Iodine, natural abundance

ISOTOPES There are a total of 145 isotopes of iodine. Only one (1-127) is stable and accounts for 100% of iodine s natural abundance on Earth. All the other 146 isotopes are radioactive with half-lives ranging from a 150 nanoseconds to 1.57x10+ years. 

Several chlorine isotopes exist with mass numbers ranging between 32 and 40. The two stable isotopes are Cl and Cl with natural abundances of 75.77% and 24.23% respectively, while the others are radioactive. Bromine also has two stable isotopes, Br and Br, with natural abundances of 50.69% and 49.31% respectively, while the others are radioactive. Iodine has only one stable isotope, and numerous radioactive ones are known. Astatine is known only as its radioisotope see Radioactive Decay). 

We have been able to enrich the isotopes from its natural abundance of 1.13% substantially by exciting the Cameron level of carbon monoxide see Figure 1 for energy levels of CO) with the 2062.4 iodine line see Figure 2 for energy levels of iodine). 

Highly enriched xenon-124 is expensive (US 150,000 per litre STP) due to its natural abundance of only 0.096%. This calls for an effective gas target and a reliable gas handling system. In the past considerable experience has been obtained in handling enriched krypton-82 gas for the production of rubidium-81 for krypton-8 Im generators. Since June 1984 this technology is being applied and further improved for the production of iodine-123 via proton bombardment of the enriched xenon-124. 

The number of elements that are known to be biologically important comprises a relatively small fraction of the 109 known elements. Natural abundance limits the availability of the elements for such use. Molybdenum (Z = 42) is the heaviest metal, and iodine (Z = 53) is the heaviest nonmetal of known biological importance. The metals of importance in enzymes are principally those of the first transition series, and the other elements of importance are relatively light sodium, potassium, magnesium, calcium, carbon, nitrogen, phosphorus, oxygen, chlorine, and, of course, hydrogen. 

Lysozyme. — Natural abundance n.m.r. spectroscopic studies have been reported for native lysozyme, lysozyme in solution with Gd ", La, Eu, Pr ", and Co, and lysozyme modified by treatment with iodine at pH 5.5. It was possible in the case of the chemically modified enzyme to assign certain resonances to specific residues in the molecule. 

Although, normally, bromine is a =1 1 mixture of Br and Br and with the advent of mass spectroscopy, it is possible to use enriched mixtures of one or the other isomer to solve the same problem, at the time, radioactive isomers were a reasonable choice. For example, Br, a P- and y-emitter, has a half-life of 57 h, while Br has a half-life of 35.7 h. Similarly, although the natural abundance of iodine is 100%, I, a y-emitter, has a half-life of 56days (See, Hughes, E. D., et al). 

Occurrence in Nature. About 99.6% of the earth s mass results from 32 of the chemical elements. The remaining 0.4% is apportioned among 64 elements, all of which are present as traces. Iodine is one of these 64. Estimates about abundance of the constituent elements of the Hthosphere place iodine 46th on a restricted Hst of 59 elements (37 very rare elements are excluded) and 61st on a Hst in which 96 elements are included. Iodine is, indeed, one of the scarcest of the nonmetaUic elements in the total composition of the earth (3). 

Although not abundant in quantity, iodine is distributed in rocks, soils, waters, plants, animal tissues, and foodstuffs (3,4). Excepting the possible occurrence of elemental iodine vapor in the air near certain iodine-rich springs, iodine never occurs free in nature. It is always found combined with other elements. 

The silver gray metal can be cut with a knife, although it only melts at 1545 °C (for comparison, iron 1538 °C). It is the rarest of the "rare earths", but is nevertheless more abundant than iodine, mercury, and silver. Thulium has few applications, especially because it is relatively expensive. The element occurs naturally as a single isotope, namely 169Tm (compare bismuth). The artificial, radioactive 170Tm is a transportable source of X-rays for testing materials. Occasionally used in laser optics and microwave technology. 

There are many natural sources of chlorine compounds, which is not surprising considering that it is the 20th most abundant element. Salt and salt water are widely available the Great Salt Lake contains 23% salt, and the Dead Sea contains about 30%. Because salt is so abundant, most minerals that contain chlorine are not important sources for economic reasons. Bromine is found in some salt brines and in the sea, as are some iodine compounds. 

F and Cl are moderately abundant elements, principal sources being fluorite CaF2 and halite NaCl, from which the very electronegative elements are obtained by electrolysis. Bromine is mainly obtained by oxidation of Br- found in salt water iodine occurs as iodates such as Ca(I03)2. Astatine is radioactive and only minute amounts are found in nature. Chlorine is used... 

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