Radioactive Decay and Radiometric Dating Radioactivity and radiometric dating Atomic structure Radioactivity Allows us to put numerical ages on geologic events Atom model Atomic structure reviewed Nucleus composed of protons and neutrons Orbiting the nucleus are electrons negative electrical charges Atomic number Identifying number Number of protons Determines the properties Mass number Protons Neutrons Nucleons Not the same as Atomic Weight 1 Periodic Table 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 188 79 Au Nuclear Notation In class activity 1 State the number of protons and neutrons Atomic mass Number neutrons protons Atomic number protons 35 17 Cl Radioactivity Spontaneous breaking apart decay of atomic nuclei 37 17 Cl 235 92 U Radioactive decay Parent atom an unstable isotope Daughter products Formed from the decay of a parent atoms Different element because of nuclear changes 2 Nucleus 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 Types of radioactive decay Alpha emission Beta emission Gamma radiation 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 Alpha emission 2 N 2 P Positive charge Large non penetrating Beta emission Gamma radiation Penetrating energetic protons Lower energy of excited daughter nucleus No charge less mass than electron Negative charge electron N P e Small but low energy minor penetration 3 Nuclear Decay Equation 226 88 4 222 Ra 2He 86 Rn Nuclear Decay Equation 3 1 H 0 e 1 Nucleons same on both sides Nucleons same on both sides Alpha decay removes No and P Beta decay In class activity 2 What type of radioactive decay is occurring in this nuclear decay equation 60 27 60 Co 28 Ni 10e 3 2 He No P e In Class Activity 3 Write a balanced nuclear decay equation for Fermium 250 that undergoes alpha decay 250 100 Fm Beta Decay Balancing Nuclear Equations Number of nucleons the same on each side of equation P protons e electrons No neutrons Keep track of protons elemental symbol Uranium238 Note the atomic number decreases to right 4 Measuring Radioactivity Radioactive decay strips electrons Ions created Geiger counter charged wire results in clicks of counter Others rely on visual reactions ions 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 Decay rate constant unaffected by external conditions Parent to daughter ratios Half Life equation Amount remaining can be calculated by R I In class activity 4 400 mg of Co 60 Half life is 5 25 years How much is left after 15 75 years 1 2n Radiometric dating Known Half life Closed system Cross checked for accuracy Yields numerical dates 5 Carbon 14 dating Half life 5760 years Recent events C 14 produced in upper atmosphere Incorporated into carbon dioxide Absorbed by living organisms Useful tool for anthropologists archeologists historians and geologists who study very recent Earth history Formation of Carbon 14 Decay of Carbon 14 C 14 dating cross and tree ring chronology Importance of radiometric dating Confirms the idea that geologic time is immense Rocks from several localities have been dated at more than 3 billion years Radiometric dating is a complex procedure that requires precise measurement 6 Geologic time scale Divides geologic history into units Originally created using relative dates Bracket events and arrive at ages Geologic Time Scale Subdivisions Eon Era Period Epoch Eon Greatest expanse of time Four eons Phanerozoic visible life the most recent eon Proterozoic Archean Hadean the oldest eon Eras of the Phanerozoic eon Cenozoic recent life Mesozoic middle life Paleozoic ancient life 7
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