Radioelements and Their Families

Before the discovery of polonium and radium there were seven empty slots in the periodic system between bismuth and uranium. While the number of newly found radioactive elements was small there were no problems with their location in the periodic system. But emanations were a baffling problem. They had identical properties and therefore could not be assigned to different boxes of the periodic system, for instance, to the two empty boxes corresponding to the unknown heavy analogues of iodine and cesium. This would be an unnatural thing to do. 

But even if we leave the enigmatical radon family alone the situation still remains unclear. In 1900 W. Crookes observed a strange phenomenon. After fractional crystallization of a uranium compound he obtained a filtrate and a precipitate. Uranium remained in the solution but it exhibited no activity. On the contrary, the precipitate did not contain uranium but exhibited a high-intensity radioactivity. On the strength of his observations Crookes made a paradoxical conclusion that uranium was not radioactive by itself, and its radioactivity was due to some admixture which he managed to separate from uranium. As if he had ill premonitions, Crookes refrained from giving the admixture any definite name and referred to it as uranium-x (UX). Later it was found that uranium restores its activity after 

Two years later E. Rutherford and F. Soddy discovered similar temporary disappearance of activity in thorium. The respective admixture was named (by analogy) thorium-x (ThX). Rutherford and Soddy attempted to find an answer to the fundamental question what happens with a radioactive element in the process of emission of radiation Does the chemical nature of the element remain unchanged or does it change They made a valuable observation that the emanation of thorium was produced by ThX rather than by thorium itself. In other words, they identified the first step of radioactive transformations  

This was the event that played the decisive part in developing the theory of radioactive decay. 

According to Rutherford and Soddy, the mechanism of radioactive decay consists in transformation of chemical elements and in their natural transmutation. This was particularly clear in the case of radium, which converted into radon after emission of alpha radiation. Somewhat later, the alpha particle was found to be a doubly ionized helium atom. The decay of radium gave rise to two new elements, namely, radon and helium  

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