1. Aim: What is a half life of an element?
Do Now: Solve the following in your notes:
                                                               
212Po He +____
142Pm +____ Nd

2. Half Life
The half-life is the amount of time it takes for half of the atoms in a sample to decay.
The half-life for a given isotope is always the same .
Number of half-lives elapsed
Fraction remaining
Percentage remaining
1/1
100 1
1/2 50 2
1/4 25 3
1/8 12 .5 4
1/16 6
.25 5
1/32
.125
1/64
.563 7
1/128
.781

3. Practice Problems
1. The half-life of Th-227 is 18.2 day. How many days are required for 0.70 of a given
sample to decay?
2. The half-life of Rn-222 is day. What was the original mass of a sample of this
isotope if g remains after day?
Try this yourself:::::
3. The half-life of Pa-234 is 6.75 hr. How much (what fraction) of a sample of this
isotope remains after hr?

4. Homework # 5 1. Write nuclear equations for the following processes:
a. Ra-226 decays by α-emission.
b. The result of the β-decay of an isotope is Mg-22.
2. Carbon-14 has a half-life of 5730 y. How much of a 144 g sample of carbon-14 will
remain after × 104y?
3. If the half-life of uranium-235 is 7.04 × 108 y and g of uranium-235 remain after
2.82 × 109 y, how much of the radioactive isotope was in the original sample?