Radioactivity is the spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. Natural radioactivity is exhibited by several elements, including uranium, radium, radon gas, and radon's daughters. The radiation produced is of three types: the alpha particle with relatively weak penetration power, which is a nucleus (two protons and two neutrons) of an ordinary helium atom; the beta particle with moderate penetration power, which is a high-speed electron or, in some cases, a positron (the electron's antiparticle); and gamma radiation, which is a type of electromagnetic radiation with very short wavelengths resulting in very high penetration power. The rate of disintegration of a radioactive substance is commonly designated by its half-life, which is the time required for one half of a given quantity of the substance to decay.
For example, if you had a two liter bottle (think of the large soda bottle in the fridge) that was filled with radon gas and then tighly sealed, at the end of one half-life (approximately 92 hours or almost 4 days) there would only be one liter left in the bottle.
Another issue to consider is the unusual property of the radioactive decay chain of uranium/radium/radon. What makes this seem unusual is that a gas is produced from a radioactive solid element (a rock) and then the radioactive gas changes back into radioactive heavy metallic particles. This process and their atomic size (extremely small) makes the transport of radioactive atoms through a relatively static environment possible. In other words, radon's extended half-life (it takes about a month for a specific amount of it to decay to almost nothing) provides enough time for the gas to migrate through cracks and crevices in building foundations, then into the internal air volume where it changes into the more harmful radioactive heavy metals.
This gas and the resulting very small metallic particles (so small that they will float in air) move quickly through a building or home, contaminating the air. An analogy that makes this easier to understand is to think how easily some people can detect the presence of a smoker in another part of the building or the cooking of coffee or bacon in the kitchen on Sunday morning. In other words, almost nothing will stop this gas from moving from the basement to other parts of a house if it makes its way into the basement in the first place. Be sure you visit our Radon FAQ page for answers on how to stop this gas before it enters your home.