200 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



were unable to unite to form a molecule — in agreement with a well- 

 known property of this gas. 



While there is room for much difference of opinion as to the in- 

 terpretation of the rather revolutionary assumptions made by Bohr 

 to explain the structure of the simple atoms and molecules, there can 

 be no doubt of the great interest and importance of this first attempt 

 to deduce the structure of the simple atoms and to explain the origin 

 of their spectra. The agreement of the properties of such theoretical 

 structures with the actual atom.s is in several cases so remarkable 

 that it is difficult to believe that the theory is not in some way an 

 expression of the actual facts. While much work will be necessary 

 before we can hope to understand the structure of any but the 

 simplest atoms, a promising beginning has been made in the attack 

 on this most difficult and fundamental of problems. 



There seems to be little doubt that the more marked physical and 

 chemical properties of an atom are to be attributed to a few outlying 

 electrons in the atomic structure. The position and number of these 

 valency electrons, as they have been termed by Stark, are defined 

 by the magnitude of the nucleus charge. It has previously been 

 pointed out that the loss of an alpha particle from a radioactive atom 

 clianges the position of the element two groups lower in the periodic 

 table, while the loss of a beta particle raises it one group higher. 

 Consequently it follows that the loss or gain of a unit charge from 

 the nucleus of an atom causes it to change its position from one group 

 to the next. If, for example, we follow the chemical properties of 

 successive elements when the nucleus charge increases by unity, we 

 soon reach an element which Ijelongs to the same group as the first, 

 although of much higher atomic weight. We must consequently con- 

 clude that the number and position of the outlying electrons in the 

 structure of the atom passes through successive changes which are 

 regularly repeated with increasing atomic weight. Quite apart from 

 any detailed knowledge of the electronic distribution of atoms, the 

 regular recurrence of elements of similar chemical properties with 

 increasing atomic weight is to be anticipated on the general theory 

 that an atom is an electrical structure. 



EVOLUTION OF THE ELEMENTS. 



It has long been thought probable that the elements are all built 

 up of some fundamental substances, and Front's well-known hypothe- 

 sis that all atoms are composed of hydrogen is one of the best-known 

 examples of this idea. The evidence of radioactivity certainly in- 

 dicates that the heavy radioactive elements are in part composed of 

 helium, for an atom of the latter appears as a result of many of the 

 radioactive transformations. No definite evidence, however, has been 



