Mr. Shields
Regents Chemistry
U02 L03
Nuclear Decay Series
Uranium has an atomic number greater than
83. Therefore it is naturally radioactive.
Alpha Particle
Most abundant isotope
Thorium Decay
Of course Thorium’s atomic number is also
greater than 83. So it to is Radioactive and
Goes through beta decay.
234Pa
91
+
0e
-1
Protactinium
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
U-238 Decay Series
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
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
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.
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
Nuclear Half-life
The Half life of any nuclide is independent of:
Temperature, Pressure
or
Amount of material left
Beanium decay
What does the graph of radioactive decay look like?
64 beans
Successive half cycles
This is an EXPONENTIAL
DECAY CURVE
50%
1
32 beans
2
16 beans
3
4
8 beans
4 beans
Loss of mass due to Decay
Amount of beanium
Fraction left
Half life’s
64
1
32
½
1
16
¼
2
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 life’s
8
1/8
3
4
1/16
4
22,920/5730 = 4 Half-life’s
t0
Carbon 14 is a radionuclide used to date
Once living archeological finds
Carbon–14 Half-life = 5730 years
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
2
25
3240
3
12.5
4860
20 grams
? 10 grams
? 5 grams
4
6.25
6480
5
3.125
8100
? 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:
Half lives
16g (starting amount) 8
1
4
2
2g
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
Half life
Half life
Time zero
3
2
1
2 grams
4 grams
8 grams
16 grams
Problem:
 How many ½ life periods have passed if a
sample has decayed to 1/16 of its original
amount?
Time zero
First half life
Second half life
Third half life
Fourth half life
1x original amount
½ original amount
¼ original amount
1/8
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 2:
Step 1:
25 grams
50
100 g
200 g
400 g
4 half lifes
3 half lifes
2 half lifes
1 half life
time zero
Elapsed time = # HL
Half-life
40 years = 4 HL
Half-life
Half life = 10 years