SOME CONTEMPORARY ADVANCES IN PHYSICS— X 119 



Were this generalK- true we could sa>- ihat each atom in llie jji-riodic 

 table is like the residue of the next atom following it; and that the 

 mth electron in the «th atom is revolving in the same sort of orbit 

 as the outermost electron of the mth atom, for every value of >i and 

 for e\'ery value of m less than that value of «. 



However, it is not always true. To take another specific instance, 

 consider the two elements potassium (Z = 19) and calcium (Z = 20). 

 The spectrum KI. which is due to a nineteenth electron joining a 

 group of 18 about a nucleus of charge +19 e, and the spectrum Call, 

 which is due to a nineteenth electron joining a group of 18 about a 

 nucleus of charge +20 e, are dissimilar. The dissimilarity is not 

 quite so great as to affect the normal states of the two systems, K and 

 Ca+, composed of nuclei of charge 19 e and 20 e each surrounded by 

 19 electrons; both have as normal state an 5-state, apparently of 

 total quantum-number 4; it is inferred that in each, the 19th electron 

 revolves in a ;zi orbit. If we consider, however, the first of the ^-states 

 (to which the total quantum-number 3 is commonly assigned), we 

 see that in the KI spectrum it has a much larger energy-\alue than 

 the Normal State, while in the Call spectrum it has nearly the same 

 energy-value. A short leap of the imagination leads to the conclusion 

 that if we could examine the spectrum produced by a 19th electron 

 joining a group of 18 about a nucleus of charge +21 e, the (/-state in 

 question would have a smaller energy-value than any 5-state. In 

 this case it would be the Normal State itself,^ and we should say that 

 the 19th electron, in a group of 19 surrounding a nucleus of charge 

 Ze, revolves in a Wi orbit if Z = 19 or 20, but in a «3 orbit if Z = 21. 



This system of 19 electrons around a nucleus of charge 21 e is a 

 doubly-ionized scandium atom, Sc+ + . Its spectrum has not been 

 produced, so that the foregoing sentences are still somewhat specu- 

 lative. What gives them value is the inference that scandium marks 

 a sort of a breach in the regularity of the Periodic System. For 

 most of the elements in the Periodic System, it can be .said that the 

 atom consists of a residue which is like the atom of the preceding 

 element, and an additional electron; and that in its turn this atom 

 resembles the residue of the atom of the element next following. To 

 this the regular periodicity of the properties of the elements is ascribed. 

 But when we reach an element of which the atom has a residue dis- 

 tinctly different from the atom of the foregoing element, then the 

 regular variation of the physical and chemical properties is inter- 

 rupted. Scandium, as a matter of fact, is the first of a group of 



3 In the First Part of this article the impression may have been left that the Normal 

 State of every atom is an 5-state. This is not true; in some known cases the Normal 

 State is a p-state, in others an /-state. 



