Natural abundance of elements

Table 1.1. Natural Abundance of Elements in the Earth s Crust ...

The natural abundances of elements in the human body, expressed as pwcent by mass, are oxygen (O), 65 percent carbon (C), 18 percent hydrogen (H), 10 percent nitrogen (N), 3 pacait calcium (Ca), 1.6 percent phosphorus (P), 1.2 pwcent aU other elements, 1.2 percent. Calculate the mass in grams of each element in the body of a 62-kg person. 

Relation Between Atomic Structure and Natural Abundance of Elements and Isotopes 3... 

The natural abundances of elements in the human body, expressed as percent by mass, are oxygen (O), 65 percent carbon (C),... 

Atoms of elements are composed of isotopes. The ratio of natural abundance of the isotopes is characteristic of an element and is important in analysis. A mass spectrometer is normally the best general instrument for measuring isotope ratios. 

Compared to other spectroscopic methods, NMR spectroscopy is a very insensitive technique. As a general rule of thumb, the sample studied must contain at least 10 moles of target nuclei. The required sample size thus depends on the percentage of the element present in the sample, as well as on the natural abundance of the... 

The greater the mass of an individual atom, the greater the molar mass of the substance. However, most elements exist in nature as a mixture of isotopes. We saw in Section B, for instance, that neon exists as three isotopes, each with a different mass. In chemistry, we almost always deal with natural samples of elements, which have the natural abundance of isotopes. So, we need the average molar mass, the molar mass calculated by taking into account the masses of the isotopes and their relative abundances in typical samples ... 

The natural abundances of the isotopes of four elements (Cl, Cr, Ge, and Sn) illustrate the diversity of isotopic distributions. The mass number and percent abundance of each isotope are indicated. 

Figure 2-19 shows the mass spectrum of the element neon. The three peaks in the mass spectrum come from three different isotopes of neon, and the peak heights are proportional to the natural abundances of these isotopes. The most abundant isotope of neon has a mass number of 20, with 10 protons and 10 neutrons in its nucleus, whereas its two minor isotopes have 11 and 12 neutrons. Example illustrates how to read and interpret a mass spectmm. 

A sample of lead atoms is analyzed by mass spectrometry. The bar graph in the margin shows the results. Use information from the graph to write the elemental symbol that represents each Pb isotope and estimate the natural abundance of each. List the number of protons and neutrons for each isotope. 

Tin is an unusual element in that it has three magnetically active isotopes, all spin-Vi. However, tin-115 has a natural abundance of only 0.35%, and is never studied. The other two, tin-117 and tin-119, occur in similar amounts (7.61 and 8.58% respectively). Spectra of the latter are normally recorded, as it is about 25% stronger. Tetramethyltin is taken as the zero-point, and the total chemical shift range is about 3000 ppm. 

Despite the presence of the A + 1 isotope for hydrogen and the A + 4 isotope for sulfur these elements are considered A and A + 2 correspondingly, as the natural abundances of these isotopes are very low and may be detected only in the case of a large number of H and S atoms in a sample molecule. 

Formally oxygen is also an A + 2 element. However, the natural abundance of the lsO isotope is only 0.2% of that of the main isotope ieO. That is why reliable calculation of the number of oxygen atoms based on the intensity of the isotopic peaks is hardly possible. Nevertheless, it is quite possible to estimate this number. Thus, if the intensity of the M + 2 peak in the mass spectrum of a sample with a low number of carbon atoms is higher than 0.5% relative to M+, this compound may contain one or more oxygen atoms. 

Nitrogen is an A + 1 element, while the natural abundance of 15N isotope constitutes about 0.4% of 14N. Since besides nitrogen only carbon contributes notably to the intensity of the A + 1 peak, it is possible to estimate the number of nitrogen atoms in the molecule on the basis of the nitrogen mle and the differences in the abundances of 13C ii 15N isotopes. 

The elements whose isotopes are routinely measured with gas inlet mass spectrometers are carbon (12C and 13C, but not 14C), oxygen (160, 170, l80), hydrogen ( H, 2H, but not 3H), nitrogen (14N and 1SN) and sulphur (32S, 33S, 34). Stable isotopes of H, C, N, O, and S occur naturally throughout atmosphere, hydrosphere, lithosphere, and biosphere. They are atoms of the same elements with a different mass. Each element has a dominant light isotope with the nominal atomic weight (I2C, 160,14N, 32S, and H) and one or two heavy isotopes (l3C, nO, 180, 15N, 33S, 34S, and, 2H) with a natural abundance of a few percent or less Table 1). 

ISOTOPES There are 21 isotopes of silicon, three of which are stable. The isotope Si-28 makes up 92.23% of the element s natural abundance in the Earth s crust, Si-29 constitutes 4.683% of all silicon found in nature, and the natural abundance of Si-30 is merely 3.087% of the stable silicon isotopes found in the Earth s crust. 

Symbol He atomic number 2 atomic weight 4.0026 a Group 0 (Group 18) inert gas element second hghtest element electron configuration Is valence 0 no chemical compound known atomic radius 0.33A isotope He-3 is found in trace concentration in He-4 natural abundance of He-3 1.37 ppm short-hved radioisotopes He-5, He-6, and He-8 are known. 

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