4/27/2017 Isotopes and Half Life the light bulb is a reminder you need to do something on your notes page

Half-Life of a Radioisotope The time for the radiation level to fall (decay) to one-half its initial value Imagine a situation where you are given a chocolate cake BUT you are told you can only eat ½ of the cake each day. We would say the cake has a ½ life of 1 day. Day 1 dish has 100% cake Day 2 50% cake and 50% empty dish Day 3 25% cake and 75% empty dish Day 4 12.5% cake and 87.5% empty dish Slide 1

Half-Life of a Radioisotope Radioactivity refers to the particles which are emitted from nuclei as a result of nuclear instability. Because the nucleus experiences the intense conflict between the two strongest forces in nature, it should not be surprising that there are many nuclear isotopes which are unstable and emit some kind of radiation. The most common types of radiation are called alpha, beta, and gamma radiation, but there are several other varieties of radioactive decay. Slide 2

Half-Life of a Radioisotope Daughter nucleus Parent nucleus If you had 8 g of this parent nucleus and it has a half life of 1 million years, how long would it take to produce 6 g of the daughter nucleus? Answer = 2 million years Day 1 8 g parent 1 million years later 4 g parent 4 g daughter 2 million years later 2 g parent 6 g daughter Slide 3

Determining the Half-Life of Pennies Activity The rate at which a radioisotope decays can be simulated by tossing pennies and letting them land randomly. In this lab you will use the tossing of pennies to generate a radioisotope DECAY CURVE. Procedure: 1. On the back of your graph paper, make a table that looks like the following. You will need more than 5 shakes. 2. Place 100 pennies into the dish and turn so that all the heads are facing up. 3. In your table, complete the first row. Parent element = heads up = 100 Daughter element = heads down (tails up) = 0 4. Put the lid on the dish and shake vigorously five times. Remove lid and count number of pennies with heads up (parent element) and heads down (daughter element). Record this in the second row of your table. 5. Remove the pennies that are heads down (tails) from the dish. Now you have only parent elements again (heads up) 6. Repeat steps 4 and 5 until you have no pennies left in your dish. Slide 4

Half-Life of a Radioisotope 4/27/2017 Half-Life of a Radioisotope The time for the radiation level to fall (decay) to one-half its initial value decay curve 8 mg 4 mg 2 mg 1 mg initial 1 half-life 2 3 Slide 5

Radiometric dating The oldest known tree on earth is a bristlecone pine tree growing in the California White Mountains = 4780 years old How do we determine how old this tree is? Slide 6

Using Radioisotopes to Determine the Age of a Rock Slide 7

Using Radioisotopes to Determine the Age of a Fossil What radioactive element would you use to determine the age of anything that was once alive (organic)? Slide 8

Common Isotope Pairs This table can be found in your data booklet There are many radioisotopes that can be used for dating. 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. This table can be found in your data booklet Slide 9

Radiocarbon Dating All living organisms on the planet have Carbon atoms - because they’re organic. Carbon dating measures the ratio of carbon-12 to carbon-14. Stable carbon-12 and radioactive carbon-14 exist naturally in a constant ratio. Carbon-14 is formed at a nearly constant rate in the upper atmosphere by the bombardment of nitrogen-14 with neutrons from cosmic radiation. The carbon-14 is eventually incorporated into atmospheric carbon dioxide. When an organism dies, carbon-14 stops being created and slowly decays. The ratio of C-14:C-12 decreases with time We use this to carbon date. Use the graph - What is the half life of C-14? A: 5730 years Slide 10

Carbon’s Half-life Carbon dating only works for organisms less than 50 000 years old. Why? What carbon’s half life? The half-life for carbon-14 is 5,730 years. Using carbon dating, these cave paintings of horses, from France, were drawn 30, 000 years ago. Slide 11

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, How many grams will you have in 29yrs? ___5 g_____________ Decay curves show the rate of decay for radioactive elements. What percentage of Strontium-90 will be left after 2 half lives? A = 25% Slide 12

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 K-40 : Ar-40 shows age of rock. Slide 13

The Potassium-40 Clock What percentage of potassium-40 Is left after 3.9 billion years? A = 12.5 % If you had 100 g of Potassium-40, how many g of Argon-40 would be produced after 5.2 billion years? A: 93.75 g Slide 14

Potassium-40 Clock What is it?: A tool to determine how old the earth is Looks at molten rock and analyzes how much K-40 is inside compared to Ar-40 When rock is produced from magma, all gasses are driven out; including the noble gas Ar-40 At this point the potassium radioisotope clock is set to ZERO! After long periods of time, the K-40 (PARENT ISOTOPE) in this rock is converted to Ar-40 (DAUGHTER ISOTOPE) through radioactive decay Slide 15

Potassium-40 Clock Notice the percent of each Potassium isotope in a banana. Would you use this isotope to determine the age of this banana? Slide 16

Uranium-235 decay Use the graph to determine the half life of U-235 A = 710 million years Slide 17