1. Half-Life
7.2

2. A large unstable isotope will decay many times until it becomes a stable daughter isotope. If we take all the decays that the parent isotope undergoes, we get a decay series.

3. Radioactive decay occurs at a certain speed, or rate, for each kind of radioactive isotope.
as a sample decays, the amount of radioactive isotopes that remain become less and less
radioactive isotopes decay at a _________________ rate
constant

4. 100% 50% 25% 12.5% 0% 50% 75% 87.5% 1 50% 50%/yr half-life
1 year → year → year → year
P P P P P P P P P P
D D D D D D D D D D
P P P P P D D D D D
P P D D D D D D D
P: ______
D: ______
What is the rate of radioactive decay?
Every _____ year(s), ________ of the parent isotope decays → ___________
This is called a ______________
100%
50%
25%
12.5% 0%
50%
75%
87.5% 1
50%
50%/yr
half-life

5. half
Half-life: the amount of time it takes for _________ of the nuclei in a sample of radioactive isotope to decay.
Each half-life means that ________ of the radioactive isotope has decayed.
50%
100%
50%
25%
12.5%
6.25%

6. After 2 half lives, 25% of U-235 remains
100%
1/1
50%
1/2
25%
1/4
12.5%
1/8
6.25%
1/16
6.25% or 1/16
1420/710 = 2 half lives
After 2 half lives, 25% of U-235 remains

7. 2.6
3.9
5.2 5 6
21/7 = 3
(5)(7) = 35
90/45 = 2
(4)(45) = 180 s

8. To find the amount of parent isotope remaining after x half-lives, multiply the original amount by the % remaining.
100%
50%
100 x ½ = 50 g 58
25%
100 x ½ x ½ = 25 g 87
12.5%
100 x ½ x ½ x ½ = 12.5 g
116
6.25%
100 x ½ x ½ x ½ x ½ = 6.25 g

9. 1
50%
32 g 2
25%
16 g
116
6.25%
4 g

10. 1 half-life so 128(0.50) = 64 g
2 half-lives so 128(0.50)(0.50) = 32 g
128(0.50)(0.50)(0.5) = 16 g

11. What is the half-life of the class? _____________
How many people are remaining after 2 half-lives? _____________

12. 2.5 years
6 years
8.6 years
12.5%
20/2.5 = 8 half-lives

13. 100% 0%
1:0 1
50%
50%
1:1 2
25%
75%
1:3 3
12.5%
87.5%
1:7 4
6.25%
93.75
1:15

14. P:D = 1:7 means 3 half-lives
3 x 7 years = 21 years old
P = 100 g – 75 g = 25 g
P:D = 25 : 75
= 1 : 3
= 2 half-lives
2 x 7 years = 14 years have passed

15. Carbon-14
5730
dies

16. 25 g
50 g
(25/200)(100)=12.5%
25% 3 2
(3)(5730)=17190 yrs
11460 yrs