Chemistry Do Now 10-9-17 Directions: Take out your Do Now sheet and begin.   If Uranium 238 undergoes 5 alpha decays, what will be the final element? If Uranium 238 undergoes 5 beta decays, what will be the final element? If Uranium 238 undergoes 5 gamma decays, what will be the final element?

Homework - Copy Pd-100 has a half-life of 3.6 days. If one had 6.02 x 1023 atoms at the start, how many atoms would be present after 20 days?

Vocabulary Words Alpha decay Beta decay Gamma decay Nuclear fission Stability Protons Electrons Neutrons Mass number Atomic number Alpha particle Beta particle Half-life Radioisotope

Objectives Students will know how and why isotopes will breakdown through alpha, beta or gamma decay by taking notes, performing a close reading and completing a worksheet on nuclear decay. Mastery Level: 21/30 (70%) or better on the nuclear decay worksheet Students will know how to solve half-life problems by performing a close read, lab on radiocarbon dating and solving half-life problems using a half-life chart. Mastery Level: 7/10 (70%) or better on the half-life worksheet

Engage Show the students the YouTube video “Radiocarbon Dating” Source: https://www.youtube.com/watch?v=2io5opwhQMQ

Explore Students will perform a close read on the article titled “What is Carbon Dating” Source: http://mocomi.com/carbon-dating/

What is half-life? Explain

7.2 Half-life

Vocabulary Daughter isotope Decay curve Half-life Parent isotope Radiocarbon dating

7.2 Half-life It can be difficult to determine the ages of objects by sight alone. Radioactivity provides a method to determine age by measuring relative amounts of remaining radioactive material to stable products formed. See pages 302 - 304

7.2 Half-life Carbon dating measures the ratio of carbon-12 and carbon-14. Stable carbon-12 and radioactive carbon-14 exist naturally in a constant ratio. When an organism dies, carbon-14 stops being created and slowly decays. Carbon dating only works for organisms less than 50 000 years old. Using carbon dating, these cave paintings of horses, from France, were drawn 30 000 years ago. See pages 302 - 304

The Rate of Radioactive Decay Half-life measures the rate of radioactive decay. Half-life = time required for half of the radioactive sample to decay. The half-life for a radioactive element is a constant rate of decay. Strontium-90 has a half-life of 29 years. If you have 10 g of strontium-90 today, there will be 5.0 g remaining in 29 years. See pages 305 - 306

The Rate of Radioactive Decay Decay curves show the rate of decay for radioactive elements. The curve shows the relationship between half-life and percentage of original substance remaining. The decay curve for strontium-90 See pages 305 - 306

Common Isotope Pairs There are many radioisotopes that can be used for dating. Parent isotope = the original, radioactive material Daughter isotope = the stable product of the radioactive decay See page 307

(c) McGraw Hill Ryerson 2007 Common Isotope Pairs The rate of decay remains constant, but some elements require one step to decay while others decay over many steps before reaching a stable 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. See page 307 (c) McGraw Hill Ryerson 2007

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. See pages 307 - 308

The Potassium-40 Clock See pages 307 - 308

Summary A half-life is the length of time required for half the nuclei in a sample of a radioactive isotope to decay into its products.

Guided Practice Problem #1 The half-life of Zn-71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would be left after 7.2 minutes has elapsed?

Guided Practice Problem #2 Pd-100 has a half-life of 3.6 days. If one had 6.02 x 1023 atoms at the start, how many atoms would be present after 20.0 days?

Guided Practice Problem #3 Os-182 has a half-life of 21.5 hours. How many grams of a 10.0 gram sample would have decayed after exactly three half-lives?

Guided Practice Problem #4 After 24.0 days, 2.00 milligrams of an original 128.0 milligram sample remain. What is the half-life of the sample?

Guided Practice Problem #5 U-238 has a half-life of 4.46 x 109 years. How much U-238 should be present in a sample 2.5 x 109 years old, if 2.00 grams was present initially?

Guided Practice Problem #6 How long will it take for a 40.0 gram sample of I-131 (half-life = 8.040 days) to decay to 1/100 its original mass?

Independent Practice Problems 1) Sodium-24 has a half-life of 15 hours. How much sodium-24 will remain in an 18.0 g sample after 60 hours? 2) After 42 days a 2.0 g sample of phosphorus-32 contains only 0.25 g of the isotope. What is the half-life of phosphorus-32? 3) Polonium-214 has a relatively short half-life of 164 seconds. How many seconds would it take for 8.0 g of this isotope to decay to 0.25 g? 4) How many days does it take for 16 g of palladium-103 to decay to 1.0 g? The half-life of palladium-103 is 17 days. 5) By approximately what factor would the mass of a sample of copper-66 decrease in 51 minutes? The half-life of copper-66 is 5.10 minutes. 6) In 5.49 seconds, 1.20 g of argon-35 decay to leave only 0.15 g. What is the half-life of argon-35?