Radioactive Decay and Radiometric Dating Extra credit chapter 7 in Bryson See online link fixed or moodle Atom model Radioactivity and radiometric dating Atomic nucleus Radioactivity Allows us to put numerical ages on geologic events Atomic number Identifying number Number of protons Determines the properties Periodic Table Series in Periodic Table Atomic structure Nucleus composed of protons and neutrons Orbiting the nucleus are electrons negative electrical charges Mass number Protons Neutrons Nucleons Not the same as Atomic Mass Isotope Same number of protons Different number of neutrons Different mass number than another isotope of the same element Variant atom of the same element Say Gold 188 for Au 188 1 Radioactivity Nucleus Spontaneous breaking apart decay of atomic nuclei Nuclear forces Very small 10 15 to 10 14 m radius Strong interaction binds nucleons Protons repel one another Neutrons counteract this More neutrons than protons in larger atoms Radioactive decay Parent atom an unstable isotope Daughter products Formed from the decay of a parent atoms Different element because of nuclear changes Beta emission Types of radioactive decay Alpha emission Gamma radiation Alpha emission Beta emission 2 N 2 P Positive charge Large non penetrating Gamma radiation Penetrating energetic photons Lower energy of excited daughter nucleus No charge less mass than electron Negative charge electron N P e Small but low energy minor penetration Very strong at small distances 10 15 m Weakens at 10X that distance 10 14 m Elements at 82 are unstable because they are big Radioactive Nuclear Decay Equation Alpha Decay Ra 226 Rn 222 Total nucleons stays same but on daughter product side Alpha decay removes 2No and 2P 2 Nuclear Decay Equation Beta Decay H 3 He 3 Balancing Nuclear Equations P protons e electrons No neutrons Keep track of protons elemental symbol decay decreases by two by releasing them with two neutrons so atomic number goes down by 2 atomic mass goes down by 4 Total nucleons stays same Beta decay No P e decay increases protons by releasing electron so atomic number goes up by 1 atomic mass stays the same Measuring Radioactivity Radioactive decay strips electrons from atoms Ions created Geiger counter charged wire results in clicks of counter Others rely on visual reactions of ions R I R remaining 1 2n I initial n number of half lives NAMES Pa protactinium Th thorium Ra radium Rn radon Po polonium At astatine Pb lead Bi bismuth Tl thallium Parent to daughter ratios of H 3 Half Life Decay is random for any radioactive atom BUT Predictable for mass of material One half of unstable parent material daughter product HALF LIFE L1 2 Decay rate constant unaffected by external conditions Half Life equation Amount remaining can be calculated by Decay of U 238 Calculate amount from half life Start with 400 mg of Co 60 Half life is 5 25 years How much is left after 15 75 years To use equation at right First calculate how many half lives 3 half lives 1 R I n 2 5 25 yr yr 5 25 halflife 15 75 75 years 15 75 years 15 years halflife halflife 5 25 yr Calculate amount from half life 400 mg of Co 60 Half life is 5 25 years How much is left after 3 half lives Calculate amount R 400mg 1 50mg 233 3
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