In 1913–14 Henry Moseley showed that atomic number is the quantity that characterizes a chemical element. He did so using X-ray spectroscopy, which he interpreted in terms of Rutherford’s model of the atom and Bohr’s quantum theory of atomic structure, with Moseley’s law expressing this quantitatively. Thus he showed that atomic number is the positive charge of the nucleus and the number of electrons in an atom of an element. He was killed in World War I.