This document discusses absolute dating and radiometric dating. Absolute dating determines the numeric age of a geologic object by measuring the radioactive decay of isotopes with long half-lives. Radiometric dating compares the percentages of radioactive parent isotopes to stable daughter isotopes produced after half lives to calculate the age of a sample. Examples of practice problems show how to calculate the number of half lives needed for a sample amount to decay and determine half life from initial and final isotope amounts.

1. ABSOLUTE
DATING

2. ABSOLUTE DATING/AGE:
•The numeric age of a geologic object or event

3. RADIOMETRIC DATING:
•A method of determining the absolute age of
an object by comparing the relative
percentages of an unstable radioactive (parent)
isotope and a stable (daughter) isotope

4. ISOTOPE:
•A version of an element where the only
difference is more or less neutrons
•(ex: Carbon 12 – 6 protons, 6 neutrons; Carbon
14 – 6 protons, 8 neutrons)

5. HALF LIFE:
• The average time
required for half a
sample of a
radioactive isotope to
break down by
radioactive decay to
form a stable
daughter isotope via
radioactive decay

6. PRACTICE PROBLEM 1:
• A sample contains 1,000 grams of an isotope that
has a half-life of 500 years. How many half-lives will
have to pass before the sample contains less than
40g of the parent isotope? Show work:
Half Life 0 1 2 3 4 5
Amount 1000g 500 250 125 62.5 31.125
Time 0 yrs 500 1000 1500 2000 2500

7. PRACTICE PROBLEM 2:
• The half-life of 238U is 4.5 billion years. How many
years would 16g of it take to decay into 0.5 grams
of 238U and 15.5 grams of its daughter isotopes?
Show work below:
Half Life 0 1 2 3 4 5
Amount 16g 8 4 2 1 .05
Time 0 yrs 4.5 by 9.0 by 13.5 by 18 by 22.5 by

8. PRACTICE PROBLEM 3:
•64 mg of a substance decays to 8 mg in 84
seconds. What is the half-life of the substance?
Show work:
Half Life 0 1 2 3
Amount 64 mg 32 16 8 mg
Time 0 s ? ? 84 s
•84s / 3 half-lives = 28 seconds per half life