Half-Life Determining the Age of a Material

How to Determine the Age of Something

Types of Dating  Absolute (exact number)  Relative (approximate by putting in order; 1 st, 2 nd, 3 rd, …)  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 Before Parent Reactant After Daughter Product The percent or fraction of change over time Change  Different  Decay  Bigger Smaller 

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 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 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)