105-15 9-28-2011 Geologic time

definite ages defining the actual age of a particular

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Unformatted text preview: h geologic studies of field relationships (laws of superposition, cross-cutting relationships, etc.). Definite (±) ages – defining the actual age of a particular geologic rock unit or event (termed “absolute age dating” and relying on the presence of radioactive elements). Elements and their isotopes • The number of protons in an atom's nucleus is called its atomic number – that number defines the “element” • # Protons + neutrons called atomic weight (mass) • The number of neutrons can vary for a particular element • Atoms of the same element with different numbers of neutrons are called isotopes. Some are radioactive. Eg., carbon (atomic # = 6) has 3 isotopes with atomic weights (# P + # N) of 12, 13, 14 What is radioactivity? Some of the naturally occurring elements on planet Earth, and some varieties (isotopes) of others, have atoms with physically unstable nucleii. To achieve nuclear stability, the atoms spontaneously disintegrate by emitting high energy particles (alpha, beta) and energetic radiation (gamma rays). This spontaneous emission if referred to as radioactivity. Alpha particles = 2 protons and 2 neutrons; beta particles are energetic electrons; gamma rays are formed during the emission of alpha and beta particles. Such changes in the “parent” nucleii of radioactive elements result in new daughter “nucleii”, because the new nucleii now have a different # of protons. Expulsion of an alpha particle means loss of 2 neutrons and 2 protons = 2 less atomic number Loss of a beta particle converts a neutron into a proton — no change in atomic mass, but 1 more atomic number Examples of unstable atomic nucleii and the transition Capture of an from parent to daughter elements. Some daughter electron converts a proton elements are themselves radioactive. The rate of transition into a neutron = 1 from parent to daughter element is called less atomic number . the “half-life” Radioactive parent nucleus pp p p p p Decay process Daughter nucleus pp pp p p Proton Neutron p (a) Alpha decay pp p p p p Atomic mass decreases by 4; atomic number decreases by 2 p p pp p p p p Alpha particle Emission of 2 protons and 2 neutrons (alpha particle) pp p p p pp Atomic mass not changed much; atomic number increases by 1 because Neutron becomes proton Beta particle (b) Beta decay An electron (beta particle) is ejected from the nucleus pp p p p p p pp p p p p p Beta particle (c) Electron capture pp p p p p Atomic mass not changed much; atomic number decreases by 1 electron combines with a proton to form a neutron Using radioactivity in dating • Parent – an unstable (radioactive) isotope • Daughter product –an element resulting from decay of parent (it may also be unstable) • Half-life – time required for one-half of the parent isotope in a sample to decay into the daughter product HALF-LIFES OF MAJOR RADIOACTIVE ELEMENTS • IODINE - 131 • STRONTIUM - 90 PLUTONIUM - 239 • URANIUM - 235 • URANIUM - 238 • CARBON - 14 ~ 8 DAYS 28 YEARS 24,000 YEARS 713,000,000 YEARS 4,500,000,000 YEARS 5730 YEARS Carbon -14 Nitrogen - 14 N loses e —> P with same atomic #...
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