Radioactivity and Half-Life
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Radioactivity
• An unstable atomic nucleus
emits a form of radiation
(alpha, beta, or gamma) to
become stable.
• In other words, the nucleus
decays into a different atom.
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Radioactivity
• Alpha Particle – Helium
nucleus
• Beta Particle – electron
• Gamma Ray – high-energy
photon
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Half-Life
• Amount of time it takes for
one half of a sample of
radioactive atoms to decay
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• Daughter
isotope
• Decay curve
• Half-life
• Parent isotope
• Radiocarbon
dating
Half-life
• It can be difficult to determine the ages of objects by sight
alone.
– Radioactivity provides a method to determine age by
measuring relative amounts of remaining radioactive
material to stable products formed.
See pages 302 - 304
Half-life
• 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.
Using carbon dating, these cave paintings of horses,
from France, were drawn 30 000 years ago.
See pages 302 - 304
• Half-life measures the rate of radioactive
decay.
– Half-life = time required for half of the
radioactive sample to decay.
– The half-life for a radioactive element is a
constant rate of 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
• Decay curves show the
rate of decay for
radioactive elements.
– The curve shows the
relationship
between half-life and
percentage of
original substance
remaining.
The decay curve for strontium-90
See pages 305 - 306
Medical Applications of Half-Life
Nuclide
Half-Life
Area of Body
I–131
8.1 days
Thyroid
Fe–59
45.1 days
Red Blood Cells
Sr–87
2.8 hours
Bones
Tc–99
6.0 hours
Heart
Na–24
14.8 hours
Circulatory System
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Half-Life Calculation #1
• You have 400 mg of a
radioisotope with a half-life
of 5 minutes. How much will
be left after 30 minutes?
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Half-Life Calculation #2
• Suppose you have a 100 mg
sample of Au-191, which has
a half-life of 3.4 hours. How
much will remain after 10.2
hours?
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Half-Life Calculation # 3
• Cobalt-60 is a radioactive
isotope used in cancer
treatment. Co-60 has a half-life
of 5 years. If a hospital starts
with a 1000 mg supply, how
many mg will need to be
purchased after 10 years to
replenish the original supply?
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Half-Life Calculation # 4
• A radioisotope has a half-life
of 1 hour. If you began with
a 100 g sample of the
element at noon, how much
remains at 3 PM? At 6 PM?
At 10 PM?
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Half-Life Calculation # 5
• How many half-lives have
passed if 255 g of Co-60
remain from a sample of
8160 g?
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Half-Life Calculation # 6
• Suppose you have a sample
containing 400 nuclei of a
radioisotope. If only 25
nuclei remain after one hour,
what is the half-life of the
isotope?
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Half-Life Calculation # 7
• If a radioactive element has
diminished by 7/8 of its
original amount in 30
seconds, what is its half-life?
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Answers to Half-Life Calculations
• Half-Life Calculation #1
– 6.25 mg
• Half-Life Calculation #2
– 12.5 mg
• Half-Life Calculation #3
– 750 mg
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Answers to Half-Life Calculations
• Half-Life Calculation #4
– 12.5 g, 1.5625 g,
0.09765625 g
• Half-Life Calculation #5
– 5 half-lives
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Answers to Half-Life Calculations
• Half-Life Calculation #6
– 15 minutes
• Half-Life Calculation #7
– 10 seconds
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