Chlorine common isotopes

Certain elements, such as chlorine, occur in several very common isotopes, so their average atomic mass isn t close to a whole number. Other elements, such as carbon, occur in one very common isotope and several very rare ones, resulting in an average atomic mass that s very close to the whole-number mass of the most common isotope. 

Chlorine occurs in two common isotopes. It appears as j Cl 75.8% of the time and as jJCl 24.2% of the time. What is its average atomic mass ... 

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. 

Chlorine has two common isotopes, C1 and Cl, which occur naturally in a 3 1 ratio. Thus, there are two peaks in a 3 1 ratio for the molecular ion of an alkyl chloride. The larger... 

For example, the presence of bromine can be determined easily, because bromine causes a pattern of molecular ion peaks and isotope peaks that is easily identified. If we identify the mass of the molecular ion peak as M and the mass of the isotope peak that is two mass units heavier than the molecular ion as M -t- 2, then the ratio of the intensities of the M and M+2 peaks will be approximately one to one when bromine is present (see Chapter 8, Section 8.5, for more details). When chlorine is present, the ratio of the intensities of the M and M + 2 peaks will be approximately three to one. These ratios reflect the natural abundances of the common isotopes of these elements. Thus, isotope ratio studies in mass spectrometry can be used to determine the molecular formula of a substance. 

You must be careful when studying molecules containing chlorine or bromine atoms, since these elements have two commonly occurring isotopes. Chlorine has isotopes of 35 (relative abundance = 75.77%) and 37 (relative abundance = 24.23%) bromine has isotopes of 79 (relative abundance = 50.5%) and 81 (relative abundance = 49.5%). When these elements are present, take special care not to confuse the molecular ion peak with a peak corresponding to the molecular ion with a heavier halogen isotope present. 

For four of the elements—oxygen, sulfur, chlorine, and bromine—the principal heavier isotope is two mass units greater than the most common isotope. 

The two most common isotopes of the element chlorine are chlo-rine-35 and chlorine-37, but the atomic weight that is given in the periodic table for chlorine is 35.453. Explain why this atomic weight is not the same as the weight of either of these isotopes. 

Molecules containing chlorine or bromine can readily be identified as these halogens have two common isotopes in a known ratio. 

A common mistake for beginners in mass spectrometry is to confuse average atomic mass and isotopic mass. For example, the average atomic mass for chlorine is close to 35.45, but this average is of the numbers and masses of Cl and Cl isotopes. This average must be used for instruments that cannot differentiate isotopes (for example, gravimetric balances). Mass spectrometers do differentiate isotopes by mass, so it is important in mass spectrometry that isotopic masses be used... 

Table 1.1 Isotopic abundance of common elements. Interesting to note is that chlorine and bromine have t /o naturally intense isotopes.

PROBLEM 2.5 Chlorine, one of the elements in common table salt (sodium chloride), has two main isotopes, with mass numbers 35 and 37. Look up the atomic number of chlorine, tell how many neutrons each isotope contains, and give the standard symbol for each. 

Chlorine has three isotopes in nature 35C1—common and stable 36C1—very rare and radioactive, with a 301,000-year half-life and 37C1—less common and stable. Chlorine-36 concentrations are commonly expressed in units of 107 atoms/1 of water. The measurement is performed in specialized laboratories with dedicated accelerators. Measurements can be done on 1-1 water samples. 

Salinization is often observed in heavily pumped coastal wells. This observation is commonly based solely on chlorine concentration data, and is almost always attributed to seawater encroachment, which is made possible because of overpumping. The real cause of salinization has to be investigated in each case with the aid of complete chemical and isotopic analyses. 

A plot of chlorine isotopic composition versus depth shows a few additional aspects of the data from two shield environments (Figure 11(a)). Canadian saline waters generally occur at depths below 500 m. Saline or brackish waters are not commonly found in shallow Canadian Shield systems (Frape and Fritz, 1987). Both the Canadian and the Fennoscandian data show a considerable variation in isotopic signature for any particular depth. If the data points were coded for specific sites, there would still be no discernible trends with depth. Finally, some Fennoscandian samples appear to approach the signature of the Baltic Sea, similar to that for the... 

One radioactive isotope of chlorine is used in research. That isotope is chlorine-36. This isotope is used because compounds of chlorine occur so commonly in everyday life. The behavior of these compounds can be studied if chlorine-36 is used as a tracer. A tracer is an isotope whose presence in a material can be traced (followed) easily. 

Besides deuterium and tritium, isotopes commonly used in organic chemistry include i C, available a > CH30H and Ba CO 0, as Ha 0 as 5NO3, N02 Cl, as chlorine or chloride as iodide. 

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