Early on, physicists figured out that many heavy elements (such as uranium, thorium, and radium) decay into simpler things. However, physicists were very unhappy for a while because this posed several nasty problems.
	Uranium's mass is 238.0508 atomic mass units (AMU). It can decay into Thorium (234.0436 AMU) and an alpha particle (4.0026 AMU). Uranium's mass minus the mass of its decay products is 0.0046 AMU. How do you account for this loss of mass?
	Half of a given lump of uranium-238 will decay in 4,460,000,000 years. However, there is no way to know when an individual atom will decay. Why will an atom, just sitting there, decay according to some set probability?

Keep reading to find the answers to these exciting questions!