Naturally occurring isotopes abundance

The 1981 report on atomic weights includes a complete review of the natural isotopic composition of the elements and also tabulates the relative atomic masses for selected radioisotopes. This information is required for the conversion of spectroscopic data from that corresponding to specific isotopes to the naturally occurring isotopic abundance. 

Table 4.3 Some Naturally Occurring Isotopic Abundances...

Carbon 12, the most abundant naturally occurring isotope, has zero spin and thus cannot be studied by NMR. On the other hand, its isotope carbon 13 has an extra neutron and can be its low natural occurrence (1.1%) nevertheless makes the task somewhat difficult. Only pulsed NMR can be utilized. 

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

The upper part of the figure illustrates why the small difference in mass between an ion and its neutral molecule is ignored for the purposes of mass spectrometry. In mass measurement, has been assigned arbitrarily to have a mass of 12.00000, All other atomic masses are referred to this standard. In the lower part of the figure, there is a small selection of elements with their naturally occurring isotopes and their natural abundances. At one extreme, xenon has nine naturally occurring isotopes, whereas, at the other, some elements such as fluorine have only one. 

A diagrammatic illustration of the effect of an isotope pattern on a mass spectrum. The two naturally occurring isotopes of chlorine combine with a methyl group to give methyl chloride. Statistically, because their abundance ratio is 3 1, three Cl isotope atoms combine for each Cl atom. Thus, the ratio of the molecular ion peaks at m/z 50, 52 found for methyl chloride in its mass spectrum will also be in the ratio of 3 1. If nothing had been known about the structure of this compound, the appearance in its mass spectrum of two peaks at m/z 50, 52 (two mass units apart) in a ratio of 3 1 would immediately identify the compound as containing chlorine. 

The masses of the naturally occurring isotopes for lanthanum and cerium are shown. For lanthanum, the isotope at 138 is only present in 0.09% natural abundance and is isobaric with Ce. For this reason the isotope La is used to measure the amount of lanthanum. Similarly, Ce and Ce are present in low abundance "Ce is present in greatest abundance and is used to measure the amount of cerium. Another isotope of cerium, C, although quite abundant, is isobaric with Nd and is therefore not used for measurement. 

Isotopic ion. Any ion containing one or more of the less abundant naturally occurring isotopes of the elements that make up its structure. 

Isotopic molecular ion. A molecular ion containing one or more of the less abundant naturally occurring isotopes of the atoms that make up the molecular structure. Thus, for ethyl bromide there exist molecular isotope ions such as CCHjBi, C2H4DBi , C2H5 Bi, C2H5 Bi, etc. 

Molecular ion. An ion formed by the removal (positive ions) or addition (negative ions) of one or more electrons from a molecule without fragmentation of the molecular structure. The mass of this ion corresponds to the sum of the masses of the most abundant naturally occurring isotopes of the various atoms that make up the molecule (with a correction for the masses of the electrons lost or gained). For example, the mass of the molecular ion of the ethyl bromide CzHjBr will be 2 x 12 plus 5 x 1.0078246 plus 78.91839 minus the mass of the electron (m ). This is equal to 107.95751p -m, the unit of atomic mass based on the standard that the mass of the isotope = 12.000000 exactly. 

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). 

Lead (13 ppm) is by far the most abundant of the heavy elements, being approached amongst these only by thallium (8.1 ppm) and uranium (2.3 ppm). This abundance is related to the fact that 3 of the 4 naturally occurring isotopes of lead (206, 207 and 208) arise primarily as the stable end products of the natural radioactive series. Only (1.4%)... 

Table 21.1 summarizes a number of properties of these elements. The difficulties in attaining high purity has led to frequent revision of the estimates of several of these properties. Each element has a number of naturally occurring isotopes and, in the case of zirconium and hafnium, the least abundant of these is radioactive, though with a very long half-life ( Zr, 2.76%, 3.6 x 10 y Hf, 0.162%, 2.0 X 10 5 y). 

Some of the important properties of Group 5 elements are summarized in Table 22.1. Having odd atomic numbers, they have few naturally occurring isotopes Nb only 1 and V and Ta 2 each, though the second ones are present only in very low abundance 0.250%, Ta 0.012%). As a consequence (p. 17) their atomic weights have been determined with considerable precision. On the other hand, because of difficulties in removing all impurities, reported values of their bulk properties have often required revision. 

Uranium (symbol U atomic number 92) is the heaviest element to occur naturally on Earth. The most commonly occurring natural isotope of uranium, U-238, accounts for approximately 99.3 percent of the world s uranium. The isotope U-235, the second most abundant naturally occurring isotope, accounts for another 0.7 percent. A third isotope, U-234, also occurs uatiirally, but accounts for less than 0.01 percent of the total naturally occurring uranium. The isotope U-234 is actually a product of radioactive decay of U-238. 

Bromine is a red-orange liquid with an average atomic mass of 79.90 amu. Its name is derived from the Greek word bromos (fipofios), which means stench. It has two naturally occurring isotopes Br-79 (78.92 amu) and Br-81 (80.92 amu). What is the abundance of the heavier isotope ... 

Gallium has two naturally occurring isotopes 69Ga, with atomic mass 68.9257 amu, and 71Ga, with atomic mass 70.9249 amu. The percent abundance of 69Ga can be estimated to be which of the following ... 

Copper has two naturally occurring isotopes. Cu-63 has an atomic mass of62.9296 amu and an abundance of 69.17%. What is die atonic mass of the second isotope What is its nuclear symbol ... 

The atomic weight increases regularly across the row except for the inversion at cobalt and nickel. We would expect the atomic weight of Ni to be higher than that of Co because there are more protons (28) in the Ni nucleus than in the Co nucleus (27). The reason for the inversion lies in the distribution of naturally occurring isotopes. Natural cobalt consists entirely of the isotope 2 Co natural nickel consists primarily of the isotopes Ni and Ni, the 58-isotope being about three times as abundant as the 60-isotope. 

There are four naturally occurring isotopes of Ra " " Ra (ti/2 = 5.8 a) and " Ra (3.7 d) in the Th series, (1600 a) in the series, and Ra (11.7 d) in the series (Table 1). The data for Ra are more limited, since it is generally present in low concentrations due to the low abundance of The differences in half lives and the connections across the different decay series have been used to infer a variety of groundwater and water-rock interaction features. For the short-lived Ra isotopes, the dominant input term to groundwater is recoil, rather than weathering, and steady state concentrations are often achieved (see Section 2.2). 

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