CONSTITUTION OF MATTER RUTHERFORD. 195 



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 remark- 

 ably 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 num- 

 bers 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 

 Avhich have all the nucleus charges from 1 for hydrogen to 92 for 

 lead. There is, however, another very important consequence that 

 follows from this conception of the atom. Disregarding for a mo- 

 ment the atomic weight which depends mainly on the structure of the 

 nucleus, the main physical and chemical properties of the atom are 

 determined by the nucleus charge and not by the atomic mass. This 

 must obviously be the case, for the number and distribution of elec- 

 trons around the nucleus is determined bj^ the electric forces between 

 the electrons and the nucleus, and this is dependent on the magni- 

 tude of the nucleus charge, which may be regarded as a point charge. 

 AVithout entering into the difficult question of the actual distribu- 

 tion of the exterior electrons in any atom, it is obvious that the 

 number and position of the outlying electrons of the atomic struc- 

 ture, Avliich probably mainly influence the chemical and physical 

 properties of the atom, are determined by the charge on the nucleus. 

 No doubt if the electrons are in motion, their positions relative to the 

 nucleus and possibly also their rates of vibration will be slightly 

 influenced by the mass of the nucleus as well as its charge, but the 

 general evidence indicates that this effect must be very small. 



We thus see that there is in the structure of every atom a quantity 

 which is more fundamental and important than its atomic weight. 



