Unlike physical "systems"--bacteria, animals, men and stars--unstable isotopes do not individually have a predictable life span. There is no way of forecasting when a single unstable nucleus will decay. Nevertheless, it is possible to get around the random behavior of an individual nucleus by dealing statistically with large numbers of nuclei of a particular radioactive isotope. In the case of thorium-232, for example, radioactive decay proceeds so slowly that 14 billion years must elapse before one-half of an initial quantity decayed to a more stable configuration. Thus the half-life of this isotope is 14 billion years. After the elapse of second half-life (another 14 billion years), only one-fourth of the original quantity of thorium-232 would remain, one eighth after the third half-life, and so on. Most manmade radioactive isotopes have much shorter half-lives, ranging from seconds or days up to thousands of years. Plutonium-239 (a manmade isotope) has a half-life of 24,000 years. For the most common uranium isotope, U-238, the half-life is 4.5 billion years, about the age of the solar system. The much scarcer, fissionable isotope of uranium, U-235, has a half-life of 700 million years, indicating that its present abundance is only about 1 percent of the amount present when the solar system was born. Note 5: Oxygen, Ozone and Ultraviolet Radiation Oxygen, vital to breathing creatures, constitutes about one-fifth of the earth's atmosphere. It occasionally occurs as a single atom in the atmosphere at high temperature, but it usually combines with a second oxygen atom to form molecular oxygen (O2). The oxygen in the air we breathe consists primarily of this stable form. Oxygen has also a third chemical form in which three oxygen atoms are bound together in a single molecule (03), called ozone. Though less stable and far more rare than O2, and principally confined to upper levels of the stratosphere, both molecular oxygen and ozone play a vital role in shielding the earth from harmful components of solar radiation. Most harmful radiation is in the "ultraviolet" region of the solar spectrum, invisible to the eye at short wavelengths (under 3,000 A). (An angstrom unit--A--is an exceedingly short unit of length--10 billionths of a centimeter, or about 4 billionths of an inch.) Unlike X-rays, ultraviolet photons are not "hard" enough to ionize atoms, but pack enough energy to break down the chemical bonds of molecules in living cells and produce a variety of biological and genetic abnormalities, including tumors and cancers.