Warm-Up 10/20 + 10/21 No lab book warm-up…instead, pick up 3 post-its off the handouts table.

Post-It Review Last time: What can rock layers and fossils tell us about what the earth used to be like? Write three answers to this question on your post-its (one per post-it) Wait for the next slide 

Fossil and Rock Layers Post-Its Get up and exchange post-its with three other people (so you have three new ones). Stop and explain your post-it before giving it away. Return to your assigned seat. At your table, group your post-its based on similarities. Be prepared to share!

Evidence For Evolution How do we know evolution happens? Fossil record a. Relative and radiometric dating DNA comparisons Body structures Comparing embryos Antibiotic resistance and Viruses

Learning Targets I can determine the mathematical age of fossils I can explain why the ratio of parent:daughter isotopes changes as a fossil ages

Paper Demonstration A half-life is how long it takes for a sample of radioactive atoms to break down by half. Pretend this paper is radioactive atoms. Time how long it takes for them to break in half. Half life = _____________ seconds.

Using Half-Life to calculate How long would it take for the paper to rip in half 10 times? (the half- life is always the same). If 50 seconds have passed, how many times has the paper been torn in half? Did the paper disappear when it torn in half?

Radiometric Dating Background An atom is the smallest unit of matter (all stuff) An isotope an atom that has a different number of neutrons (in the nucleus) Ex: Carbon 12 is normal; Carbon 13 and 14 are isotopes. Still called Carbon, though.

Radiometric Dating Background Some isotopes are unstable and tend to fall apart easily (decay) When an isotope decays, it turns into a different isotope Ex: Carbon 14 decays into Nitrogen 14

Half-Lives The unstable isotopes decay at a predictable rate A “half-life” is how much time it takes for half of the original isotope to decay into the new isotope For Carbon 14, the half-life is 5730 years Checkpoint: After 5730 years, how much of the Carbon 14 in a fossil is still C14? How much is now Nitrogen 14? (answer in %)

Which is the parent? Which is the daughter?

Clock in Rocks, part one You need: A book The Clocks in Rocks handout To do: Read EVERYTHING on pages 288-291 Complete #1 (key words and meaning of key words)

Calculating the Age of Fossils At the moment a fossil dies (time 0), the Carbon 14 it contains has not decayed at all into Nitrogen 14 (it is 100% C14, 0% N14) After one half-life, it is 50% C14 and 50% N14 After two half-lives, it is 25% C14 and 75% N14 After four half-lives, it is _____ C14 and ____ N14 See my diagram on the board

Now do #2-6 in your packet

Radioactive Dating Game! You need a cup and a M n’M (yes, one). When I say go, put your hand over your cup and shake it up and down. When I say stop, look in your cup. If the M is face-down, you have decayed. You are out of the game until the end of the trial- please sit down. You are now the “daughter” atom

Solving Radiometric Dating Problems Half-lives Time (years) Mass (g) 0 (today) 1 2 3 4 Knowing the half-life of a parent isotope and how much mass of the isotope is left, we can calculate how old it is!

Example How old are the rocks and bacteria fossils? You find evidence of bacteria that lived in old lava rocks. You measure the amounts of Uranium 238 (parent) and Lead 206 (daughter) in the rocks. The sample contains 75% parent isotopes and 25% daughter isotopes. The half-life of Uranium 238 is 4.8 billion years. How old are the rocks and bacteria fossils?