Thorium mass number range

In today s parlance, we call the radium emanation radon-222. (Like radium, the word radon comes from the Latin radius, for ray or beam. ) The alpha decay of radium-226 produces radon-222 and helium-4. The thorium emanation is radon-220, but the decay scheme from thorium-232 is more involved (see Problem 19.11). Rn-222 and Rn-220 are the two longest-lived isotopes of radon, the heaviest and rarest of the noble gases. (There are now 36 known isotopes of radon with mass numbers ranging from 193 to 228.) For many years, Dorn was generally credited as the sole discoverer of radon. However, as noted above, Ernest Rutherford and his co-workers, particularly Frederick Soddy, should be given at least equal billing. 

ISOTOPES There are 37 isotopes of radon. All are radioactive. None are stable. They range in mass numbers from Rn-196 to Rn-228. Their half-lives range from a few microseconds to 3.8235 days for Rn-222, which is the most common. It is a gas that is the result of alpha decay of radium, thorium, or uranium ores and underground rocks. 

Radium is element number 88, in which all of its isotopes are radioactive hence, what little radium is found on Earth is mostly as a trace element in uranium ores. The most common isotope has a mass number of 226 with a half-life of 1,604 years. The second longest-lived isotope is radium 228, with a half-life of 5.77 years. The other isotopes have much shorter half-lives ranging from microseconds to days. Radium is constantly being formed as part of the radioactive decay series of uranium and thorium. Because it decays so quickly, however, only minute quantities of radium ever exist at any one time. 

When pure, thorium is a silvery white metal. In air, it tarnishes slowly, becoming gray and finally black. Thorium has isotopes ranging in mass number from 210 to 237, all isotopes being radioactive. Much of the internal heat in the earth s crust has been attributed to thorium (and uranium). Thorium is a potential atomic fuel source, because bombardment of Th with slow neutrons yields the fissile isotope There... 

An interesting variant of Group I is the determination of thorium in monazite concentrates.73 Here the variations that may occur in the chemical composition of the matrix leave its x-ray absorbance virtually unaltered. This simplicity is possible because the principal individual rare-earth elements present in the samples lie in the range of atomic numbers from 57 to 60, a range so small as to preclude marked variations in the over-all mass absorption coefficient. 

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