Isotopes Fractionate
I have promised my students not to throw technical terms around without explanations. For the sake of organization or readability, explanations may not be given when some terms are first mentioned, but discussed in a subsequent chapter. I have, for exampl
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Isotopes Fractionate
12.1
Isotopes I have promised my students not to throw technical terms around without explanations. For the sake of organization or readability, explanations may not be given when some terms are first mentioned, but discussed in a subsequent chapter. I have, for example, used the expression isotope tracers a few times. What are isotopes and how do they trace? Isotopes were not discovered until 1913, because the atomic weights of chemical elements are not whole numbers as William Prout predicted in 1815. Prout's theory makes sense, because the weight of an atom is in its nuclear particles, neutrons and protons, each of which is one unit of the atom's total mass; the mass of an electron is, for all practical purposes, nil. T. W. Richards, one of the best chemical analysts of all time, proved in 1913 beyond any doubt that the atomic weights of elements are not whole numbers. But he was also upset to find that weights of one and the same element can be different Lead produced by the radioactive decay of uranium, Richards discovered, has a different weight from that of ordinary lead. The results puzzled Richards, but a young British chemist, Frederick Soddy, proposed a brilliant solution: A chemical element is not made up of one kind of atom, but is a mixture of two or more isotopes. The word isotope is derived from the Greek words that mean "same place". The "place" is the pigeon hole in the periodic table of chemical elements, each reserved for one chemical element which has the same number of protons and of electrons. The pigeon hole is also reserved for two or more isotopes of that element which possess different numbers of neutrons. All chlorine atoms, for example, have 17 protons in their nucleus, but one of the elements' isotopes has 18 neutrons, giving it an atomic weight of 35, while the other has 20 neutrons, giving it an atomic weight of 37. The pigeon hole for chlorine is reserved for both of the isotopes, chlorine 35 and chlorine 37, and for other isotopes which are radioactive and short-lived. Chlorine is ordinarily a mixture of the two stable isotopes. Chlorine 35 constitutes always 75.77% and chlorine 37.23%
K. J. Hsü, Physics of Sedimentology © Springer-Verlag Berlin Heidelberg 2004
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Chapter 12 . Isotopes Fractionate
of the mixture. The atomic weight with this ratio of its two isotopes turns out to be 35.45, the inconvenient number we encountered in our chemistry exercises before computers were invented. Isotopes are used as tracers in geology. A groundwater hydrologist, for example, may spike groundwater with a radioactive isotope, or radio-isotope, and trace the flow path through the detection of this isotope in water samples. Some radio-isotopes are, in fact, natural tracers. Carbon-l4 is, for example, produced in the stratosphere by cosmic radiation. Seawater acquires its C-14 during its exchange with the atmosphere and this radio-isotope decays as it descends and circulates as a bottom current. The amount of C-14 still left in a seawater sample gives an age, which is
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