Naturally occurring isotopes and their abundances

Appendix 5 Naturally occurring isotopes and their abundances... 

Chapter 4, Atoms and Elements, introduces elements and atoms and the periodic table. The names and symbols of element 114, Herovium, FI, and 116, Livermorium, Lv, have been added to update the periodic table. Atomic numbers and mass number are determined for isotopes. Atomic mass is calculated using the masses of the naturally occurring isotopes and their abundances. Trends in the properties of elements are discussed, including atomic size, electron-dot symbols, ionization energy, and metallic character. 

The upper part of the figure illustrates why the small difference in mass between an ion and its neutral molecule is ignored for the purposes of mass spectrometry. In mass measurement, has been assigned arbitrarily to have a mass of 12.00000, All other atomic masses are referred to this standard. In the lower part of the figure, there is a small selection of elements with their naturally occurring isotopes and their natural abundances. At one extreme, xenon has nine naturally occurring isotopes, whereas, at the other, some elements such as fluorine have only one. 

ISOTOPES OF NATURALLY OCCURRING ELEMENTS AND THEIR ABUNDANCES... 

The molar masses of elements are determined by using mass spectrometry to measure the masses of the individual isotopes and their abundances. The mass per mole of atoms is the mass of an individual atom multiplied by the Avogadro constant (the number of atoms per mole). However, there is a complication. Most elements occur in nature as a mixture of isotopes we saw in Section B, for instance, that neon occurs 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. All molar masses quoted in this text refer to these average values. Their values are given in Appendix 2D. They are also included in the periodic table inside the front cover and in the alphabetical list of elements inside the back cover. 

Titanium has five naturally occurring isotopes. Table 2 contains a listing of these isotopes and their abundances (where m/z is the mass per unit chaige). It also contains a match for ions derived from Ti02 at 400°C. The match is reasonable because of the high percentage of isotope 80 in comparison with all of the other isotope abundances. 

Some of the important properties of Group 5 elements are summarized in Table 22.1. Having odd atomic numbers, they have few naturally occurring isotopes Nb only 1 and V and Ta 2 each, though the second ones are present only in very low abundance 0.250%, Ta 0.012%). As a consequence (p. 17) their atomic weights have been determined with considerable precision. On the other hand, because of difficulties in removing all impurities, reported values of their bulk properties have often required revision. 

Symbol B atomic number 5 atomic weight 10.811 a Group III A (Group 13) metalloid element atomic volume 4.70 cc/g-atom electron affinity 0.277 eV electronic configuration Is22s22pi valence state +3 naturally occurring stable isotopes are B-10 and B-11 and their abundance 19.57% and 80.43%, respectively. 

There are four naturally occurring isotopes of iron. Their abundances and atomic masses are fisted in Table 2 together with data for other man-made isotopes of iron whose atomic masses have been determined with precision. The most abundant of the natural isotopes is Fe, and this is also the most stable nuclear configuration of all the elements in terms of nuclear binding energy per nucleon. Its stability, viewed in terms of nuclear equihbria established in the last moments of supemovae, perhaps explains its widespread occurrence in the cosmos. The isotope Fe has several applications, most notably in Mossbauer spectroscopy, and it has proved to be... 

Seven naturally occurring isotopes of dysprosium are known. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element s name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope. The four most abundant isotopes of dysprosium are dysprosium-161, dysprosium-162, dysprosium-163, and dysprosium-164. 

You can calculate the atomic mass of any element if you know its number of naturally occurring isotopes, their masses, and their percent abundances. The following Example Problem and Practice Problems will provide practice in calculating atomic mass. 

Appropriate tracers to investigate SOM turnover rates are C and C, the two naturally occurring isotopes of C. The radioactive C atom is cojitinuously formed in the atmosphere by solar radiation and from the remaining "C in organic compounds their age can be estimated. The mean natural abundance of "C is constant however, small variations in the C/ C ratio identify sources and processes involved in the formation of organic molecules. The best-known examples are the isotopic difference between heavy C4 plants and light C, plants and the isotopic enrichment of food chains. 

Problem 2.10. Neon has three naturally occurring isotopes of mass number (to the closest atomic mass unit) of 20.0, 21.0, and 22.0. Their abundances (percent of those found in nature) are 90.92 percent, 0.257 percent, and 8.82 percent. What is the atomic weight of neon ... 

Most elements occur in nature as mixtures of isotopes. We can determine the average atomic mass of an element, usually called the element s atomic weight, by using the masses of its isotopes and their relative abundances ... 

The lanthanoids resemble each other much more closely than do the members of a row of tf-block metals. The chemistry of the actinoids is more complicated, and in addition, only Th and U have naturally occurring isotopes. Studies of the transuranium elements (those with Z > 92) require specialized techniques. The occurrence of artificial isotopes among the /-block elements can be seen from Appendix 5 all the actinoids are unstable with respect to radioactive decay (see Table 25.5), although the half-lives of the most abundant isotopes of thorium and uranium ( Th and h = 1.4 X lO and 4.5 x 10 yr respectively) are so long that for many purposes their radioactivity can be neglected. 

Uranium is composed of three naturally occurring radioactive isotopes having the mass numbers 238, 235, and 234 all of which decay by emitting alpha particles from their nuclei. All of the naturally occurring isotopes of uranium can also decay by spontaneous fission, but virtually all spontaneous fission events in a uranium-bearing mineral are due to which is by far the most abundant isotope of uranium at 99.2745% by number. 

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