Review: Types of decay 1. Alpha

2. Beta

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

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

Decay Curves: shows the rate of decay for radioactive elements shows the relationship between half-life & percentage of original substance remaining See pages 305 - 306

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 302 - 304

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!

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

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 307 - 308