4io 



TABLE 632. 

 PERODIC SYSTEM AND THE RADIOACTIVE ISOTOPES.' 



< Indicates the loss of an alpha particle (producing He) ; the element becomes more electro-positive and the atomic 

 weight decreases by 4, position changing 2 columns to the left. 



/ Indicates beta radiation (loss of electron); the element becomes more electro-negative, atomic weight remains 

 the same, position changes one column to the right and up. 



Isotopes of an element have the same valency and the same chemical properties (solubility, reactivity, etc.), al- 

 though their atomic weights may differ. The isotopes of Bi are, e.g., RaE, ThC, AcC, RaC. 



In the upper half of the table are the elements possessing high electro-potential, simple spectra, colorless ions. The 

 properties are analogous in the vertical direction (groups). In the lower half are the elements with low electro-poten- 

 tial, complex spectra, colored ions and tending to form complex double salts, the general properties of the elements 

 being more pronounced in the horizontal direction (periods). 



On the left side of the table are the electro-negative elements, those of the upper half forming strong acids, those 

 of the lower half weak oxyacids. 



On the right side of the table are the electro-positive elements, forming bases, oxysalts, sulfides, etc. 



The center of the lower half is occupied by the amphoteric elements forming weak acids and bases, many complex 

 compounds and double salts, many insoluble and mostly colored compounds. 



A very striking point, however, is, as already mentioned, that the similarity among the elements in the upper half is 

 in the verticaUdirection, and in the lower half in the horizontal direction. This justifies the use of the expressions 

 group-relation and period-relation. 



* Table adapted from Hackh, J. Am. Chem. Soc. 40, 1023, 1918, Phys. Rev. 13, 169, 1919. 



The following isotopes have been determined by means of mass-spectra. A ton, Phil. Mag. 40, 633, 1920; Na- 

 ture, 106, 468, 1920. The columns give symbol, min. number of isotopes, masses in order of intensity. Numbers in 

 brackets are provisional. 



1.008 F i 19 A 



As 



H 

 He 

 B 

 C 



N 

 O 



F 



Ne 

 Si 

 P 



s 



Cl 



19 



20, 22, (21) 



28, 29, (30) 



31 



32 



35. 37. (39) 



36 



40, 



75 



Br 2 79, 81 



Kr 6 84, 86, 82, 83, 80, 78 



X 5, (7) 129, 132, 131, 174, 136, (128, 130?) 

 Hg (6) (197-200), 202, 204 



SMITHSONIAN TABLES. 



