Moseley used X-ray spectroscopy to determine the atomic number of various elements. He discovered that the frequency of X-rays emitted by an element was directly proportional to its atomic number, rather than its atomic weight. This provided a more precise and accurate method for determining the order of elements in the periodic table, which had been based on atomic weight up to that point.

Moseley's work was groundbreaking and revolutionized our understanding of the atom and its structure. It also paved the way for the development of modern periodic table, which is now organized by atomic number.