steps. Moseley found that for the K radiation the frequency of the radiation was proportional to where N was a whole number which varied by unity in passing from one element to the next of higher atomic weight and a constant about unity. From silver to gold, the spectra given by the L radiations of elements were compared. These spectra consist of about five lines, of which two are relatively very strong. It was found again that the spectra were similar in type and that the frequency of a given line diminished by definite steps in passing from one element to another. The frequency of the radiation in this case was proportional to where b was a constant and N a whole number. Moseley concluded that the value of N in these expressions was the atomic number, i.e., the number of the element arranged in order of increasing atomic weight. Taking aluminium as the 13th element, he found that succeeding elements were expressed by the value of N 14, 15, 16, 17, etc., up to 77 for gold.
There appears to be little doubt that the X-ray spectrum of an element arises from the vibrations of the rings of electrons deep in the atomic structure outside the nucleus. Quite apart from the very interesting question of the mode of origin of these very high frequency spectra, it is seen that the fundamental modes of vibration of the distribution of electrons are simply connected with the square of a number, which varies by unity in passing from one element to the next. There appears to be no doubt that the atomic number represents the number of units of positive charge carried by the nucleus, which on account of the atomic nature of electricity can only vary by whole numbers and not by fractions.
It is obvious that the study of X-ray spectra reveals at once whether any atomic number is missing, and also affords a remarkably simple method of settling the number of elements possible in the rare earth group about which there has been so much difference of opinion. Moseley concluded that from aluminium to gold, only three possible elements were missing which should have atomic numbers 43, 61, 75, and only one element of number 61 appears to be missing in the rare earth group. The frequencies of the X-ray spectra of these missing elements can be calculated with certainty, and these data should prove an invaluable aid in a search for these missing elements. It has long been known that nickel and cobalt occupy an anomalous position in the periodic table when arranged according to atomic weights. This difficulty is now removed, for Moseley found that when arranged in order of nucleus charge, both elements fall into the position to be expected from their chemical properties.
Nucleus Charge and Chemical Properties
It is established by the work of Moseley that the elements can be defined by their nucleus charge, and that probably elements exist which
