Nuclear Decay Series

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Nuclear Decay Series
Isotopic Half-Lifes
Mr. Shields Regents Chemistry U02 L03
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Nuclear Decay Series
Uranium has an atomic number greater than 83.
Therefore it is naturally radioactive.
Alpha Particle
Most abundant isotope
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Thorium Decay
Of course Thoriums atomic number is also greater
than 83. So it to is Radioactive and Goes through
beta decay.
Protactinium
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U-238 Decay Series
Protactinium decays Next and so on until we reach
a stable Non-radioactive Isotope of
lead Pb-206 Atomic No. 82
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U-238 Decay Series
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Decay Series
U-238 IS NOT the only radioactive isotope
that Has a specific decay series. All
radioisotopes have specific decay paths they
follow to ultimately reach stability
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Decay Series Time Span
The next Question you might consider asking is
how long does this decay process take?
The half life of U-238 is about 4.5 billion years
which is around the age of the earth so only
about half of the uranium Initially present when
the earth formed has Decayed to date.
Which leads us into a discussion of Nuclear Half
life
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Nuclear Half-life
Unstable nuclei emit either an alpha, beta or
positron particles to try to shed mass or
improve their N/P ratio.
But can we predict when a nucleus
will Disintegrate? The answer is NO for
individual nuclei But YES if we look at large
s of atoms.
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Nuclear Half-life
Every statistically large group of
radioactive nuclei decays at a predictable
rate. This is called the half-life of the nuclide
Half life is the time it takes for half (50) of
the Radioactive nuclei to decay to the
daughter Nuclide
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Nuclear Half-life
The Half life of any nuclide is independent
of Temperature, Pressure or Amount of
material left
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Beanium decay
What does the graph of radioactive decay look
like?
This is an EXPONENTIAL DECAY CURVE
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Loss of mass due to Decay
Amount of beanium 64 32 16 8 4 Fraction left 1 ½
¼ 1/8 1/16 Half lifes 1 2 3 4
If each half life took 2 minutes then 4 half
lives would take 8 min. The equation for the No.
of half lives is equal to T (elapsed) / T (half
Life) 32 minutes / 4 minutes 8 half lifes
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22,920/5730 4 Half-lifes
t0
Carbon 14 is a radionuclide used to date Once
living archeological finds Carbon14 Half-life
5730 years

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Half-Lives

• In order to solve these half problems a table
  like
• the one below is useful.
• For instance, If we have 40 grams of an original
• sample of Ra-226 how much is left after 8100
  years?

½ life period original remaining Time Elapsed Amount left
0 100 0 40 grams
1 50 1620 yrs 20 grams
2 25 3240 ?
3 12.5 4860 ?
4 6.25 6480 ?
5 3.125 8100 ?
10 grams
5 grams
2.5 grams
1.25 grams
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Problem

• A sample of Iodine-131 had an original
• mass of 16g. How much will remain in 24
• days if the half life is 8 days?

Step 1 Half lifes T (elapsed) / T half life
24/8 3   Step 2 16g (starting amount)
8 4 2g Half lives 1 2 3
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Problem

• What is the original amount of a sample of H3 if
  after 36.8years 2.0g are left ?

Table N tells us that the half life of H-3 is
12.26 yrs. 36.8 yrs / 12.26 yrs 3 half
lives. Now lets work backward Half life 3 2
grams Half life 2 4 grams Half life 1 8
grams Time zero 16 grams
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Problem

• How many ½ life periods have passed if a sample
  has decayed to 1/16 of its original amount?

Time zero 1x original amount First half life ½
original amount Second half life ¼ original
amount Third half life 1/8 Fourth half life
1/16
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Problem

• What is the ½ life of a sample if after 40 years
  25 grams of an original 400 gram sample is left ?

Step 2 Elapsed time HL Half-life 40 years
4 HL Half-life Half life 10 years
Step 1 25 grams 4 half lifes 50 3 half
lifes 100 g 2 half lifes 200 g 1 half life 400
g time zero