Dating Rocks and Remains

Radioactive Decay:

Vocabulary• Half-Life: The amount of time it takes for

half of a radioactive isotope to decay.

• Radioactive Decay: The breakdown of unstable molecules into stable molecules.

• Parent Isotope: The initial, unstable element.

• Daughter Isotope (Product): The stable element an unstable isotope decays into.

Radioactive Decay

• Having a different number of neutrons than protons in the nucleus of an atom makes it unstable.

• The nucleus begins to break apart, giving off energy, in an effort to lose the extra neutron.

• Each isotope decays at its own rate, called its half life.

An isotope is an element with a different number of neutrons then protons in its nucleus.

Half-LifeHalf-Life: The amount of time it takes for half of a radioactive substance to decay into a stable form or daughter elements.

Each radioactive element has its own half-life. [See your ESRT cover for half lives.]

Absolute DatingThe method of obtaining an exact date for a geologic sample is called Radio-Metric Dating.

The idea is that radioactive elements decay in a predicable way, at a predicable rate. That is to say, it decays at the same rate REGARDLESS of any outside factors.

A ratio of the amount of original radioactive material, or Parent Element, to the amount of decayed remains or Daughter Elements, is used to determine how long something has been undergoing decay.

At the end of one half-life, half of the mass of the original radioactive isotope will remain.

Example: 64 grams of Ur238 to start• Undergoes 1 half-life or 4.5 X 109 yrs passes

• 32 grams of Ur 238 would be left.• After 2 half-lives, only 16 grams would remain.

Basically, for each half-life multiply the original mass by .5 for each of the half-lives that have passed.• After 3 half-lives only 8 grams would remain.• After 4 half-lives only 4 grams would remain.

What is the total amount of uranium 238 that will remain of a 1 gram sample after 4.5 X 109 years of radioactive decay? After 9.0 X 109 yrs?

Check Your ESRT:

- Half-life of Ur238…4.5 X 109

• After 1 half-life (4.5 X 109 years) = .5 grams

• After 2 half-lives (9.0 X 109 years) = .25 grams

• Now, when a radioactive element decays, it must become something, right?• (Law of conservation of matter . . .)

• For example ~ Rb87 becomes Sr87

• If we start with 100g of Rb87, after 4.9 x 1010 years (one half-life) we will have 50g of Rb87

and 50g of Sr87

• After 9.8 x 1010 years (two half-lives) we will have 25g of Rb87 and 75g of Sr87

• After 14.7 x 1010 years (three half-lives) we will have 12.5g of Rb87 and 87.5g of Sr87