Half Life
The TIME it takes for HALF of the atoms in a
sample to decay into a daughter product
Half lives are given in Table N:
What is the half life of C-14 ?
U-238 ?
Au-198 ?
Half Life
The shorter the half life, the more unstable
the element is, and the faster it decays
The longer the half life, the more stable an
element is, and the slower it decays
Half Life
=
8.07 days
Half life problems
Cr-51 is unstable.
If you have 52 grams of Cr-51 in July 1, how
many grams will you have on July 28?
Half life problems
Answer:
First look up the half life in Table N =
28 days
Since the time it takes for HALF of the
atoms to decay is 28 days, on July 28 only
HALF of the atoms will be left.
Half of 52 grams is 26 grams
Cr-51 half life = 28 days
How much of the 52 grams will be remaining after
168 days?
Day:
Half
lives:
0
28
56
74
112
140
168
0
1
2
3
4
5
6
grams:
52
26
13
6.5
3.25 1.63
.815
If you had 52g to start, and ended
up with .813g after 168 days, what
fraction of the original amount
remains?
168 days = 6 half lives
½ remains after 1 half life
¼
after 2 half lives
1/8
after 3 half lives
1/16
after 4 half lives
1/32
after 5 half lives
1/64
after 6 half lives
If you have .75 g of Cr-51
remaining after 168 days, how
much was in the original sample?
Days
168
140
112
74
Grams
.75
1.5
3
6
56
12
28
0
24
48