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: 0 28 56 74 112 140 168

Half

lives: 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 life¼ after 2 half lives1/8 after 3 half lives1/16 after 4 half lives1/32 after 5 half lives1/64 after 6 half lives

If you have .75 g of Cr-51 remaining after 168 days, how

much was in the original sample?

168 140 112 74 56 28 0Days

Grams .75 1.5 3 6 12 24 48