The document discusses radioactive decay and nuclear fission. It explains that the half-life of an isotope is the time it takes for half of its atoms to decay. It provides examples of isotopes like radium-226 and discusses where they come from. The strong nuclear force holds atomic nuclei together, and some heavy nuclei are radioactive because they can reach a lower potential energy state through decay. Nuclear fission occurs when an unstable nucleus splits into smaller nuclei, often emitting free neutrons which can then trigger further fissions in a chain reaction.

1. Today: More about half-life, why atoms are radioactive, fission Diagram of neutron-induced U-235 fission

2. Half-life is the average amount of time for ½ of the atoms to decay 100% remaining 50% remaining 25% remaining 12.5% remaining Time Isotope with long half-life Isotope with short half-life “ Exponential Decay”

5. Why are some atoms radioactive? First: Some brainstorming Form small groups, with a spokesperson How many different forces in nature can you think of? Can you classify them into similar groups?

14. Why are some atoms radioactive? The nuclear (residual strong) force presents an activation barrier Quantum mechanics allows some probability that an alpha particle can “tunnel” through the barrier Once through, electrostatic force dominates (imagine ripping apart velcro)

17. As nuclei get larger, electrostatic force “wins” Strong force large between close protons. Strong force much lower between distal protons But electrostatic force still very high! (Nucleons on the surface feel less strong attraction.)

18. You can imagine large nuclei like large soap bubbles…very floppy Small Medium Large