Conventional atomic mass

Atomic Mass Intervals and Conventional Atomic Masses... 

For those elements with standard atomic masses given as intervals, the lUPAC also provides conventional atomic mass values (Table 2.2). These conventional values can be used when we need a specific, representative value of the atomic mass. The values have been selected so that, for materials normally encountered, the atomic mass would be within in an interval of plus or minus one in the last digit. 

TABLE 2.2 Conventional Atomic Masses and Atomic Mass Intervals for Selected Elements... 

From a table of atomic masses (inside the front cover), we see that the atomic mass of lithium is reported as an atomic mass interval [6.938, 6.997]. The conventional atomic mass value (from Table 2.2) is 6.94. Because the values in this range, and the conventional atomic mass value, are all much closer to 7.01600 u than to 6.01512 u, lithium-7 must be the more abundant isotope. 

These results indicate that the percent isotopic abimdance of Br varies between 50.516% and 50.817%. Because of this variation, the atomic mass of bromine is best expressed as an atomic mass interval. When a representative value of the atomic mass of Br is required, we wordd use the conventional atomic mass (Table 2.2)... 

PRACTICE EXAMPLE B Use data from Example 2-5 and the conventional atomic mass of Li (Table 2.2) to... 

Use the conventional atomic mass of boron to estimate the fractional isotopic abundances of the two naturally occurring isotopes, i°B and B. These isotopes have masses of 10.012937 u and 11.009305 u, respectively. 

The two naturally occurring isotopes of nitrogen have masses of 14.0031 and 15.0001 u, respectively. Use the conventional atomic mass of nitrogen to estimate the percentage of atoms in naturally occurring nitrogen. 

The conventional atomic mass is provided for elements that have their atomic masses defined in terms of an atomic mass interval and may be used in situations when a representative value of the atomic mass is required. (See also atomic mass interval.)... 

Atomic masses in this table are relative to carbon-12 and are rounded to five figures, as recommended by the International Union of Pure and Applied Chemistry (lUPAC) Pure Appl. Chem., Vol. 85, No. 5, pp. 1047-1078, 2013. For certain radioactive elements the numbers listed (in parentheses) are the mass numbers of the most stable isotopes. For H, Li, B, C, N, O, Mg, Si, S, Cl, Br, and Tl, the conventional atomic mass, a... 

The abundance of a trace element is often too small to be accurately quantihed using conventional analytical methods such as ion chromatography or mass spectrometry. It is possible, however, to precisely determine very low concentrations of a constituent by measuring its radioactive decay properties. In order to understand how U-Th series radionuclides can provide such low-level tracer information, a brief review of the basic principles of radioactive decay and the application of these radionuclides as geochronological tools is useful. " The U-Th decay series together consist of 36 radionuclides that are isotopes (same atomic number, Z, different atomic mass, M) of 10 distinct elements (Figure 1). Some of these are very short-lived (tj j 1 -nd are thus not directly useful as marine tracers. It is the other radioisotopes with half-lives greater than 1 day that are most useful and are the focus of this chapter. 

The quantity mle is the conventional notation for the mass-to-charge ratio of the molecular ions, with the mass a multiple of the atomic mass constant tnu and the charge a multiple of the fundamental charge e. 

The most abundant isotope of carbon has a mass of 12 atomic mass units, 12C. A less abundant stable isotope is 13C. And much less abundant is the radioactive isotope t4C, also called radiocarbon. It is convenient to express the abundances of these rare isotopes in terms of ratios of the number of atoms of the rare isotope in a sample to the number of atoms of the abundant isotope. We call this ratio r, generally a very small number. To arrive at numbers of convenient magnitude, it is conventional to express the ratio in terms of the departure of r from the ratio in a standard, which I call. v, and to express this departure in parts per thousand of s. Thus the standard delta notation is... 

For example, to determine the empirical formula of di-n-octylphthalate, the daughter spectrum of the containing molecular ion (392) was obtained (Figure 7). The relative peak areas of adjacent peak pairs at m/z 149 and 150 is 2 1. This indicates that the M+1 ion is twice as likely to lose a atom as retain it. Thus the ratio of the number of carbon atoms lost to those retained is 2 1. Since the identified phthalate substructure contains 8 carbons, the unknown compound (di-n-octylphthalate) must contain 24 carbon atoms. These data, along with the molecular weight of 390 as determined from the conventional Cl mass spectrum of the unknown was fed into the empirical formula generator and the output was one empirical formula C24H38O4. 

For a value to be traceable it must be related to stated references. By definition and convention the stated references are taken to include SI [6] reference values (e.g., atomic mass values), reference materials (RMs), as well as primary, reference, and standard methods. It is sometimes stated that chemical measurements are traceable to the mole. This is an incomplete statement as chemical measurements are simultaneously traceable to a number of references, inter alia, the mole, kg, meter, etc. Whilst it is considered desirable to employ high level references, such as the SI, where feasible, this is not always necessary in terms of fit for purpose criteria. Neither is it possible to relate all types of analyte (fat, fiber, protein, pH, etc.) to the SI. The key issue is that the references should be stated and fit for purpose. 

If, by convention, the mass of the most common Isotope of oxygen is taken as 16.000 atomic mass units (amu), the two chlorine isotopes assume relative masses of 35 and 37 atomic mass units. 

Older texts and even recent publications use the term Motnic weight instead of atomic mass. The proper term is atomic mass, but for years the convention was to use atomic weight, and old habits die hard. 

The numbering of groups (the vertical columns, also known as families) follows two different conventions, both of which should be familiar. In the system commonly used in North America, Roman numerals and letters are used to denote the various groups. The alternate system, devised by the International Union of Pure and Applied Chemistry Convention (lUPAC— the same group responsible for certifying atomic masses and element names) in 1985, numbers the Groups from 1... 

First, we must define S C and the concept of isotope fractionation (cf. Deines et al. 1974 Faure 1991 Stumm and Morgan 1996). The atomic weight of natural carbon is 12.011. Of this, about 98.9% occurs as the stable isotope C with a nominal atomic weight of exactly 12 atomic mass units (amu) and 1.1% as stable C with an atomic weight of 13 amu. A variety of natural processes can lead to small changes in these proportions, which are conventionally expressed in terms of 5 C where... 

The reciprocal Angstrom unit is also conventionally used for momentum (A ) throughout the neutron scattering literature. There was neither a common optical spectroscopic alternative (as there was to justify the adoption of the wavenmnber, cm, for the unit of energy) nor chemical reason (as there was to justify the adoption of the atomic mass unit, u or amu, for the unit of mass). For quantities not defined by lUPAC we have mostly used symbols consistent with the neutron scattering literature [14,15] but have, on rare occasions, been forced to invent our own symbol for the sake of clarity. We provide a table of symbols and units (p. xix). 

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