Radiometric Dating


Isotopes are atoms of an element that have the same number of protons, but a different number of neutrons in the nucleus. The forces that bind protons and neutrons together in the nucleus are usually strong. However, in some isotopes, the nuclear force is not sufficiently strong and nuclei are unstable. These radioactive isotopes decay at a constant rate and form daughter isotopes and can be used to chart prehistoric events. Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied. One such method is called carbon dating, which is limited to the dating of organic (once living) materials.



  1. There are four (4) bags represent four (4) different “fossils”.
  2. Determine how many half-lives the isotope has gone through and therefore the age of the “fossil”.
  3. (initial # of parent isotopes = # of parent isotopes + # of daughter isotopes)
    (# of parent isotopes÷ initial # of parent isotopes × 100 = _____ %)
  4. Graph the rate of decay for U-235 and U-238.
  5. (y-axis: # of parent remain)
    (x-axis: number of ahlf-life)
  6. Determine the number of half-lives by locating the % on the graphs.


"Fossil" # # of Parent Isotope # of Daughter Isotope % of Parent Isotope # of Half-Lives Age of "Fossil"
1 ??? ??? ??? ??? ???
2 ??? ??? ??? ??? ???
3 ??? ??? ??? ??? ???
4 ??? ??? ??? ??? ???


1. Rank the fossils from oldest to youngest. ???
2. Draw a picture of their relative position in a cross-section layer of soil. ???
3. alpha (α) decay:
a. plutonium–238
b. bismuth–210
c. americium–241
d. thorium–230
4. beta (β) emitters: (electron)
a. carbon–14
b. thorium–230
c. uranium–235
d. radon–222
5. Write a radioactive decay chain for the two (2) Uranium isotopes used in this lab. ???

Physics in April