Find a rock's age from radioactive decay

Grade 9 Science Worksheet - Building Confidence

Circle the best answer.

1. 1.

   What is a half-life, by Ari's reading (USGS)?

   • The time for the whole parent to decay
   • The time for half of the parent to decay
   • A time that changes with sample size
   • Half the age of the rock
2. 2.

   A rock has 100 g of a parent isotope. How much is left after one half-life, by Bo's math?

   • 25 g
   • 50 g
   • 0 g
   • 75 g
3. 3.

   Two samples of potassium-40, one big and one small. A big sample and a small sample of potassium-40 are compared. Which has the longer half-life, by Kira's reading?

   • They have the same half-life
   • The small sample
   • It changes with temperature
   • The big sample
4. 4.

   What fraction of the parent is left after three half-lives, by Jun's math?

   • 1/3
   • 1/6
   • 1/8
   • It has fully decayed
5. 5.

   After two half-lives, what fraction of the parent remains, by Luz's math?

   • It has fully decayed
   • 1/2
   • 1/3
   • 1/4

6. Read this first

   Half-lives (USGS)

   Parent	Daughter	Half-life
   uranium-238	lead-206	4.5 billion years
   uranium-235	lead-207	704 million years
   potassium-40	argon-40	1.25 billion years
   carbon-14	nitrogen-14	5,730 years

7. 6.

   What is the half-life of carbon-14, by Milo's reading?

   • 4.5 billion years
   • 1.25 billion years
   • 5,730 million years
   • 5,730 years
8. 7.

   Which clock fits a volcanic rock about 2 billion years old, by Pia's reading?

   • Any clock gives the same result
   • Counting rock layers
   • Carbon-14
   • Uranium-238 or potassium-40
9. 8.

   A sample has equal numbers of parent and daughter atoms (1:1). How many half-lives have passed, by Tala's reading?

   • Half of one
   • One
   • Zero
   • Two
10. 9.

    A sample has 1 parent atom for every 3 daughter atoms (1:3). How many half-lives have passed, by Zee's math?

    • One
    • Two
    • Three, from the 3
    • Four
11. 10.

    a sandstone layer between two volcanic ash layers. Which layer can geologists date directly to find a sedimentary layer's age, by Esme's reading (NPS)?

    • The sandstone's own sand grains, as its age
    • The fossil shells, with uranium
    • Volcanic ash layers above and below it
    • Nothing can be dated