Lesson 44: Half Life
The
half life
of an element is the time it will take
half
of the parent atoms to transmutate into
something else (through alpha or beta decays, or another process).
•
This amount of time varies from just 10
22
s to 10
28
s … that's
10
21
years!
•
The total number of atoms in a sample stays the same, it's
just that some of the atoms have changed to different
elements.
Example 1
: The half life of
C
6
14
is 5730 years.
Explain
what you would expect to happen over a long
period of time.
Imagine a sample of carbon that originally had 100 of these carbon14 atoms. In reality we
would need the sample to have many more atoms, since statistics are really only reliable for
large numbers.
During the first few hundred years or so we would notice that some of the carbon14 atoms
have transmutated into some other element. In fact, a lot of them have changed. Since we
started with a lot of the carbon14 atoms, there is the greatest chance of seeing quite a few
change. It would be like throwing 100 quarters into the air; since there are so many, you've got a
really good chance of seeing a 5050 split between heads and tails when they hit the ground.
By the time 5730 years have passed, we would expect to only have 50 carbon14 atoms
remaining. Remember, the half life is the time it takes for half of them to change. There are still
the same total number of atoms, just not as many carbon14 as we started with.
Some people think that if we wait another 5730 years, all of the carbon14 will be gone.
.. nope!
Remember, half life is the time it takes for half the atoms to decay. So, after the next 5730 years
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 Fall '08
 Staff
 Radioactive Decay, HalfLife, half lives

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