THE TRANSFORMATIONS OF ELEMENTS 541 



Section 4. — Radioactive Elements and their Properties 



No fewer than twenty-four new elements have been dis- 

 covered by radioactive methods ; uranium and thorium were 

 known long before their activity was observed. Of these 

 twenty-six elements, five are said to be raylcss ; in them no 

 activity has been detected by the instruments in use at present. 

 The others emit one or more kinds of ray ; nine or ten give 

 a particles alone. An appreciable life-period is possessed by 

 nine only — uranium, thorium, radium, polonium, radio-lead, 

 mesothorium and radiothorium, actinium, and probably ionium. 

 The remainder are none the less true elements, although their 

 short life may never permit them to be examined by other than 

 radioactive means. 



If the recent work of Boltwood is correct, and ionium is 

 the true intermediate product between uranium X and radium, 

 these twenty-six elements fall naturally into three large groups, 

 namely, the elements uranium, thorium, and actinium, and their 

 respective disintegration products. Using Rutherford's method 

 of classification (from Radioactivity, pp. 449-50) in a slightly 

 modified form, the elements can be grouped as shown below. 

 Only the atomic weights of three — uranium, radium, and 

 thorium — are known with any certainty. If the a particle is 

 a helium atom (of atomic weight 4), then the atomic weights 

 of the other elements, calculated by deducting 4 for each a 

 particle evolved, may approximate to those given in brackets. 

 The present state of our knowledge does not allow any stress 

 to be placed on this hypothesis. In the table on the next page 

 Ur = uranium, Th = thorium, Ra = radium, Act = actinium. 



The nomenclature of the elements is due to Rutherford ; his 

 original idea was to name the first derivatives so as to show 

 their parentage ; hence Ur X, Th X, Act X (ex-uranio, ex-thorio, 

 ex-actinio). The discovery of radiothorium and radioactinium 

 upset this system. It will be observed that each group contains 

 one gas or emanation ; the elements derived from it are named 

 in order of formation A, B, C, and so on. The graphic repre- 

 sentation in the last column, due to Rutherford, illustrates the 

 method of disintegration, the atomic contractions due to loss 

 of a particles being considerably exaggerated in order to 

 emphasise the theory. The brackets show those elements 

 which differ in mass content by not more than one electron. 



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