Can anyone explain isotope decay before 1230 pm EST on Thursday February 18 2010?

A nucleus consists of protons and neutrons. Some arrangements of protons and neutrons are unstable so the nucleus with fall apart to make a more stable nucleus. For example polonium - 210 is unstable with a half life of about 140 days. It has 84 protons and 126 neutrons. When it decays the nucleus releases an alpha particle which is essential a helium nucleus (2 protons and 2 neutrons and no electrons). Polonium - 210 minus an alpha particle yields a the stable element lead - 206 which has 82 protons and 124 neutrons. Unstable nuclei will decay by alpha, beta or gamma emission until they form stable nuclei. All nuclei heavier than lead - 210 are unstable and will generally decay to form lead. There are many isotopes lighter than lead that are unstable too and just decay to form a more stable nuclei.