Half-LifeDetermining the Age of a Material

How to Determine the Age of Something

Types of Dating Absolute (exact number) Relative (approximate by putting in

order; 1st, 2nd, 3rd, …)

Tree Rings (Dendrochronology) Layers of Earth (Stratigraphy) Radioisotope (Radiometric)

Radioisotope Dating (Radioactive Decay) Mass can (not) be created nor destroyed

Radioactive decay means that the substance is changing (smaller by giving stuff away)

Half-life is a measurement of change (it is an average, not exact amount)

Half-life means the time it takes for ½ of something to turn into something else (not disappear)

The Process of Change

BeforeParent

Reactant

AfterDaughterProduct

The percent or fraction of change over

time

Change Different

Decay BiggerSmaller

Common Isotope Pairs Decay is

predictable

Each parent will decay into a specific daughter

Can be 1 step or a series (many) of steps

Half-Life The half-life is an approximation. A statistical

average. It is how we measure the change in decay

Google map says it takes 42 minutes to drive from Vancouver to Surrey

The average Canadian home has 1.9 children and 2.25 vehicles

Canadian Life Expectancy is 80.93 years

The chance of flipping a coin 5 times and getting all heads (each time) is 1/32

One generation is approximately 30 years

Half-Life The half-life is the rate of radioactive

decay for a given isotope

It is equal to the time required for ½ of the nuclei to decay (change)

Rate graph curve or line (w/ slope) x-axis = time Y-axis = amount of material (percent or

fraction)

Rate = percentage of material / time

DEMONSTRATION

Decay Curve

Decay Curve

Decay Table

Radioisotope Dating (Age) Because the process of decay can be

predicted or timed we can determine the age of things

If it takes 2 minutes for 500 grams of ice to turn into 250 grams at 22° Celsius

Then what can we predict:

We can Predict: How much we used to have (past) How much we will have (future) How old is it (past) How long will it live for (future) How old is something (unknown) compared to

something else (known)

Assume the rates (half-life) to be constant Use the percentage or fraction of change (not the

whole number) over time

½ or 50% of the ice melts, not 2.5 grams

Carbon Dating When things are

alive the ratio of C-14 to C-12 are equal

But, after death C-14 decays.

Calculating the ratio will give you the age

Initial: =

After: >

Potassium Clock When rocks are

formed by lava, all gasses are freed no Argon gas

But, after the rock is formed (hardened or cooled) then potassium decays into argon gas

Gas in rock tells age

Initial: = 0

After: > 0

Limitations/Dangers The age is only as accurate as the range

of the isotope

Can not measure things that are older than the isotope or things that will be around longer than it.

Have to be careful with initial conditions

Therefore we have to be careful in choosing an appropriate isotope to use

Types of Isotopes (General) Carbon-14 (5730 years): for life cycles

(organism’s remains) Potassium-40 (1,300 million years): for

the age of the earth Uranium-238 (4,468.3 million years):

makes plutonium Uranium-235 (703.8 million years): for

reactors & Weapons

Types of Isotopes (Medical) Sodium-24 (15 h): for the study of

electrolytes within the body. Chromium-51 (28 d): used to label red

blood cells

Iodine-123 (13 h): thyroid function Iodine-125 (60 d): used in cancer

(prostate and brain), Iodine-131 (8 d): organ imaging (photos)