Iodine atomic mass

Simple fragmentation of the molecular ion of iodobenzene gives a fragment ion, CjH,. The difference in measured masses between the molecular and fragment ions gives the mass of the ejected neutral iodine atom. 

Finally, accurate mass measurement can be used to help unravel fragmentation mechanisms. A very simple example is given in Figure 38.2. If it is supposed that accurate mass measurements were made on the two ions at 203.94381 and 77.03915, then their difference in mass (126.90466) corresponds exactly to the atomic mass of iodine, showing that this atom must have been eliminated in the fragmentation reaction. 

As the number of elements increased, so did attempts to organize them into meaningful relationships. Johann Dobereiner (1780-1849) discovered in 1829 that certain elements had atomic masses and properties that fell approximately mid-way between the masses and properties of two other elements. Dobereiner termed a set of three elements a triad. Thus, chlorine, bromine, and iodine form a triad Dobereiner proposed several other triads (lithium-sodium-potassium, calcium-strontium-barium). Dobereiner recognized that there was some sort of relationship between elements, but many elements did not fit in any triad group, and even those triads proposed displayed numerous inconsistencies. 

Fig. 55. The probability that a pair of iodine atoms remain unreacted at a time f after they were formed with an initial separation of 0.7 nm. The encounter distance is 0.37 nm. A Morse potential energy getween the iodine atoms is imposed and the temperature was chosen as 300 K. The diffusion coefficient is 5 X 10 9 m2 s1 and iodine atom has a mass of 0.127 kg mol . Monte Carlo techniques were used to calculate the survival probability, with-----, toe — 2xl013s 1, cjc = 1013s l --------,...

The product we monitor is again the I atom using femtosecond-resolved mass spectrometry (the other product is the Bzl species). We also monitor the initial complex evolution. The initial femtosecond pulse prepares the system in the transition state of the harpoon region, that is, Bz+h. The iodine atom is liberated either by continuing on the harpoon PES and/or by electron transfer from iodine (I2-) to Bz+ and dissociation of neutral I2 to iodine atoms. We have studied the femtosecond dynamics of both channels (Fig. 17) by resolving their different kinetic energies and temporal behavior. The mechanism for the elementary steps of this century-old reaction is now clear. 

Iodine has a lower atomic mass than tellurium (126.90 for iodine versus 127.60 for tellurium) even though it has a higher atomic number (53 for iodine versus 52 for tellurium). Explain. 

Although there is only one naturally occurring isotope of iodine, 1271, the atomic mass is given as 126.9045. Explain. 

Ans. The atomic masses indicated on the Periodic Table of the Elements are averages, but they are calculated relative to the mass of 12C. The mass number for iodine s naturally occurring isotope is 127, which is a total of the number of protons and neutrons, not true masses. 

As the atomic mass increases, so do the Van der Waals forces between the molecules. This causes the molecules to be held together more tightly as the atomic masses increase. Iodine, the heaviest of the halogens listed, has the greatest mass and the greatest Van der Waals forces and exists as a solid. 

The oxidizing power of halogens decrease with increasing relative atomic mass Iodine is a mild oxidant, while the iodide ion often acts as reducing agent. Some oxidations with halogens, used in qualitative analysis, are as follows ... 

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. 

In the periodic table, locate two pairs of elements besides iodine and tellurium that are out of order, based on their atomic masses. 

The mass of iodine in a certain compound is almost exactly twice that of the only other element—copper. Using their atomic masses, determine the formula of the compound. 

Molecular dynamics simulations have shown that for isolated reactants rotational excitation contributes to the enhanced reactivity (cf. Fig. 5, Ref. 97). In the kinematic limit, initial reagent rotational excitation is needed for a finite orbital angular momentum of the relative motion of the products. This is intuitively clear for the H2 -f I2 —t 2 HI reaction, where there is a large change in the reduced mass. The rather slow separation of the heavy iodine atoms means that rotational excitation of HI is needed if the two product molecules are to separate. This is provided by the initial rotational excitation of the reactants. The extensive HI rotation is evident in Fig. 9 which depicts the bond distances of this four-center reaction on a fs time scale. 

Another change Mendeleev made based on chemical analogy and intuition was placing iodine (T) after tellurium (Te), even though the atomic mass of iodine was less than tellurium. This anomaly, along with the difficulty of where to place the inner transition metals, were problems that would soon be definitively solved. At the time of the periodic table s construction, little was known of atomic structure. With further scientific discoveries such as the existence of protons and the existence of electronic shells, these mysteries were explained and placed into their current places in the periodic table. 

Second, the elements do not always fit neatly in order of atomic mass. For example, Mendeleev had to switch the order of tellurium, Te, and iodine, I, to keep similar elements in the same column. At first, he thought that their atomic masses were wrong. However, careful research by others showed that they were correct. Mendeleev could not explain why his order was not always the same. 

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