Half-Life & Carbon Dating
pg. 621-625
LO: I understand how radioactive isotopes can be used to determine the age of
matter.
Review of Radioactivity1. Why are atoms unstable?• Unbalanced nuclear forces—Electrostatic Charge Repulsion & Strong Nuclear
Force• The more protons in nucleus more neutrons needed to maintain stability.
2. Radioactive particles involvedAlpha Beta Gamma Neutron Positron
3. Types of Nuclear Reactions• Radioactive Decay• Nuclear Fusion—stars• Nuclear Fission—chain reactions (atomic bombs, etc.)
What we know about radioactivity
Radioactive (unstable) atoms exist everywhere:• Outer space bombardment into Earth’s
atmosphere• Matter on Earth• Air
All atoms above Z=83 (Bismuth) are radioactive
What is half-life?
Half Life: The time it takes for half of the parent nuclide to decay to the daughter nuclide.
100% 50% 25%
1st HL 2nd HL
50 yrs 50 yrs
Half-Life ExamplesCarbon-14 has a half-life of 5730 yrs. How old is the fossil if it
contains only 12.5% of the Carbon-14 relative to living organisms?
Radiocarbon Dating
Radiocarbon Dating: A technique used to estimate the ages of fossils and artifacts due to the carbon-14 contained in them. Not useful for metals.
Carbon-14: A radioactive isotope formed from nitrogen bombardment in the atmosphere by a neutron particle.
Carbon-14 dating organisms
• Plants take in Carbon-14 via photosynthesis• Living organisms eat plants which incorporate
Carbon-14 into animal tissues• Upon death, the organism stops incorporating new
Carbon-14 (half-life of 5730 yrs)• The amount (%) of Carbon-14 in the organic tissues
indicates their age (relative to living organisms)• Not dependable when dating objects beyond 50,000
years
How old is the artifact?
Question: A wooden boat discovered near Egypt had a Carbon-14 content of approximately 12.5% of that found in living organisms. How old is the boat?
Solving Half-Life Problems
A radioactive sample contains 2.45 g of an isotope with a half-life of 3.8 days. How much of the isotope in grams remains after 11.4 days?