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Mr. Shields Regents Chemistry U02 L03
Nuclear Decay Series
Uranium has an atomic number greater than83. Therefore it is naturally radioactive.
Most abundant isotope
Alpha Particle
Thorium DecayOf course Thorium’s atomic number is alsogreater than 83. So it to is Radioactive andGoes through beta decay.
234Pa + 0e91 -1
Protactinium
U-238 Decay Series
Protactinium decaysNext and so on untilwe reach a stable Non-radioactive Isotope of lead
Pb-206 Atomic No. 82
U-238 Decay Series
Decay Series
U-238 IS NOT the only radioactive isotope thatHas a specific decay series.
All radioisotopes have specific decay paths they follow to ultimately reach stability
Decay Series Time Span
The next Question you might consider askingis how long does this decay process take?
The half life of U-238 is about 4.5 billionyears which is around the age of theearth so only about half of the uraniumInitially present when the earth formed has Decayed to date.
Which leads us into a discussion of Nuclear Half life
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 willDisintegrate?
The answer is NO for individual nuclei
But YES if we look at large #’s of atoms.
Nuclear Half-life
Every statistically large group of radioactivenuclei 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 theRadioactive nuclei to decay to the daughterNuclide
Nuclear Half-life
The Half life of any nuclide is independent of:
Temperature, Pressureor
Amount of material left
Beanium decay
64 beans
32 beans
16 beans
8 beans4 beans
Successive half cycles
1
2
34
50%
What does the graph of radioactive decay look like?
This is an EXPONENTIALDECAY CURVE
Loss of mass due to Decay
Amount of beanium 64 32 16 8 4Fraction left 1 ½ ¼ 1/8 1/16Half life’s 1 2 3 4
If each half life took 2 minutes then 4half lives would take 8 min.
The equation for the No. of half livesis equal to:
T (elapsed) / T (half Life)32 minutes / 4 minutes = 8 half life’s
Carbon 14 is a radionuclide used to date Once living archeological finds
Carbon–14 Half-life = 5730 years
22,920/5730 = 4 Half-life’s
t0
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 grams1 50 1620 yrs 20 grams2 25 3240 ?3 12.5 4860 ?4 6.25 6480 ?5 3.125 8100 ?
10 grams
5 grams
2.5 grams
1.25 grams
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 life’s = T (elapsed) / T half life = 24/8 = 3 Step 2: 16g (starting amount) 8 4 2gHalf lives 1 2 3
Problem: What is the original amount of a sample of H–
3 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 gramsHalf life 2 4 gramsHalf life 1 8 gramsTime zero 16 grams
Problem: How many ½ life periods have passed if a
sample has decayed to 1/16 of its original amount?
Time zero 1x original amountFirst half life ½ original amountSecond half life ¼ original amountThird half life 1/8Fourth half life 1/16
Problem:
What is the ½ life of a sample if after 40 years 25 grams of an original 400 gram sample is left ?
Step 1:25 grams 4 half lifes50 3 half lifes100 g 2 half lifes200 g 1 half life400 g time zero
Step 2:
Elapsed time = # HLHalf-life
40 years = 4 HLHalf-life
Half life = 10 years