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7.2 Half-Life
Carbon DatingThe Rate of Radioactive Decay
Using a Decay CurveCommon Isotope Pairs
The Potassium-40 Clock
Carbon Dating
• We can measure the radioactivity in plants or animals and how it has changed
• All organisms on the Earth contain carbon• Radiocarbon dating: the process of determining the
age of an object by measuring the amount of carbon-14 remaining.
• A living organisms have approximately the same ratio of carbon-12 to carbon-14
• When an organism dies the carbon-14 atoms decay over time
The Rate of Radioactive Decay
• The rate of radioactive decay can be compared using a quantity called half-life
• Half-life: the time required for half of the radioactive (unstable) material to decay into a more stable material.
• The shorter the half-life, the faster the decay• The half-life of strontium-90 is 29 years, if you have
20g of strontium-90 in 29 years you will have 10g
Using a Decay Curve
• All radioactive decay rates follow a similar pattern called a decay curve
• Decay curve: a curved line on a graph that shows the rate at which radioisotopes decay.
• The difference between different isotopes is the length of their half-life
Common Isotope Pairs
• Parent isotope: the isotope that undergoes radioactive decay
• Daughter isotope: the stable product of radioactive decay
• Each isotope can be used for radiocarbon dating• The dating range is different depending on the half-life
of the parent isotope
The Potassium-40 Clock
• Potassium-40/Argon-40 has a half life of 1.3 billion years (as shown in common isotope pairs list)
• Argon-40 is found trapped in molten rocks in the form of a gas as potassium-40 decays
• The ratio between the mass of potassium=40 and argon-40 determines the age of the rock– If there are equal masses of potassium-40 and argon-40
the rock is 1.3 billion years old(1 half-life)
Questions1. Give the definition of the following:
b. Half-lifec. Decay curved. Radiocarbon datinge. Daughter isotopef. Parent isotope
2. What is the ratio of carbon-14 to carbon-12 in a living organism?3. What happens to the ratio of carbon-14 to carbon-12 when an
organism dies?4. If you have 10g of an element with a half-life of 10 years for much of
said element would you have in 20 years5. What is the daughter isotope of Uranium-238?6. What is the half-life of potassium-40/argon-40?
Answers1. Definitions
b. Half-life: the time required for half of the radioactive (unstable) material to decay into a more stable material.
c. Decay curve: a curved line on a graph that shows the rate at which radioisotopes decay.
d. Radiocarbon dating: the process of determining the age of an object by measuring the amount of carbon-14 remaining.
e. Daughter isotope: the stable product of radioactive decayf. Parent isotope: the isotope that undergoes radioactive decay
2. There is an equal amount of carbon-12 to carbon-143. The ratio of carbon-14 to carbon-12 decreases. There becomes less carbon-
14 as it decays4. 2.5g5. Lead-2066. 1.3 billion years