Radioactivity

Check Your Understanding

• If there is 100 g of carbon 14 at time zero, how much carbon 14 will there be after 5730 years (one half life)?
• 50 g. There has been one half-life, so only half the amount is left.
• How much will be left in another 5730 years (11460 years total)?
• 25 g. The amount is cut in half again, so ¼ of the original amount remains.

Check Your Understanding

• If a sample of a radioactive isotope has a half-life of one year, how much of the original sample will be left at the end of two years?
• One quarter (or half of a half). Half decayed during the 1st year, and half of the half remaining decayed during the 2nd year.

Dating

• Scientists often use radioactive half-life information to date older materials
• Carbon 14 is used to date organic materials that have been dead for centuries
• Ex: dinosaur bones, wooden artifacts
• Uranium 238 and 235 are used to date much older objects that were never living
• Ex: dating the age of Earth

Check Your Understanding

• Why can’t Uranium 238 be used to date younger objects like skyscrapers?
• Not enough Uranium has undergone radioactive decay to measure a change. The half-life of U-238 is 4.5 billion years!
• BrainPop Carbon Dating