Activity 7.2 – Half Lives
Ensure you have the ½ lives animation open while answering the questions
below. Type your answers directly below each question. Follow the instructions
carefully, then answer the questions.
Purpose:
To use an animation of different radioactive isotopes to learn about radioactive
decay.
Materials:
-
½ lives animation
graph paper or Microsoft excel
Instructions:
1) Click on Carbon 14. Slide the slider on the bottom across to carbon 14.
What is one ½ life of carbon 14 equal to? What fraction of Carbon 14 has
not decayed at this point? What is the ratio of parent to daughter product
at this point? 5730 years, ½, 1:1
2) Carbon – 14 is used to date items that were once living like fossils. If
a fossil was found that was about 25% parent and 75 % daughter (or a 1:3
ratio), how old was this fossil? About 11000 years old
3) Click on iodine – 131. Slide the slider across to one ½ life.
How does the ½ life of iodine – 131 compare to carbon -14? What is one
½ life of iodine - 131 equal to? Much shorter, 8.07 days
4) Slide the slider all the way over to 4 half lives. Note that the amount of
original sample (the parent) goes down by ½ each time. Complete the
blanks in the table below for iodine – 131, use the animation to help.
# of ½ lives
Time
% Parent
% Daughter
Parent:Daughter Ratio
0
1
2
3
4
0
8 days
16 days
24 days
32 days
100
50
25
12.5
6.25
0
50
75
87.5
93.75
1:1
1:3
1:7
1:15
5) You will now graph some of the data from your completed table.
***You will want to check with your teacher that your table is correct before
graphing****
On a full piece of graph paper, or using a graphing program if you wish,
follow these instructions:
a. On the x axis (horizontal) you will graph the number of days
b. On the y axis (vertical) you will have percentage from 0 to 100
c. First plot the % parent from the third column of your table. Draw a
smooth, best fit curve (DO NOT connect the dots). Next plot the %
daughter from the fourth column of your table. Draw a smooth,
best fit curve (DO NOT connect the dots). Use a different colour to
draw this line.
d. Upload this graph with this activity.
-
TIPS FOR GRAPHING:
ensure you have an overall title and titles and units for your axis.
Each tick mark or square must be of an equal value.
Your graph should be at least ½ page big
Questions:
1) What is a ½ life? The time it takes for ½ of a radioactive isotope to decay
into its daughter product
2) Why would iodine – 131 and nitrogen -16 NOT be useful for dating really
old objects? ½ lives very short
3) You want to find the age of a really old rock. Why would carbon-14 not be
useful? Carbon-14 only useful for things that were once living.
Conclusion:
In one or two well-written paragraphs written in the third person, write a
conclusion. Discuss the major results of this activity, the science behind it, what
you have learned and any difficulties in this activity.
DO NOT:
- simply repeat the procedures,
- use I, we, names etc
Third Person Writing: for example, instead of saying "I learned in this lab," try
saying "This lab demonstrated..." Instead of saying "I think ½ lives are used for..."
say "1/2 lives are used for...”