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112lect8 - GY 112 Lecture Notes D Haywick(2006 1 GY 112...

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GY 112 Lecture Notes D. Haywick (2006) 1 GY 112 Lecture Notes Radiometric dating Lecture Goals : A) Radioactivity B) Types of Radiometric dating C) Mass spectometers Textbook reference: Levin 7 th edition (2003), Chapter 1; Levin 8 th edition (2006), Chapter 2 and 3 (plus stuff from Doug’s personal experience) A) Radioactivity This lecture might prove tricky if you have not had chemistry before. All you need is a rudimentary understanding of the subject (e.g., the level we do in GY 111), but for some people, even that is too much. Fear not. Chemistry and in particular, geochemistry, are pretty straightforward if you have a good imagination. Chemical elements are arranged into a table ( Periodic Table; see above image) according to their atomic weight and their chemical properties. Each successive element has one more proton in its nucleus and one more electron in “orbit” around the nucleus then the previous one. There may also be additional neutrons added to the nucleus and this is where some variability in specific elements can occur. Not all atoms of a particular element may contain the same number of neutrons. Take hydrogen for example. The vast majority of the hydrogen we find on the planet Earth consists of a single proton and a
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GY 112 Lecture Notes D. Haywick (2006) 2 single electron. ( 1 H). But some atoms of hydrogen have a nucleus that contains 1 proton and 1 neutron (see image to left from http://www.emc.maricopa.edu /faculty/farabee/ BIOBK/BioBookCHEM1.html). This atom retains the same charge (neutrons are neutral), but since neutrons weigh about the same as a proton, the weight of this “heavy” hydrogen is doubled ( 2 H). Atoms with the same number of protons but different numbers of neutrons are called isotopes . Isotopes are generally identified through their atomic number, but in the case of hydrogen, the isotopes are actually named (almost like they were distinct elements). 2 H is commonly called deuterium and is given the chemical symbols D. Heavy water, which is used as a neutron absorber in nuclear power plants is D 2 O. A third isotope of hydrogen is also known. Its nucleus consists of 1 proton and 2 neutrons ( 3 H). This isotope is called tritium (T) and is different from the other two in being unstable. The nucleus of tritium is too heavy and gradually breaks apart. The decay is one of the forms of radioactivity and which in this case, and several other elements as well, is hazardous to life forms. Some unstable elements are, however, useful for geological interpretations of time. Using radioactive decay to establish geological time is called radiometric (or radioisotopic ) dating . Before we
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