Atomic Clock SlidesAtomic Clock The Science of Carbon-14 Dating The Carbon Connection Carbon-12 The "normal" carbon. 6 protons, 6 neutrons. It is stable and makes up 99% of the carbon in your body. Carbon-14 The "unstable" carbon. 6 protons, 8 neutrons. It is radioactive and rare, but it's the key to our clock. "All living things are made of carbon. As long as you're eating and breathing, you're taking in both C-12 and C-14." Ratio stays constant while alive When Life Stops... 1 Organism stops taking in new carbon. 2 Carbon-14 begins to decay into Nitrogen-14. 3 The amount of Carbon-12 remains exactly the same. By comparing the ratio of C-14 to C-12, scientists can calculate how long ago the organism died. The Half-Life A half-life is the time it takes for exactly half of a radioactive sample to decay. 5,730 Years The half-life of Carbon-14 0 Half-lives = 100% C-14 1 Half-life = 50% C-14 2 Half-lives = 25% C-14 50% Gone The Rules of the Game Must be Organic It only works on things that were once alive (wood, bone, shells, cloth). It doesn't work on rocks or metals! The Age Limit After about 50,000 years, there isn't enough C-14 left to measure accurately. For dinosaur bones, we need different isotopes! Discussion: Why can't we use Carbon dating for a plastic bottle or a gold coin?
Dating Decoded Teacher GuideDating Decoded Teacher Answer Key & Facilitation Guide Lesson: Atomic Clock Grade 9 Science Key Value 5,730 Years Half-life of C-14 Effective Range Up to 50k Years Max detection limit Requirement Organic Matter Must be formerly alive Part 1: Atomic Basics 1. Isotopes: Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. C-14 has 8 neutrons, while C-12 has 6. 2. The Change: Living things consume carbon through food/photosynthesis, maintaining a constant ratio. Once dead, they stop taking in carbon. C-14 decays, but stable C-12 remains, causing the ratio to change over time. Part 2: Half-Life Table Half-Lives% RemainingTotal Years150%5,730225%11,460312.5%17,19046.25%22,920 Part 3: Scenarios Scenario A: 25% remains, which equals 2 half-lives. Age = 11,460 years. Scenario B: Younger than 5,730 years. Since 75% is greater than 50% (one half-life), the specimen has not yet reached its first half-life. Part 4: Critical Thinking The T-Rex Fossil: No. C-14 dating only works for objects up to ~50,000 years old. After 65 million years, there is zero detectable C-14 left. Scientists use other isotopes like Uranium-238 (half-life of 4.5 billion years) to date dinosaur-era rocks. Misconception Alert Students often think "Carbon dating" works on everything old. Emphasize that it cannot date metal swords, gold coins, or stone tools unless organic matter (like a wooden handle) is attached. It measures when something died, not when it was made.
Carbon Chronology WorksheetCarbon Chronology Investigating the Radioactive Record Name: Date: Part 1: The Atomic Basics 1. Define an isotope in your own words. How does Carbon-14 differ from Carbon-12 at a subatomic level? 2. Why does the ratio of Carbon-14 to Carbon-12 remain constant in a living tree, but begin to change once the tree is cut down? Part 2: Calculating the Clock The half-life of Carbon-14 is approximately 5,730 years. Complete the table below. Number of Half-LivesPercentage of C-14 RemainingTotal Years Elapsed0100%0 years12312.5%422,920 years Part 3: Solving the Mystery Scenario A: The Ancient Campfire Archeologists find charcoal from an ancient fire pit. Lab tests show that it contains only 25% of the Carbon-14 found in living wood. How many half-lives have passed, and how old is the campfire? Scenario B: The Ice Mummy A preserved body is found in a glacier. It contains 75% of its original Carbon-14. Is the mummy older or younger than one half-life (5,730 years)? Explain your reasoning. Part 4: The Carbon Limit Could a paleontologist use Carbon-14 dating to determine the age of a Tyrannosaurus Rex fossil that is 65 million years old? Why or why not?
