Atoms with abundant isotopes

The relative abundances of isotopic peaks are important for derivation of atomic compositions. It is especially convenient if the studied molecules contain atoms with unique isotopic distributions, such as when irregular changes of isotopic abundance are present. For example, as listed in Table 9.1, the abundances of sulfur isotopes follow the order In the case of potassium it is in the case of chlorine it is... 

The structures of many compounds include atoms in the forms of several natural isotopes. Table 4.1 in Chapter 4 lists the natures and relative abundances of the various isotopes of the main chemical elements constituting analyzable molecules in GC-MS coupling. The mass spectra present grouped peaks corresponding to the statistical distributions of the isotopes present. Ions consisting of the same atoms with different isotopes are known as isotopic isomers or isotopomers. 

Carbon has two common isotopes, and with relative isotopic abundances of, respectively, 98.89% and 1.11%. (a) What are the mean and standard deviation for the number of atoms in a molecule of cholesterol (b) What is the probability of finding a molecule of cholesterol (C27H44O) containing no atoms of... 

For any one element, the abundances (relative amounts) of isotopes can be described in percentage terms. Thus, fluorine is monoisotopic viz., it contains only nuclei of atomic mass 19, and phosphorus has 100% abundance of atoms with atomic mass 31. For carbon, the first two isotopes occur in the proportions of 98.882 to 1.108. 

The most abundant isotope is which constitutes almost 99% of the carbon in nature. About 1% of the carbon atoms are There are, however, small but significant differences in the relative abundance of the carbon isotopes in different carbon reservoirs. The differences in isotopic composition have proven to be an important tool when estimating exchange rates between the reservoirs. Isotopic variations are caused by fractionation processes (discussed below) and, for C, radioactive decay. Formation of takes place only in the upper atmosphere where neutrons generated by cosmic radiation react with nitrogen ... 

The boron atom in natural abundance is approximately 80% nB and 26% 10B. Identify all of die possible symmetry groups for LaBg with these isotopic species, e.g. LanB t L Bj B, La,1B410B2, LanB3,0B3 etc. A model is especially useful in this case. 

Note The mass number must not be confused with the atomic number of an element. For the heavier atoms there can be isotopes of the same mass number belonging to different elements, e.g., the most abundant isotopes of both igAr and 2oCa have mass number 40. 

Definition The molecular ion has the same empirical formula as the corresponding neutral molecule. The neutral and its molecular ion only differ by one (or more) electron(s). A singly charged molecular ion can either be a positive radical ion, M" , or a negative radical ion, M" (not in case of El). The mass of this ion corresponds to the sum of the masses of the most abundant isotopes of the various atoms that make up the molecule (with a correction for the elec-tron(s) lost or gained. Chap. 3.1.4). [8,9]... 

At one time, the hydrogen atom with one proton and no neutron was used as the standard to define 1 atomic mass unit (1 amu). Today, chemists use carbon-12, the most abundant isotope of carbon for the standard amu, which is defined as 1/12 of the C-12 atom. Therefore, the actual atomic weight for an element is in average mass units (numbers), taking into account all the isotopes (atoms) of that element. 

A neutral organic compound, composed of the most abundant isotopes of the el erne tils present, with an even number of nitrogen atoms 111 List have an even relative molecular mass, but a compound with an odd number of nitrogen atoms must have an odd relative molecular mass. 

Mass spectrometers measure the relative mass of an individual ion. with its particular combination of isotopes. We therefore use the relative atomic masses of the most abundant isotopes lo calculate the accurate relative molecular mass of an ion. 

The nominal mass of a molecule or ion is the integer mass of the species with the most abundant isotope of each of the constituent atoms. For carbon, hydrogen, and bromine, the most abundant isotopes are l2C, H, and 79Br. Therefore the nominal mass of C2H5Br is (2 X 12) + (5 X 1) + (1 X 79) = 108. 

I 3 Isotope patterns. (Caution This problem could lead to serious brain injury.) For an element with three isotopes with abundances a, b, and c, the distribution of isotopes in a molecule with n atoms is based on the expansion of (a + b + c)". Predict what the mass spectrum of Si2 will look like. 

In 1917 Harkins found that on the average elements of even atomic number (Z) are about 70 times as abundant in meteorites as those of odd Z he further noted that the first seven elements in the order of their abundance are all even-numbered and make up almost 99% of the material in meteorites. [28] Four years later he elaborated his assumption that the relative abundances of the atomic species of low atomic weight may be used as an index... of their relative stability. He now suggested several more rules, including that atoms with even A (mass number) and an odd number of electrons are extremely rare. [29] These rules were claimed to be valid for isotopes, and not merely elements. In 1931, after more data on the distribution of isotopes had been collected, he reported that even-A nuclei were much more frequent than odd-A nuclei. [30]... 

The most abundant isotope is the one with a mass number of 23 because the atomic mass is so close to this number. 

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