1. Review: Types of decay
1. Alpha

2. 2. Beta

3. 3. Gamma
REMEMBER: Gamma is just surplus energy. Gamma emission makes no change to the atomic or mass numbers.

4. Sec. 7.2 Half-Life:
the time it takes for half of a radioactive sample to decay BUT it is a constant rate!
Ex) Strontium-90 has a half-life of 29 years. So, if you have 10 g of strontium-90 today, there will be 5 g (half) remaining in 29 years.
Vocabulary:
Parent isotope: the original radioactive material
Daughter isotope: the stable product that remains after decay has happened

5. Decay Curves: shows the rate of decay for radioactive elements
shows the relationship between half-life & percentage of original substance remaining
See pages

6. Radioactive Dating: a method to determine age of objects
compares amount of parent isotope to daughter isotope
Ex) Radiocarbon dating measures the ratio of carbon-12 & carbon-14.
stable carbon-12 & radioactive carbon-14 exist naturally in a constant ratio
when an organism dies, carbon-14 stops being taken in & what remains slowly decays
See pages

7. only works for organisms < 50,000 years old
because the half-life of carbon-14 is 5730 years
after 50,000 years, there isnt enough C-14 left!

8. There are many radioisotopes that can be used for dating
some elements require one step to decay, while others take many steps before reaching a stable daughter isotope.
Carbon-14 decays into nitrogen-14 in 1 step
Uranium-235 decays into lead-207 in 15 steps.
Thorium-235 decays into lead-208 in 10 steps.
See page 307

9. 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 1.3 billion years.
Argon-40 produced by the decay of potassium-40 becomes trapped in rock.
Ratio of potassium-40 : argon-40 shows age of rock.
Take the Section 7.2 Quiz
See pages