Abundance of naturally occurring isotopes

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

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). 

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. 

C22-0019. Calculate the isotopic abundances of naturally occurring uranium 10 years ago. 

Note. Not all naturally occurring isotopes are stable - some are radioactive but with long half-lives. See (for example) WebElements [http //www.webelements.com/webelements/index. html] for abundance ratios of naturally occurring isotopes and half-lives of radioisotopes. 

You can calculate the atomic mass of any element if you know its number of naturally occurring isotopes, their masses, and their percent abundances. The following Example Problem and Practice Problems will provide practice in calculating atomic mass. 

The holder of a liquid scintillator may contain small amounts of naturally occurring isotopes. In particular, is always present (isotopic abundance of is 0.01 percent). Another isotope, C, is a constituent of contemporary organic materials. Solvents, however, may be obtained from petroleum, consisting of hydrocarbons without C. 

The mass of an ion is given in unified atomic mass units, u. One unified atomic mass unit is equal to 1 /12 of the mass of the most abundant, stable, naturally occurring isotope of carbon, The mass of is defined as exactly 12 u. The abbreviation amu, for atomic mass unit, is now considered obsolete but may still be encountered in the hterature. 

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... 

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

Table 25.1 Abundance of naturally occurring stable isotopes of important elements in stable isotope dilution assays (SIDAs). The occurrence of naturally occurring isotopes has to be considered as these can cause spectral overlaps between the analytes and the labelled standards.

From such data, we can obtain the atomic mass (also called atomic weight) of an element, the average of the masses of its namrally occurring isotopes weighted according to their abundances. Each naturally occurring isotope of an element contributes a certain portion to the atomic mass. For instance, multiplying the isotopic... 

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. 

---