, allowing prediction of an element's properties based on its position relative to atomic numbers.