The assignment of atomic numbers to each element in the periodic table is credited to Henry Moseley. In the early 20th century, Moseley conducted experiments involving X-ray spectroscopy and found a systematic relationship between the X-ray frequencies emitted by various elements and their atomic numbers. He discovered that the X-ray frequency was directly proportional to the square of the atomic number, which led him to propose that the atomic number of an element is determined by the number of protons in its nucleus. Moseley's work revolutionized the understanding of the periodic table and provided a more accurate ordering of elements based on atomic number rather than atomic mass, thus establishing the foundation for the modern periodic table.