14.4: Activity 14C  Tcharts, Decay Curves, and Halflives
 Page ID
 14848
Helpful tips to calculate a halflife:
 Draw or insert a Tchart.
 In the Tchart, label the left side with time units (number of halflives, years, ka, Ma, Ga, etc.)
 Label right side with mass units (percentage, atoms, gram, kilograms, etc.)
 Begin by ALWAYS writing zero in the first spot of the time column.
 In the mass column, begin with the given mass if provided. Recall that this is 100% of the original parent isotope.
 If the mass is the starting mass, keep dividing the number in the mass column by 2 for each halflife in the left column.
 If you are given a final mass, record this at the very bottom of the mass column.
 If the mass given is the final mass, multiply that mass by 2 until the initial time (Time 0) is reached.
 The final mass amount at the bottom of the mass column equals how much mass is left after radioactive decay has occurred.
 In the time column, add one halflife at a time until you reach the total time given in problem. The number of halflives elapsed is equal to the number of times you added a halflife in the time column.
1. Calculate the amount of parent isotope remaining for all the given halflives in this Tchart.
Number of Halflives 
Amount of Parent Isotope Remaining (%) 

0 
100 
1 

2 

3 

4 

5 

6 

7 

8 
2. Plot your findings from the Tchart on the graph below.
3. Draw the decay curve by connecting your plotted data points.
Using the above graph (Figure 14.25), answer the following questions.
4. How much of the parent isotope would be remaining after 7 halflives have passed?
a. 6.25%
b. 1.56%
c. 0.78%
d. 0.39%
5. If a radiometric element has a halflife of 425 years, how old would a rock be that only had 3.125% of the parent isotope remaining?
a. 2125 years
b. 1700 years
c. 2550 years
d. 3400 years
6. Approximately how much of the parent isotope would be remaining after 3.5 halflives?
a. 16%
b. 12%
c. 4%
d. 8%
7. Based on your graph above, approximately how many halflives have passed when only 35% of the parent isotope is remaining?
a. 0.75
b. 1.5
c. 2.1
d. 2.5
Attributions
 Table 14.4: “TChart” (CCBY 4.0; Chloe Branciforte, own work)
 Figure 14.25: “Halflife Curve Graph” (CCBY 4.0; Chloe Branciforte, own work)