Radioactive Decay and Radiometric Dating STM Chapters 7 and 8 Pages 135-142 And 157-166 Radioactivity and radiometric dating Atomic structure Radioactivity Allows us to put numerical ages on geologic events ES Chapter 11 Pages 322-327 Atomic structure reviewed Atom model 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
Periodic Table Isotope Same number of protons Different number of neutrons Different mass number than another isotope of the same element Variant of the same atom Say Gold 188 for 188 79 Au Nuclear Notation Atomic mass Number (neutrons+protons) Atomic number (protons) 35 17 Cl In class activity # 1 State the number of protons and neutrons 37 17Cl 235 92U Radioactivity Spontaneous breaking apart (decay) of atomic nuclei Radioactive decay Parent atom an unstable isotope Daughter product Formed from the decay of a parent atoms Different element because of nuclear changes
Nucleus Very small 10-15 to 10-14 m radius Nuclear force called Strong interaction binds nucleons Protons repel one another Neutrons counteract this More neutrons than protons in larger atoms Nuclear forces Very strong at small distances (10-15 m) Weakens at 10X that distance (10-14 m) Larger nuclei, therefore, are inherently unstable, because they are big Atomic numbers greater than 82 are unstable Radioactive Types of radioactive decay Alpha emission Beta emission α β Gamma radiation γ Alpha emission Positive charge 2 N + 2 P Large non-penetrating α Beta emission β Gamma radiation γ 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
Nuclear Decay Equation 226 4 88 Ra He+ 222 86 Rn 2 Nucleons same on both sides Alpha decay α removes N o and P + Nuclear Decay Equation 3 0 1H e - + 3 2He -1 Nucleons same on both sides Beta decay β N o P + + e - In class activity #2 What type of radioactive decay is occurring in this nuclear decay equation? 60 27 Co 60 28 Ni + e In Class Activity # 3 Write a balanced nuclear decay equation for Fermium-250 that undergoes alpha decay Fm Balancing Nuclear Equations Number of nucleons the same on each side of equation P + (protons) + e - (electrons) = N o (neutrons) Keep track of protons elemental symbol Uranium- 238 Note the atomic number decreases to right
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 1 2 n In class activity # 4 In class activity # 4 Calculate how many half lives.
In class activity # 4 Calculate how many half lives In class activity # 4 Calculate amount In class activity # 4 Calculate amount Radiometric dating Known Half-life Closed system Cross-checked for accuracy Yields numerical dates Carbon-14 dating Half-life 5730 years Recent events C-14 produced in upper atmosphere Incorporated into carbon dioxide Absorbed by living matter 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 Geologic time scale Divides geologic history into units Originally created using relative dates
Difficulties in dating the time scale Not all rocks are datable (sedimentary ages are rarely reliable) Materials are often used to 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 Era Subdivision of an eon Three Eras Eras of the Phanerozoic eon Cenozoic ("recent life") Mesozoic ("middle life") Paleozoic ("ancient life")