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. 19K37 X + 18Ar37

– B 87Fr220 2He4 + X

• 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, non-

radioactive 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

0 1 2 3 4 5 6

Number of Half-lives

Twizzlium Example (do 4 Half-lives)

Half-life ChartNumber of AMOUNT LEFTHalf-lives Fraction Decimal Percent

012345

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