Transcript HALF-LIFE - Mr. Penton

Chapter 7.2 – BC Science 10
 The time required for half ½ the nuclei in a sample of
a radioactive isotope to decay
 The half life for any radioactive isotope is a constant
for any radioactive isotope.
 Each radioisotope decays at a unique rate
50% of sample (4 pennies) are still radioactive
25% of sample (2 pennies) are still radioactive
12.5% of sample (1 penny) still radioactive
No more pennies are radioactive.
Think about a sample with millions of atoms…
 It can be difficult to determine the ages of objects by sight
alone.
 Radioactivity provides a method to determine age.
 Measure the relative quantities of
 remaining radioactive material
 stable products formed.
See pages 302 - 304
(c) McGraw Hill Ryerson 2007
 Carbon dating measures the ratio of carbon-12 and
carbon-14.
 Stable carbon-12 and radioactive carbon-14 exist naturally in
a constant ratio.
 When an organism dies, carbon-14 stops being created and
slowly decays.
 Carbon dating only works for organisms less than 50 000 years
old.
.
See pages 302 - 304
(c) McGraw Hill Ryerson 2007
Living things replace the carbon-12 and carbon-14 in their bodies
when they are alive.
However when living things die, they do not replace the carbon -14
as carbon-14 decays into nitrogen-14
Carbon-14 takes 5715 years to decay half of its nuclei into nitrogen 14
Carbon-12 remains unchanged.
Therefore something that has been alive within the past 50000
years contains enough nuclei of remaining carbon-14 to
measure.
The ratio of carbon-12 and carbon-14 can estimate the age of the
sample
This picture shows a skeleton and a model for C -14 decay. The arrows
represent the amount of C-14 giving off it's radiation as time passes.
Notice the amount goes down by half for every half life.
 All decay curves for any radioactive element look the
same except for the length of the half life.
 Half-life measures the rate of radioactive decay.
 Half-life = time required for half of the radioactive sample
to decay.
 Strontium-90 has a half-life of 29 years. If you have 10 g of
strontium-90 today, there will be 5.0 g remaining in 29
years.
See pages 305 - 306
(c) McGraw Hill Ryerson 2007
Decay curves show the rate of decay
for radioactive elements.
The curve shows the
relationship between halflife and percentage of the
original substance
remaining.
The decay curve for strontium-90
 Parent isotope = the original, radioactive material
 Daughter isotope = the stable product of the radioactive
decay
 The rate of decay remains constant, but some elements
require one step to decay while others decay over many
steps before reaching a stable daughter isotope.
See page 307
(c) McGraw Hill Ryerson 2007
 There are many radioisotopes that can be used for
dating.
 Carbon-14 decays into nitrogen-14 in one step.
 Uranium-235 decays into lead-207 in 15 steps.
 Thorium-235 decays into lead-208 in 10 steps.
See page 307
(c) McGraw Hill Ryerson 2007
 Radioisotopes with very long
half-lives can help determine
the age of very old things.
 The potassium-40/argon-40
clock has a
half-life of 1.3 billion years.
 Argon-40 produced by the
decay of potassium-40
becomes trapped in rock.
 Ratio of potassium-40 :
argon-40
shows age of rock.
See pages 307 - 308
See pages 307 - 308
1. If there are 50 grams of U-238 on day
zero of radioactive decay, how much
will there be after 4.5 billion years (1
Half Life)?
A) 0.0 grams
B) 10 grams
C) 25 grams
D) 50 grams
Half-Life
% Left
Mass
Time
0
1
2
100.000%
50.000%
25.000%
50
25
12.5
4.5 BY
3
12.500%
6.25
4
6.250%
3.25
5
3.125%
Based on the graph, 2 half-lives equals
A) 4.5 billion years.
B) 9 billion years.
C) 12.5 billion years.
D) 18 billion years.
Half-Life
% Left
Mass
Time
0
100.000%
50
1
50.000%
25
4.5 BY
2
25.000%
12.5
9 BY
3
12.500%
6.25
4
6.250%
3.25
5
3.125%
Use the chart to determine the half-life of
Carbon-14.
A) 5,000 years
B) 5700 years
C) 10,000 years
D) 11,400 years
 1. How long will
it take 200 grams of
Plutonium 239 (half life 24,400 years)
to decay to 25 grams?
 1. How long will it take 200 grams of
Plutonium 239 (half life 24,400 years) to
decay to 25 grams?
Half-Life
% Left
Mass
Time
0
100.000%
200 g
0 Years
1
50.000%
100 g
24,400 Y
2
25.000%
50 g
48,800 Y
3
12.500%
25 g
73200 Y
4
6.250%
5
3.125%
 2. How many grams of
iodine 131 (half
life 8 days) would be left after 24 days
if you start with 25 grams?
 1. How many grams of iodine 131 (half life
8 days) would be left after 24 days if you
start with 25 grams?
Half-Life
% Left
Mass
Time
0
100.000%
25 g
0 Days
1
50.000%
12.5 g
8 Days
2
25.000%
6.25 g
16 Days
3
12.500%
3.125 g
24 Days
4
6.250%
5
3.125%
A half-life is the
length of time
required for half the
nuclei in a sample of
a radioactive isotope
to decay into its
products.
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