466 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



properties of the elements is evidently checked at this point 

 and only goes forward again after some 40 units have been 

 added to the atomic weight. 



There is reason to think (see later, p. 492) that there 

 may be sixteen elements between barium and tantalum, of 

 which fourteen are now known ; if this be correct, the total 

 number of elements in this period would be thirty-two. 



(5) A period composed of radioactive elements. The 

 last period is a mere fragment, but includes four well-defined 

 elements of known atomic weight, occupying alternate 

 columns in the first octave of Period VII. and correspond- 

 ing closely in their properties with the earlier members of 

 these four families. These four elements are 



Niton Nt 222 Thorium Th 234 



Radium Ra 226 Uranium Ur 238 



They differ from the lighter elements in that they are all 

 RADIOACTIVE, i.e. they constantly liberate energy, which 

 appears to be derived from- the DISINTEGRATION of the atom. 

 In the absence of any known synthetic process, a radio- 

 active element must either decay very slowly, or must be 

 reproduced continuously from some element of greater 

 atomic weight. Uranium and thorium, the two elements 

 of greatest atomic weight, show very slight radioactivity and 

 are credited with a very slow rate of decay. All other radio- 

 active substances appear to be derived, directly or indirectly, 

 from these two PARENT ELEMENTS. 



Some of these radioactive elements, or, more briefly, 

 RADIO-ELEMENTS, 1 which enjoy a relatively long life, are 

 found in appreciable quantities in the ores from which the 

 parent-elements are derived, and can be separated by ordi- 

 nary chemical methods. Thus the Bohemian pitchblende 

 of Joachimsthal (crude U 3 O 8 ) has given radium, polonium 

 and actinium, in the uranium series, whilst thorium- 



1 Soddy, The Chemistry of the Radio- Elements, Parts I and II, 

 London, 1914. 



