H. G. J. Moseley

The Discovery of the Proton (H. G. J. Moseley)

Shortly before the first World War, A. van den Broek's observation was explained by H. G. J. Moseley, who studied the frequencies of the x-rays given off by cathode-ray tubes when the electrons strike the anode. Moseley found that the frequencies of these x-rays depend on the metal used to form the anode.

Aluminum is the first metal when the elements are arranged in order of increasing atomic weight. It was therefore the lightest element for which Moseley was able to obtain results. Because it is the thirteenth element in the periodic table, aluminum has an atomic number of 13. (By convention, the symbol for atomic number is a capital "Z.") Moseley found a relationship between the frequencies of the x-rays given off by a cathode-ray tube and the atomic number of the metal used to form the anode.

Moseley argued that the frequencies of the x-rays given off by the anode when it was hit by electrons in the cathode-ray tube should depend on the charge on the nucleus of the atom emitting these x-rays. He therefore concluded that the atomic number of an element is equal to the positive charge on the nucleus of its atoms. Aluminum, for example, must have a net charge of +13 on the nucleus of each atom.

Less than six months after completing this work, Moseley volunteered to serve in the British army during World War I and became one of the more than 8.5 million casualties of this war when he was killed during the battle at Gallipoli in August 1915. Shortly after the war, in 1920, Rutherford proposed the name proton for the positively charged particles in the nucleus of an atom.