The fossil below is that of a Coelophysis and was found in upstate N.Y.

Approximately how old is the rock?

Scientists made the geologic time scale using fossils and call it a relative time

scale, they can give a range.

Radioactive Decay: Scientists have devised a way of using the decay rate

of Radioactive Isotopes (Atoms).

Atoms are supposed to have the same number of protons and neutrons

No Big deal. Eventually it breaks down into its normal stable self

Carbon 14 is unstable (parent), while Nitrogen 14(daughter) is stable.

Carbon 14 is unstable: It has an uneven number of protons

and electrons

Once the original element changes it can never go back. Whatever is lost by the

“parent” is gained by the “daughter”

This breakdown is predictable, and is measured in half life which is the time required for ½ (50%) the radioactive element to change to the decay

product.

If no time has passed that means you have 100% of your original element and 0% new element.

If ONE half life has passed that means you have 50% original and

?

•

After every HALF LIFE, you cut the original product in HALF.

So after one half life you have 50/50

How much of the original is left after TWO half lives?

Keep cutting the original in half.

Number of Half lives

Original Product Decay Product

0 100% 0%

1 50% 50%

2 25% 75%

3 12.5% 87.5%

4 6.25% 93.75%

Unstable Stable

You can tell how old something is by counting how many parent and

daughter isotopes you have.

For example equal amounts of C 14 and N 14 means it has gone through 1

half life. 5,700 years.

How many years is 2 half lives?

How much C14 is left?

Half Lives

Original remaining # of years

0

1

2

3

If 100 grams of pure C 14 starts to decay, how much N 14 will be

created in 11,400 years? How many ½ lives is that?

A 200 gram rock sample contains 25 grams of K 40, what is the age of

this rock?

A rock sample contains 75% Argon and 25% Potassium. How many half lives have

passed, and how long did it take?

After three half-lives, how much of the original sample of uranium-238 remains?

If an original sample of radioactive uranium-238 had a mass of 400 grams, what is the total amount of the uranium-238 sample that would be left after 9 billion years?

If the half-life of the radioactive material is 1,000 years, what is the age of the sample represented by the diagram?

Draw what you think the box will

look like after 3 half lives