12C carbon

Carbon, a common element in the outer crust of the earth, and the main component of all biological and organic substances, occurs in three isotopic forms carbon-12 or C-12 for short (whose chemical symbol is 12C), carbon-13 or C-13 (13C), and carbon-14 or C-14 (14C) (see Fig. 8 and Table 66). 

Molecular weight the average weight of a molecule measured in units equivalent to 1/12 of Carbon 12 (12C). 

Relative mass (m) mass of an atom, a molecule or an ion divided by one-twelfth of the mass of the 12C carbon atom. It is thus dimensionless. Thus m = (mass of a molecular or ionic species)/(mass of 12C) x (1/12). 

Table 2.1 may be useful for calculating the number of carbon, bromine, chlorine, and sulfur atoms in the molecular formula. This table shows that for every 100 12C atoms there are 1.1 13C atoms. Also, for every 100 32S atoms, there are 0.8 33S atoms and 4.4 34S atoms. The following examples 

The atomic weight of an element is the relative mass of an average atom of the element compared with 12C, which has an atomic weight of exactly 12. Thus, since a sulfur atom has a mass jj times that of a carbon atom, the atomic weight of sulfur is 

Corey et al. investigated the kinetic isotopic effect (KIE) in asymmetric dihydroxylation. 12C/13C KIE was measured for the dihydroxylation of styrene, p-nitrostyrene, and 4-methoxy-benzoate (Figure 7).197 The observed similar 12C/13C isotopic effect of two olefinic carbons 

Not long ago this could have been determined only by extremely laborious, and often equivocal, selective degradation experiments but the coming of carbon n.m.r. spectroscopy has now made all the difference. Neither the 12C nor the 14C carbon isotopes produce an n.m.r. signal but the 13C isotope, which occurs in ordinary carbon to 

The result is a type of onion-like model of the star with an iron-nickel core in the centre. The situation is somewhat different for smaller stars the path branches at the point where carbon burning (12C +12 C) begins. While the heavier stars are not affected by this process, the smaller ones (4-8 solar masses) are completely torn apart by carbon burning. 

The chemical analysis has revealed that rather low C/O ratios are found in metal-poor extragalactic carbon stars, as found for galactic carbon stars of the solar vicinity. Furthermore, the three analyzed stars show similar s-elements enhancements [ls/Fe]=0.8-1.3 and [hs/Fe]=l.l-1.7. This leads to new constraints for evolutionary models. For instance, the derived C/O and 13C/12C ratios are lower than model predictions at low metallicity. On the contrary, theoretical predictions of neutrons exposures for the production of the s-elements are compatible with observations (see Fig. 1). Finally, from their known distances, we have estimated the luminosities and masses of the three stars. It results that SMC-B30 and Sgr-C3 are most probably intrinsic carbon stars while Sgr-Cl could be extrinsic. 

The defining characteristic of an atom of a chemical element is the number of protons in its nucleus. A given element may have different isotopes, which are nuclei with the same numbers of protons but different numbers of neutrons. For example, 12C and 14C are two isotopes of carbon. The nuclei of both isotopes contain six protons. However, 12C has six neutrons, whereas 14C has eight neutrons. In general, it is the number of protons and electrons that determines chemical properties of an element. Thus, the different isotopes of an element are usually chemically indistinguishable. These isotopes, however, have different masses. 

A further point about mass spectrometry, noticeable in the spectrum of propane (Figure 12.2), is that the peak for the molecular ion is not at the highest m/z value. There is also a small peak at M + l because of the presence of different isotopes in the molecules. Although 12C is the most abundant carbon isotope, a small amount (1.10% natural abundance) of 13C is also present. Thus, a certain 

As we saw in Chapter 2, the first quantitative information about atomic masses came from the work of Dalton, Gay-Lussac, Lavoisier, Avogadro, Cannizzaro, and Berzelius. By observing the proportions in which elements combine to form various compounds, nineteenth-century chemists calculated relative atomic masses. The modern system of atomic masses, instituted in 1961, is based on 12C (carbon-12) as the standard. In this system I2C is assigned a mass of exactly 12 atomic mass units (amu), and the masses of all other atoms are given relative to this standard. 

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