from the Physko- Chemical Standpoint. 331 



in atomic weight. Since they are chemically indistinguishable 

 from each other, they occupy the same place in the Periodic 

 Table ; and on this account Soddy named them isotopes 

 (from isos equal, and topos a place). A second type of 

 the radio-elements is exemplified by mesothorium-1, meso- 

 thorium-2, and radiothorium. These elements differ com- 

 pletely from one another in chemical character ; but they 

 ail possess the same atomic weight. For this reason the 

 name isobares* (from isos equal, and baros weight) is here 

 suggested for them. 



These isobaric elements result from the operations of /3-ray 

 changes in the radioactive series ; and the generation of one 

 element from another in this way is spontaneous and 

 irreversible. On the other hand, a somewhat similar process 

 occurs among the non-radioactive elements when an atom 

 changes its valency; but in the latter case the process is 

 controllable in the laboratory and is reversible under proper 

 conditions. The two actions, then, are not identicalf; but 

 they appear to display a certain parallelism which is of con- 

 siderable importance from the point of view of atomic 

 structure. Unless a model atom is capable of throwing light 

 upon this matter, it is evidently incomplete; and as the point 

 forms a crucial test of the theory of atomic architecture, 

 some details of it are given here, though the merest outline 

 must suffice. 



Ferrous iron and ferric iron will serve as a convenient 

 example of the effects of changing the valency of an element 

 by chemical reactions. Ferrous iron is divalent, whilst 

 ferric iron is trivalent : the absorption spectra of the two 

 materials are different from each other ; and in chemical 

 properties ferrous iron shows a close analogy with mag- 

 nesium, whilst ferric iron is akin to aluminium in its 

 reactions. A difference in chemical character such as this 

 should, according to modern ideas of the atom, involve 

 certain changes in the atomic nucleus ; but at the same 

 time it is hard to imagine that any changes in the nucleus 

 can occur in ordinary inactive elements. 



Turning to the case of the radioactive isobares, it is found 

 that a very similar state of things prevails. Mesothorium-1 

 is divalent and resembles in its chemical relations the 

 members of Group II. of the Periodic Table, which also 

 contains magnesium. Mesothorium-2 is trivalent, and shows 

 a close kinship with elements in the aluminium group. 



* Isobars would be a better word, but unfortunately it is already in 

 use in meteorology. 



t Soddy, < Nature/ xcii. p. 399 (1913) ; Fleck, Chem. Soc. Trans, cv. 

 p. 247 (1914). 



Z2 



