Do Now: To Be Collected (10 minutes)
On a separate sheet of paper:
• 1) Write the nuclear reaction of P-32.
• 2) What is the mode of decay of Pu-239?
• 3) What is represented by X in the following:
– A.
–B
37 
K
19
220 
Fr
87
X + 18Ar37
4 + X
He
2
• 4) Find the number of protons and neutrons in
an atom of 87Fr220
• What is Nuclear
Fallout?
Uranium Decay Series:
• U-238 is unstable and decays into more
stable nuclei.
• It takes 14 decay steps until a stable, nonradioactive nucleus is finally reached.
• The daughter nuclide of one step becomes
the parent nuclide of the next:
U Decay Series
Someone might ask…
• How long does it take for it to lose it’s
radioactivity?
Next topic for Nuclear Decay:
Half-Life!!
½ Life
Half Life (1/2 Life)
• What is half life?
• Half Life: The time required for something to
fall to ½ its original value
Lets time one!
In this example, Green will decompose into red. Time how long it takes
for ½ of them to turn to red
DONE
Lets time the second half life!
Ok, so it took about 8 seconds for half of the green to turn to red. Will it
be the same the second time?
How long does it take ½ the remaining green to turn red?
DONE
Half Life (1/2 Life)
• What is half life?
• Half Life: The time required for something to
fall to ½ its original value
• **No Factor (temp, pressure, or other
elements combined with it) can affect the
length of a half-life of a radioisotope!!! It is a
fixed length of time for each radioactive
nuclei**
(for element green, it was 8 seconds)
Yo Mr. D, SO WHAT?!
• Don’t really see why half-life is important and
super cool?
• Get ready for it…..
C-14
• C-14 is a naturally occurring isotope and will
be incorporated into plants during
photosynthesis
• Comparing how much C-14 is left to how
much N-14 is present will tell us how old a
sample is!
C-14
• Example:
• C-14 has a half life (T) of 5730 years
• Lets say I have a sample that has a C-14 : N-14
ratio of 1 : 1
• How old is my sample?
• BAM! 5730 years
Twizzlium Example (do 4 Half-lives)
0
1
2
3
4
Number of Half-lives
5
6
Half-life Chart
Number of
AMOUNT LEFT
Half-lives Fraction Decimal
Percent
0
1
2
3
4
5
Different Half Life problems
• Take out HW packet and the ‘Atoms’ packet
and flip to the graphic organizer for Half-life
• The formula to use is:
• # H.L. = t / T
• t= time that has gone by
• T= length of one half life
R. Chemistry Type Problems
• Many HALF-LIFE problems seem harder than
they really are.
• Here are some helpful hints and examples.
• GENERALLY, you need to figure out something
before proceeding with the problem.
R. Chemistry Type Problems
• Table N in the Reference Tables gives the
length of the half-life of some Radioisotopes.
• You may need to look up this information
sometimes to solve the problem.
Start by making a data table:
•
•
•
•
•
t (time elapsed-longer time)
T (length of Half-life)
# half-lives (size of chart)
beginning amount (beg. am’t.)
ending amount (end am’t)
• The only TRUE unknown will be the one
asked for in the question.
• The other missing information can be
found using the half-life equation or a
chart.
Different Half Life problems
• How much of a 50 g sample of N-16 will be
left after 21.6 seconds
• 1) Determine the number of half-lives
–3
• 2)Cut the amount in half as many times as
there are half lives
– 50  25  12.5  6.25g
Different Half Life problems
• HW #3)The half-life of cesium-137 is 30. years.
How much Cs-137 was present originally if,
after 120. years, 6.0 g remained?
• 1) Determine the #HL
• 2) Double the amount as many times as there
are half lives
Different Half Life problems
• HW #1) What is the half-life of a radioactive
isotope if 25% of the original mass of the
isotope remains after 20. days?
• 1) Determine #HL by determining the #of time
you can cut the original in half till you get the
amount you have
• 2)Divide the time elapsed (t) by the #HL
Different Half Life problems
• HW #8) An ancient scroll made of papyrus is
analyzed, and it is found to contain only 25%
of the steady-state concentration of C-14
found in living organisms. How old is the
material that the scroll is made of?
• 1) Determine how many times you can cut the
original amt. in half to get your final amt.
• 2) Multiply the #HL by the duration of 1 HL
With your neighbor
• Work on the HL practice, alternating roles as
you do so.
• 15 minutes (group quiz)
Back to the focus question
• What is nuclear fall out?
U-238
• Used in nuclear bombs
• Once detonated, radioactive material is
projected into the air
• Clouds of radioactive particles being blown
across the sky
• Rainfall would take them out of the sky and
into the ground
U-238
• This would contaminate the ground water,
anything left out in the open (including living
things)
• When would the water/area be safe again?
• The half life for U-238 is 4.5 billion years
• Pretty much never again.
U-238
• Soil would need to be removed before
planting again
• Water table would have to be replaced with
fresh water before any of it would be safe
…Some serious stuff
Use the rest of this time
• To make sure you finish the Half Life
Homework assignment