Radioactive Decay and Radiometric Dating Radioactivity and radiometric dating Atomic structure Radioactivity Allows us to put numerical ages on geologic events 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 atom of the same element 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 products Formed from the decay of a parent atoms Different element because of nuclear changes
Nucleus 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 Nuclear forces 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 Types of radioactive decay Alpha emission Beta emission α β Gamma radiation γ Alpha emission 2 N + 2 P + Positive charge 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 Beta Decay 60 28 Ni 0 + 1 e In Class Activity # 3 Write a balanced nuclear decay equation for Fermium-250 that undergoes alpha decay 250 100 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 # 4 400 mg of Co-60 Half life is 5.25 years How much is left after 15.75 years? Radiometric dating Known Half-life Closed system Cross-checked for accuracy Yields numerical dates
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
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")