RADIOACTIVE DECAY AND HEATING
Some atomic nuclei are not stable. If they are allowed to sit, they will spontaneously break apart into smaller nuclei releasing particles and energy. For example, the isotope Uranium 238 is unstable in that it will spontaneously break apart into Thorum 234 and a Helium with the release of some energy (see the panel on the right) with a half-life of 4.5 billion years. Uranium 238 and Thorium 234 are sometimes written in shorthand as at right. For the Uranium 238, the 92 says that its nucleus contains 92 protons and the 238 says that its nucleus is made of 92 protons and 146 neutrons. For the Thorium, the 90 says that its nucleus contains 90 protons and the 234 says that its nucleus contains 90 protons and 144 neutrons. Here, we will stick to saying that Uranium 238 decays to Thorium 234 and a Helium nucleus plus energy for clarity.
This decay has a half-life of 4.5 billion years. The meaning of half-life is that it tells you how long it takes half of the uranium 238 in a box to decay into thorium 234. This property of radioactive decay lends itself well to the age dating of rocks and other materials.
