1. The half-life of a radioactive element is the time it takes for half of its atoms to decay into something else
If you have 100 grams of a substance and the half life is 2 days, then 50 grams will remain after 2 days.

2. Half-Life= the time it takes for half of the atoms in a sample to decay
The half-life for a specific isotope is the same no matter how much of it you have

3. Solving Half Life Problems
Set up a table like the following:
For each half-life elapsed:
Cut the mass remaining in half
Increase the time elapsed by an amount equal to the half-life
Add one to the number of half-lives
You don’t always have to fill out the entire table to solve the problem

4. Ex: Co-60 5.26 years 160 grams 1/2 (50%) 10.52 yrs 80 grams 1/4 (25%)
1/8
21.04 yrs
20 grams
1/16
Add one half-life’s worth of time each time
Cut the mass in half each time
Cut the fraction in half each time

5. Practice 7.646 days 3.823 days 15 grams 1/2 (50%) 7.646 days 7.5 grams
1/4 (25%)
days
3.75 grams
1/8 (12.5%)

6. Practice 2 grams 16 grams 7.13 sec 8 grams 1/2 (50%) 14.26 sec 4 grams
1/4 (25%)
21.39 sec
2 grams
1/8 (12.5%)
14 grams

7. Practice 3 half lives 0 yr 50 grams 213,000 yr 25 grams 1/2 (50%)
1/4 (25%)
639,000 yr
6.25 grams
1/8 (12.5%)

8. Practice 32 grams 0 hr 32 grams 12.36 hr 16 grams 8 grams 24.72 hr

9. Practice
10 min
0 min
80 mg
10 min
40 mg
20 min
20 mg
30 min
10 mg