Name: ___________________________________________
Regents Earth Science
Half Life Regents Review
1. A fossil formed 11,400 years ago. Which
percentage of the original amount of carbon14 remains in the fossil?
(1) 100%
(3) 25%
(2) 50%
(4) 12.5%
Date: ___________
Period: _____
4. A bar graph of the radioactive decay of
carbon-14 is shown below.
2. The table below shows the radioactive decay
of carbon-14. Part of the table has been left
blank.
After 22,800 years, approximately what
percentage of the original carbon-14
remains?
(1) 15%
(3) 6.25%
(2) 12.5%
(4) 3.125%
3. Radioactive decay of 40K atoms in an igneous
rock has resulted in a ratio of 25 percent 40K
atoms to 75 percent 40Ar and 40Ca atoms.
How many years old is this rock?
(1) 0.3x109 y
(3) 2.6x109 y
(2) 1.3x109 y
(4) 3.9x109 y
The solid black sections of the bars on the
graph represent the percentages of
(1) carbon-14 from the original sample
that has not decayed
(2) uranium-238 from the original sample
that has not decayed
(3) nitrogen-14 decay product resulting
from the radioactive decay
(4) lead-206 decay product resulting
from the radioactive decay
5. How old is a bone that has 12.5% of the
original amount of radioactive carbon-14
remaining?
(1) 5,700 years
(3) 17,100 years
(2) 11,400 years
(4) 22,800 year
6. The igneous intrusion contains the radioactive isotope potassium-40, which is used in radioactive
dating to determine the age of rocks. State one property of potassium-40 that allows it to be useful
in the radioactive dating of rocks.
_________________________________________________________________________
Base your answers to questions 7 and 8 on the
data table and information below and on your
knowledge of Earth science. The data table
shows the radioactive decay of carbon-14 and
the age of fossil remains, in years (y). Part of
the table has been left blank.
7. Identify the decay product when carbon-14
undergoes radioactive disintegration.
____________________________
8. The carbon-14 in the fossil remains of a
mastodont has undergone five half-lives of
radioactive decay. Calculate the age of these
fossil remains.
________________________ years
Base your answers to questions 9 through 12 on the graph below and on your knowledge of Earth science.
The graph shows the rate of decay of the radioactive isotope carbon-14 (14C).
9. Complete the flow chart below by filling in the boxes to indicate the percentage of carbon-14
remaining and the time that has passed at the end of each half-life.
10. Identify the decay product formed by the disintegration of carbon-14.
____________________________
11. Explain why carbon-14 cannot be used to accurately determine the age of organic remains that are
1,000,000 years old.
_________________________________________________________________________
_________________________________________________________________________
12. State the name of the radioactive isotope that has a half-life that is approximately the same as the
estimated time of the origin of Earth.
____________________________