1. Interpret Graphs 1 2
A half-life (t ) is the time required for one-half of the nuclei in a radioisotope sample to decay to products.
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2. Lead-212 has a half-life of about 10 hours
Lead-212 has a half-life of about 10 hours. How much lead-212 is left after 20 hours if the original sample had 8 grams? ____ grams. 2
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3. Half-Lives of Some Naturally Occurring Radioisotopes
Half-Life
Comparing Half-Lives
Half-lives can be as short as a second or as long as billions of years.
Half-Lives of Some Naturally Occurring Radioisotopes
Isotope
Half-life
Radiation emitted
Carbon-14
5.73 × 103 years b
Potassium-40
1.25 × 109 years
b, g
Radon-222
3.8 days a
Radium-226
1.6 × 103 years
a, g
Thorium-234
24.1 days
Uranium-235
7.0 × 108 years
Uranium-238
4.5 × 109 years
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4. Some with long-term half-lives are good for dating materials
Half-Life
Comparing Half-Lives
Some with long-term half-lives are good for dating materials
Some radioisotopes with short half-lives, are useful in medicine.
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5. Half-Life
Exponential Decay Function
You can use the following equation to calculate how much of an isotope will remain after a given number of half-lives.
A = A0  1 2 n
A stands for the amount remaining.
A0 stands for the initial amount.
n stands for the number of half-lives. (doesn’t necessarily have to be an integer; n = t / t1/2)
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6. The half-life of phosphorus-32 is 14. 3 days
The half-life of phosphorus-32 is 14.3 days. How many milligrams of phosphorus-32 remain after days if you begin with 2.5 mg of the radioisotope?
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7. The half-life of phosphorus-32 is 14. 3 days
The half-life of phosphorus-32 is 14.3 days. How many milligrams of phosphorus-32 remain after days if you begin with 2.5 mg of the radioisotope?
n = days × = 7 half-lives
1 half-life
14.3 days
= (2.5 mg) = 2.0 × 10–2 mg
A = A = (2.5 mg)
( ) 1 2 n 7
( )
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