ISOTOPES – RADIOACTIVE DATING

ISOTOPES – WHAT ARE THEY? Isotopes of an element have the same atomic number, but different atomic mass because they have different numbers of neutrons** - let’s review atomic nuclear material – atomic mass = protons + neutrons So Carbon 14 and Carbon 12 – explain, how that is different from the normal. Consider Oxygen with 3 isotopes – 16, 17 and 18 – explain what has happened. What is the mass?, # of protons?, and # of neutrons? have students look at periodic table to begin to connect different types of elements and their atomic weights.

Draw this in your journal: identify what makes an isotope an isotope.

HOW DO WE USE ISOTOPES IN SCIENCE? Why does it work for “carbon dating”? - carbon 12 is the stable isotope vs carbon 14 being unstable when an organism dies, there is a certain ratio of C12 and C14, but the longer it has been dead, the less C14 there is because it has not been replenished (only replenishes when living) The rate that it “decays” at is called it’s half-life rate. What does that really mean? reiterate again how it works with living organisms; this is a long learning curve in changing the misconceptions

There are many misconceptions about how this works; reiterate, # of protons determines the elements identity; # of neutrons determines the different masses of same element; In living organisms we use carbon dating - we look at the ratio of what has changed with carbon ratio from living to what is left in the remains as a way to determine when it was last alive.

Hydrogen has isotopes also;

WHAT IS AN ELEMENT'S HALF-LIFE? Each element has its own unique half- life Carbon 12 is stable, C14 = 5730 yrs Oxygen has 3 isotopes, O16, O 17, O18, O 15 = 122 sec for its half life Iodine 131 = 8 days Plutonium 238 = 88 yrs Hydrogen 3 = 12 yrs Uranium 238 = 4.47 x 10 9 yrs , so 4.5 billion years from now there will be only half of the uranium currently on the planet. So let's put that info to use now: Based on the Isotope weight what element does Iodine become; Hydrogen? review each of the isotopes; have students write in their journals

PRACTICE PROBLEM : DRAW A HALF LIFE GRAPH Using iodine 131 for example – which has a half life of 8 days, Lets start with 100%, how much would be left after 1 half life?, how much after a 2nd half life? A third? What would the graph look like? Sketch it in your journals. When iodine decays it becomes xenon 131? Explain how this happens present this version 1st, it is one of the simplest ways; the other is simply taking halves of the numbers then counting the half life; the third is on a following slide with exponents

HALF LIFE PROBLEM Problem: 228Ac has a half life of 6.13 hours. How much of a 5.0 mg sample would remain after one day? Solution: -determine the number of half lives that have elapsed. number of half lives = 1 half life/6.13 hours x 1 day x 24 hours/day number of half lives = 3.9 half lives 2. For each half life, the total amount of the isotope is reduced by half. 3. Amount remaining = Original amount x 1/2(number of half lives) Amount remaining = 5.0 mg x 2-(3.9) Amount remaining = 5.0 mg x (.067) Amount remaining = 0.33 mg Answer: After 1 day, 0.33 mg of a 5.0 mg sample of 228Ac will remain.

HOW DO WE USE THESE ELEMENTS? Current uses for Radioactive isotopes radioisotopes and uses.pdf radioactive-dating-game PHET simulation isotopes-and-atomic-mass Lab word problems review

PHET ANIMATION ANALYSIS Based on the Phet animation re: Isotopes in nature – which elements are stable ( no other isotopes) and which have more than one other isotope naturally occurring? Which isotopes are used to age living organisms? Which are best to identify age of non-living features? Why? Explain in 3 sentences how isotopes are used for aging features on earth.

Paleo Misconception probe how do seashells get to the top of Mountains in montana? Fossil fuels take ____________ to make? and are made from the recycled material of _____________ How long does it take for a fossil of a fish/ woolly to become a fossil

Fossil basics

Fossil tutorial via Plato:

Index fossil definition: A distinctive fossil, common to a particular geological period, that is used to date rocks of that period.

Relative dating vs Absolute If we are simply looking at fossils and using them to estimate the age of the rock - which type of dating is that? How could we check to be sure what the more precise age of the rock or fossil is? Can fossils and rocks also give us indicators of climate at the time? write down some ideas

Fossil formation process Make notes on your lab sheet for each of the following processes. you will need to consider these as you do the fossil walk:

PERMINERALIZATION OR PETRIFICATION Porous materials such as bones, wood, or shells get filled in with a mineral-rich water and fill the pores with a new chemical compound.

Trace Fossils show traces left in the rock by an animal, such as; 1) Tracks - animal footprints made in soft sediment that later formed solid sedimentary rock. 2) Burrows - animal trails made in soft sediment that later formed solid sedimentary rock. 3) Coprolites - Fossil dung (feces) and stomach contents.

MOLDS AND CASTS Molds: Skeleton imprints sediment and organism is later dissolved, leaving just the imprint behind Casts: Sediments fill in an already-created mold creating a rock in the shape of the mold

Unaltered remains The organism is preserved by some material- Example: Insect in amber, Wooly Mammoth frozen in ice, tar pits?

Preservation Examples: Original remains can be preserved in ice or in amber (tree sap). Both ice and amber protects the organism from decay (oxygen free environment) and from pressures that would crush the organisms. The entire animal has been preserved, even the soft parts which usually decay and disappear. Examples: (1) Woolly Mammoths preserved in ice in Alaska and Siberia. (2) Insects preserved in tree sap (amber). Cane in Jurassic Park.

REPLACEMENT Original body material is replaced atom by atom with new material Very slow allowing intricate details to be preserved. IE: Pyrite for organic material in organic walls of organisms Cretaceous Crato Formation, Brazil (Martill, 1993)

RECRYSTALLIZATION Changes the molecular structure of the original materials and are reformed as coarser crystals. Original skeletal material is reorganized into bigger crystals and the fine details of the internal structure is destroyed

CARBONIZATION Fossil is reduced to a thin sheet of carbon. Many soft parts are fossilized in this way.

Carbonization occurs when fine sediment encloses delicate matter such as leaves in a oxygen poor environment. As time passes, pressure squeezes out the liquid and gaseous components of the organism leaving behind a thin residue of carbon.

Mold and Cast often preserve a replica of a plant or animal in sedimentary rocks. an organism is buried in sediment and then dissolved by underground water leaving a hollow depression or an impression, called a mold. The mold shows only the original shape and surface markings of the organism; it does not reveal the internal structure. When minerals or sediment fills the hollow depression or impression it forms a cast.

Potential Fossil vs. Non- Fossil Look over the list of items and identify which have the potential to form fossils and what kind is most likely. Raisin Seashell Small bone Small tooth Insect wing Tree Leaf Flower Peanut Orange Seed Stick Hair

Which fossilization method is which? A. amber B. cast and mold C. freezing D. imprint E. mineralization F. petrification G. tar pit H. trace fossil 1. 2. 3. 4. 5. 6. 7. 8.

What type of conditions needed to be occurring during the life of this organism to be preserved?

Theories and Evidence Consider the following video, and outline notes re: validity, evidence, perspective

Please take notes during the video: you will be writing a position essay regarding the validity of their argument, evidence. It will be due tomorrow.