Using Science to Measure Time Relative dating (e.g., fossil records) Electronic dating Quartzite crystals Nuclear time clocks
Nuclear Radiation In terms of physics and chemistry, nuclear radioactivity is pretty cool… its’ just unfortunately not as sexy as The Simpsons may have you believe at first.
How an Atom Works Atoms have protons, neutrons, and electrons. The protons and neutrons are located inside the center of the atom (nucleus) Protons normally repel other protons. (Why?) The force that holds together protons can “accidentally” be broken. What gets released?
Types of Nuclear Radiation Alpha decay Beta decay Gamma decay Shoots off 2 protons & 2 neutrons Changes a neutron into a proton (or vice versa) Shoots off a high pocket of energy (no change)
Why Does this Even Matter? Alpha decay Beta decay Gamma decay Shoots off 2 protons & 2 neutrons Changes a neutron into a proton (or vice versa) Shoots off a high pocket of energy (no change)
Example: Polonium’s Decay Process Alpha decay Beta decay becomes lead Becomes Astatine or Becomes bismuth
Using Radioactivity for Finding Time All elements can undergo nuclear radiation except for two (which two do not decay?) Since even 1 mg of any element has at least 10 20 atoms in it, we can find the precise moment in time that that sample was made. All things decay with a certain “reverse J-curve” pattern called a “half life”
What Is Half Life? Half life is the time it takes for exactly one-half of a sample of any element to decay. Stable atoms have half-lives of millions or billions of years. Radioactive atoms have half-lives that are very short.
C-14 = 5,730 years U-235 = 704 million years K-40 = 1.25 billion years Common Half Lives
Further Evidence Uranium-235 and its decay product (lead) are both created in very different scenarios. If you find a mixture of uranium/lead, what can you conclude about the lead? The same goes for potassium-40 and calcium. If you find a mixture of potassium that also has calcium, what can you conclude about the sample?
Questions An archaeologist finds a clay pot where 12.5% of the expected carbon-14 is remaining. In Antarctica, a sample of potassium was found where only 6.25% of the expected potassium-40 was left. C-14 = 5,730 years K-40 = 1.25 billion years