Hemingway
Name:
7.2 Half-life and Radioactive Decay
•
It can be difficult to determine the ages of objects by sight alone.
 _______________ provides a method to determine age by measuring relative amounts
of remaining radioactive material to stable products formed.
Carbon Dating
• All organisms contain ___________
 Plants use CO₂ to make food, Organisms eat plants
• Carbon has two main ____________
 Carbon-_____and carbon-____
• __________________ measures the ratio of carbon-12 and carbon-14.
 When______, an organism has stable carbon-12 and radioactive carbon-14 naturally in
a ___________________
 When an organism _________, carbon-14 stops being created and slowly _________
 Ratio of carbon-14 decreases with time
 This ratio can be used to _________________ of remains of that organism
 Carbon dating only works for organisms less than _____________________
(radiocarbon dating)
The Rate of Radioactive Decay
• _______________ measures the rate of radioactive decay.
 Half-life = time required for _________of the radioactive sample to __________
 The half-life for a radioactive element is a ___________rate of decay.
 Ex:
 Strontium-90
• has a half-life of _________years. If you have 10 g of strontium-90 today
there will be _______ remaining in 29 years.
Decay Curves
• Decay curves show the rate of decay for radioactive elements.
 The curve shows the relationship between ___________ and percentage of
__________________ remaining.
 Graphs will looks the same with the only difference being __________of half life
 Each type of radioisotopes decay at a _______________rate
 Example
 _____________ half life =3.8 days
 Carbon-14 half life = 5730 yrs
 Uranium-238 half life= _______________ \
Practice Problems # 1-4 pg 306
Hemingway
Name:
Common Isotope Pairs
• There are many radioisotopes that can be used for dating.
 _____________________ = the original, radioactive material
 ____________________ = the stable product of the radioactive decay
 The rate of decay remains constant, but some elements require ___________ to decay
while others decay over __________ before reaching a___________ daughter isotope.
 Carbon-14 decays into nitrogen-14 in one step.
 Uranium-235 decays into lead-207 in 15 steps.
 Thorium-235 decays into lead-208 in 10 steps.
The Potassium-40 Clock
• Radioisotopes with very long half-lives can help determine
the age of very old things.
 The potassium-40/argon-40 clock has a half-life of _____________________
 Argon-40 produced by the decay of potassium-40 becomes trapped in rock.
 Ratio of potassium-40 : argon-40 shows ___________
 As parent isotope___________, daughter isotope _____________
Practice Problems # 1-3 pg 309
Workbook pg 132-136 ALL