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

List of Common Isotope Pairs

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