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#1) Thorium-232 has a half-life of 14 billion years - If there are 4.53 grams of Thorium-232 in a rock...

#1) Thorium-232 has a half-life of 14 billion years
- If there are 4.53 grams of Thorium-232 in a rock sample, how much would be left 14 billion years from now?
- How many grams would you have left after 28 billion years?
#2) A sample starts with 600 grams and has a half life 20 years. How old is the rock if there are 18.75 grams left?
#3) Explain what “half-life” means in your own words.
#4) Which statement best describes relative dating?
A. Finding the exact age of a rock in years
B. Comparing rock layers to see which one is older or younger
C. Measuring the amount of carbon in a fossil
D. Using radioactive decay to find the age
#5) Which method gives the exact age of a rock in years?
A. Relative dating
B. Fossil comparison
C. Radiometric (absolute) dating
D. Layer stacking
#6) How does absolute dating differ from relative dating?

Answer

  1. After 14 billion years, half of the Thorium-232 would remain. Therefore, 2.265 grams (half of 4.53 grams) would be left.

  2. After 28 billion years, another half-life would pass, leaving half of 2.265 grams, which is 1.1325 grams.

  3. The rock is 60 years old. This is calculated by determining how many half-lives have passed to reduce 600 grams to 18.75 grams. 600 to 300 (1 half-life), 300 to 150 (2 half-lives), 150 to 75 (3 half-lives), 75 to 37.5 (4 half-lives), 37.5 to 18.75 (5 half-lives). 5 half-lives × 20 years per half-life = 100 years.

  4. “Half-life” is the time required for half of the radioactive atoms in a sample to decay.

  5. B. Comparing rock layers to see which one is older or younger

  6. C. Radiometric (absolute) dating

  7. Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events.

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