Iodine placement

Similar electron accessibility generates similar chemical behavior. For example, iodine has many more electrons than chlorine, but these two elements display similar chemical behavior, as reflected by their placement in the same group of the periodic table. This is because the chemistry of chlorine and iodine is determined by the number of electrons in their largest and least stable occupied orbitals 3 S and 3 p for chlorine and 5 S and 5 p for iodine. Each of these elements has seven accessible electrons, and this accounts for the chemical similarities. 

Consider the proper placement of tellurium and iodine in the periodic table, as shown in Figure 1-3. Te has the heavier atomic weight. The chemical properties of tellurium are like those of selenium because both are semi-metallic elements that form compounds like those of sulfur. Iodine resembles bromine because these elements are nonmetallic halogens that form compounds like those of chlorine. Therefore, the order in the table cannot be based solely on atomic weight. 

Besides new insight into the reactivity of free radicals, methods for die production of carbon-centered free radicals have also seen major improvements in die last several years. One very common new mediod is to use tin-based reagents as radical chain carriers. Trialkyltin radicals readily abstract bromine or iodine from carbon to produce a carbon-centered free radical. Placement of a bromide or iodide substituent on a substrate dius permits formation of a carbon-centered free radical at diat position using tin-based mediodology. This process was initially developed for die reduction of alkyl halides, and it remains an excellent synthetic method for diat purpose. The complete chain mechanism for die reduction is shown. 

Several other elements seemed out of order. For example, their atomic masses placed iodine (1) before tellurium (Te), but their chemical properties required the opposite order. Mendeleyev concluded that the atomic masses must have been determined incorrectly and put these two elements in positions reflecting their properties. We now know that the periodic properties of the elements are based on their atomic numbers, not their atomic masses, which explains Mendeleyev s difficulty with the placement of certain elements. 

Dewald CL, Jensen CC, Park YH, Hanks SE, Harrell DS, Peters GL, Katz MD. Vena cavography with CO(2) versus with iodinated contrast material for inferior vena cava filter placement a prospective evaluation. Radiology 2000 216(3) 752-7. 

The second innovation Mendeleev made involved the relative placement of tellurium and iodine. If the elements are listed in strict order of their atomic masses, then iodine should be placed before tellurium, since iodine is lighter. That would place iodine in a group with sulfur and selenium and tellurium in a group with chlorine and bromine, an arrangement that does not work for either iodine or tellurium. Therefore, Mendeleev rather boldly reversed the order of tellurium and iodine so that tellurium falls below selenium and iodine falls below bromine. 

The question of the placement of tellurium takes on some importance in view of its being one of the few elements that belong to a reversed pair, the other element in this case being iodine. Many pioneers of the periodic system reversed the positions of tellurium and... 

Even in the absence of natural color compounds can be detected using iodine vapor and concentrated sulfuric acid. With iodine vapor, aromatic compounds form bro vn iodine complexes. Spraying the plate with concentrated sulfuric acid with subsequent placement in an oven holding 250°C, oxidation products of the compounds, formed during the heating in the presence of sulfuric acid, are detected as colored spots. Most of involatile organic compounds can be detected in this way. 

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