ABSOLUT DATING

Explanations collected from three online presentations

• For example: – If a rock has a parent/daughter ratio of 1:3 , the

remaining parent proportion is 25% – 25% = 2 half lives

Determining Age

– If half life is 57 milliion years then the rock is 57 million years x 2 =

114 million years old

Half-LifeElement

Uranium-238 4.5 x 10 years9

Carbon-14 5730 years

Bismuth-210 5.0 days

Polonium-214 1.6 x 10 sec- 4

Radioactive Half-LifeThe time it takes for one-half

of a radioactive sample to decay

Look at factors of 2 One half-life (1/2)Two half-lives (1/4)

Three half-lives (1/8)For Example: A material has decreased by ¼ of its original amount it has

gone through two half-lives

N-14

N-1477

N-14

N-14

C-14

C-14

C-1486

C-14

CO214

Carbon-14 is a radioactive isotope that is naturally incorporated from

carbon dioxide into living organisms, the amount remains

relatively constant during the life of the organism

When the living organisms dies the carbon 14 is no longer being replaced in the organism and

will start to decay. The amount of loss from the that compared to living organisms can be used

to determine when the organism died.

22,920 years ago

17,190 years ago

11,460 years ago

5730 years ago

Present

Calculate Age

Problem:The carbon-14 radioactivity in

the bones of a body was measured to be 1/8 of that compared to a living person

How long ago did the person live?

Calculate Age

Calculation of Age:The carbon-14 has decreased by 1/8 which is three half lives (1/2

times 1/2 times 1/2 = 1/8)

Carbon-14 half life = 5730 years

3 times 5730 = 17,190 years

Present

One Half-Life5730 years ago

Two Half-Lives11,460 years ago

Three Half-Lives17,190 years ago

Radioactive Decay

• Radio Isotope is an isotope that undergoes radioactive decay. It naturally breaks down into a different element called the decay product.

• Half –life: the time it takes for ½ of the original amount of atoms to decay to the decay product.– Note: Each element decays to a different decay product

Each radioactive isotope has a specific decay product and rate of decay (half-life).

See page one of the reference table

The half-life of a radioactive nuclide (atom) is the amount of time it takes for half of that nuclide to decay into the decay product.

The half-life of Carbon-14 is 5730 years

After 5730 years, ½ the mass of an original sample of Carbon-14 remains unchanged.

After another 5730 years, ¼ (half of the half) of an original sample of Carbon-14 remains unchanged.

The half-life of a radioactive nuclide cannot be changed.

Half- Life

Determining how much of a radioactive isotopes remains unchanged after a period of time.

• Determine how many half-lives have gone by (Time/half-life)

• Halve the mass of the starting material for each half-life period that goes by.– How much of a 20.g sample of 14C remains

unchanged after 17,100 years?– The half-life period is 5,700 yrs. So 17,100 years is 3

half-lives (17,100/5,700). Half the mass three times.

5,700 yrs

5,700 yrs

5,700 yrs

20 g 10 g 5 g 2.5 gEach arrow represents one half-life

Absolute Age

• The ratio between the radioactive element and the decay product is the decay-product ratio.

• Using the decay product ratio, a scientist can determine the products absolute age by calculating the number of half lives that have past.

Regents Question #23 August 2008

1 (the whole) 1/2 1/4

Each arrow represents one half-life

With each half-life ½ of the previous amount decays, so that after two half-lives ¼ of the original amount remains

Regents January 2010

Selecting the best Radioactive Element

• The scientists must choose the best element to use for dating:– Carbon-14 is common in living organisms but has a

short half live and is not useful for samples older than 50,000 years. Few atoms will be left after 10 half lives

– U-238 has a half life of 4.5 billion years. Useful for very old samples. But samples too young may not have enough Lead-206 to measure