7.2 - Half Life

Half Life

Radiocarbon Dating
Finding age of object by measuring amount of
carbon-14 remaining
Compare ratio of C-14 C
to C-12C
When an organism dies, C-14 C
decays and is not
replaced

Half-life
life
Time required for half the nuclei in a sample to decay
Eg: : Strontium-90 has T 1/2 of 29 years
# half lives
Elapsed time
% Strontium-90
present
Amount
Strontium-90

Decay Curve
Curved line on graph
that shows the rate at
which radioisotopes
decay

Decay Curve
Iodine-131 has T 1/2 of 8 days. If we start with
20g of Iodine-131, how much remains after 24
days?
A sample of rock has 64g of radioisotope. How
much remains after 4 half lives?

Pg 306 #1-4
Practice Problems

Common Isotope Pairs
Parent isotope = undergoes decay
Daughter isotope = stable product of decay
Parent Daughter
Parent isotope decreases, daughter isotope
__________.
Increases

1. What is the ratio of argon-40 to
potassium-40 after two half lives?
2. What is the ratio of argon-40 to
potassium-40 after 3.9 billion years?
3. When there is more parent than daughter
isotope, what does that tell you about the
age of the rock?

Pract-
Pg 311 #6, 7, 9, 10, 11, 12