Relative abundance of naturally occurring isotopes

RELATIVE ABUNDANCES OF NATURALLY OCCURRING ISOTOPES Table 4.18 Relative Abundances of Naturally Occurring Isotopes... 

Nuclidic Masses and Relative Abundances for Naturally Occurring Isotopes of Some Important Elements... 

Accurate atomic weight values do not automatically follow from precise measurements of relative atomic masses, however, since the relative abundance of the various isotopes must also be determined. That this can be a limiting factor is readily seen from Table 1.3 the value for praseodymium (which has only 1 stable naturally occurring isotope) has two more significant figures than the value for the neighbouring element cerium which has 4 such isotopes. In the twelve years since the first edition of this book was published the atomic weight values of no fewer than 55 elements have been improved, sometimes spectacularly, e.g. Ni from 58.69( 1) to 58.6934(2). 

Furthermore, isotope analysis is relevant for determining the atomic weight (Ar(E)) of elements. The Ar(E) is the average of all masses of all naturally occurring stable isotopes (taking into account the abundances of isotopes) of a chemical element (see Appendix I10). By consideration of the masses of isotopes (/ ,) and the known relative abundances of all stable isotopes (Xi) with i = 1 to n of a selected chemical element, the average atomic weight (Ar(E)) of this element can be calculated ... 

We might at first think that the peak would be the peak of highest mie value. This is not so, however. Most elements occur naturally as several isotopes generally the lightest one greatly predominates, and the heavier ones occur to lesser extent. Table 13.1 lists the relative abundances of several heavy isotopes. 

The atomic weights listed in the periodic table are relative numbers ( C = 12.0000. ..) based upon the weighted average of naturally occurring isotopes (e.g., the atomic mass of chlorine is 35.45 reflecting the roughly 3 1 ratio of Cl to Cl). The isotope Cl has 17 protons (atomic number = 17) and 18 neutrons in its nucleus C1 has 17 protons and 20 neutrons. A more precise analysis combines the relative abundances and precise relative masses of the two stable nuclides of chlorine ( Cl 75.78 percent 34.968853. Cl 24.22 percent 36.965903) as follows Relative Atomic Mass of Chlorine 0.7578 (34.968853) + 0.2422 (36.965903) = 35.45 It is noteworthy that on rare occasions, lUPAC may introduce a very slight modification to the atomic mass provided for an element in the periodic table. The relative masses of the nuclides are known to... 

The element lead (Pb) consists of four naturally occurring isotopes with atomic masses 203.97302, 205.97444, 206.97587, and 207.97663 amu. The relative abundances of these four isotopes are 1.4, 24.1, 22.1, and 52.4% respectively. From these data, calculate the atomic weight of lead. 

Naturally occurring europium has an average atomic mass of 151.965 amu. If the only isotopes of europium present are Eu and Eu, describe how you would determine the relative abundance of the two isotopes. Include in your description any information that would need to be looked up. 

Boron has only two naturally occurring isotopes. The mass of boron-lOis 10.01294 amu and the mass ofboron-11 is 11.00931 amu. Calculate the relative abundances of the two isotopes. 

For other elements that occur with major relative abundances of more than one isotope in the natural state, the isotope pattern becomes much more complex. For example, with chlorine and bromine, the presence of these elements is clearly apparent from the isotopes Cl and for chlorine and Br and Br for bromine. Figure 47.2a shows the molecular ion region for the compound chlorodecane. Now, there are new situations in that C, C, C1, and Cl isotopes all have probabilities of occurring together. Thus, there are molecular ion peaks for + Cl, C + Cl, + Cl, and so on. Even so, the isotopic ratio of 3 1 for Cl to Cl is very clear... 

Ans. Hydrogen occurs as diatomic molecules, and it would be easy to separate H , H H, and 2H , but not the individual atoms. But there would be relatively little "H, since in abundance the heavy isotope accounts for only 0.015% of naturally occurring hydrogen atoms. 

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