Between 1912 and 1914, the physicist H.G.J. Moseley conducted a series of experiments where he bombarded targets made out of differ-ent kinds of metals with cathode rays. Each metal he studied emitted X-rays of a character-istic frequency, almost like a set of "fingerprints". He found that the heavier the atomic mass of the element, the shorter was the wavelength and the more penetrating were the X-rays. The pattern that emerged when the observed X-rays were organized in order of increasing frequency suggested to Moseley a regular increase in the positive charge on the nuclei of the atoms. In passing from one element to the next in the periodic table, the atomic number (Z) always varied by one unit. Occasional gaps indicated that an element was missing. In 1914, Moseley published his findings.

Moseley's efforts helped solve riddles which had perplexed Mendeleev and other scientists for many years. Both the apparent irregularities in the location of such elements as potassium and argon and the positioning of the rare earth (inner transition) elements in the periodic table could now be explained on the basis of atomic number. The modern periodic law now states that the properties of elements are a periodic function of their atomic number. Moseley's methods also permitted a complete survey of the elements and it showed that, with a few exceptions, the periodic table was complete. At age 27, Moseley volunteered for military service during World War I and became a signal officer in the British army. He was killed in action in Gallipoli in 1915.