Radioactive Dating
A . HALF LIVES.
One half life represents a period of time where half of a parent isotope decays into a daughter isotope. Remember, subsequent half-lives are based on the amount of parent isotope remaining after the PREVIOUS half-life. Since we start with 100%, you will have three significant figures per answer. Fill the values into your table, then plot your results in the graph below the table. half lives
% parent
% daughter
0 1 2
100
0
3 4 5 6 7 8 9
B. TYPES OF DECAY.
There are three types of radioactive decay: alpha(

), beta(

), and gamma(

). They are listed in order of increasing particle penetration.

decay occurs when a radioactive isotope emits a helium nucleus consisting of two protons and two neutrons. There are two types of beta decay.

+ decay occurs when a proton becomes a neutron and emits a positron(positively charged electron).

decay occurs when a neutron becomes a proton and emits an electron.
 decay usually occurs after an

or
 decay event has left the nucleus in an excited state. When the nucleus falls to a lower energy state it releases a gamma ray(photon). Using your periodic table, write the following decay reactions.
(

decay)
(
 + decay)

(

- decay)
C . DECAY TIME : GEOLOGIC TIME . The following are real world examples of isotopes scientists use to date findings/discoveries.
The number following the element is its atomic mass. The atomic mass is listed because a variety of isotopes of the following elements exist in nature…some are stable and some are not. Use this information and your chart to answer the following questions:
1.The parent isotope Carbon-14 has a half life of 5,715 years. Its daughter isotope is Nitrogen-14. a. What type of decay does Carbon-14 undergo? b. How old is a sample if 87.5% of the Carbon-14 has decayed? c. How old is a sample if 3.12% of the Carbon-14 remains?
2. The parent isotope Thorium-230 has a half life of 75,000 years. Its daughter isotope is Radium-226. a. What type of decay does Thorium-230 undergo? b. How much time has passed if 50% of the Thorium-230 has become Radium-226? c. How old is a sample that has 98.44% Radium-226?
3. The parent isotope Potassium-40 has a half life of 1.3 billion years. It has two daughter isotopes Calcium-40 and Argon-40. a. What type of decay does Potassium-40 undergo in each of its possible decay events? b. How old is a sample if 99.22% of the Potassium-40 has decayed? c. How much time has passed if the ratio of K-40 : Ca-40 : Ar-40 is 6.25% : 84.4% : 9.35% respectively?
4. The parent isotope Uranium-238 has a half life of 4.5 billion years. Its daughter isotope is Thorium-234. a. What type of decay does Uranium-238 undergo? b. How much time has passed if only .09766% of the Uranium-238 remains? c. How old is a sample that has 96.88% Thorium-234?
D . WORD PROBLEMS - Using what you have learned, attempt to solve these problems. I will award you 25 participation points per correct answer. Of course you must show your work! Treatment of significant figures is a must.
1.The half-life of Zn-71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would be left after 7.2 minutes has elapsed?
2.You have found a human buried in ice. Samples of tissue show that 65.5% of the Carbon-14 remains. What year did this human die?
3.Pd-100 has a half-life of 3.6 days. If one had 6.02 x 10 23 atoms at the start, how many atoms would be present after 20.0 days?
4.You discover a meteorite near your house with 75% of its Thorium-230 decayed. How long has this meteor been floating in space?
5.Os-182 has a half-life of 21.5 hours. How many grams of a 10.0 gram sample remains after 3 days?
6.A strange radioactive rock in your geology class has 3.125% of its Potassium-40 left. How old is this rock?
7.After 24.0 days, 2.00 milligrams of an original 128.0 milligram sample remain. What is the half-life of the sample?
8.After alien abduction, doctors find an instrument in your abdomen with only .195% of its U-238 remaining. How old is the tool?
9.
100.0 grams of an isotope with a half-life of 36.0 hours is present at time zero. When 5.00 grams remains, how much time passed?
10.While playing in a bog behind your house, you unearth a tree stump, 37.5% of its C-14 remains. What year did this tree die?
11.
How much time will be required for a sample of H-3 to lose 75% of its radioactivity? The half-life of tritium is 12.26 years.
12.
The half life of iodine-131 is 8.040 days. What percentage of an iodine-131 sample will remain after 40.2 days?