List of nuclides  This list of nuclides shows observed nuclides that either are stable or, if radioactive, have half-lives longer than one hour. At least 3,300 nuclides have been experimentally characterized (see List of radioactive nuclides by half-life for the nuclides with decay half-lives less than one hour). A nuclide is defined conventionally as an experimentally examined bound collection of protons and neutrons that either is stable or has an observed decay mode.  252 nuclides are considered stable. Many of these in theory could decay through spontaneous fission, alpha decay, double beta decay, etc. with a very long half-life, but no radioactive decay has yet been observed. Thus the number of stable nuclides is subject to change if some of these 252 are determined to be very long-lived radioactive nuclides in the future. If a decay has been predicted theoretically but never observed experimentally (either directly or through finding an excess of the daughter), it is given in parentheses. Those that have been checked for radioactivity are indicated with "> number" and show the lower limit for the half-life based on experimental observation. Such nuclides are considered to be "stable" until a decay has been observed in some fashion. For example, tellurium-123 was reported to be radioactive, but the same experimental group later retracted this report, and it presently remains observationally stable. The next group is the primordial radioactive nuclides. These have been measured to be radioactive, or decay products have been identified (Te-128, Ba-130). There are (currently) 34 of these (see these nuclides), of which 24 have half-lives longer than 100 trillion years. With most of these 24, decay is difficult to observe and for most purposes they can be regarded as effectively stable. Bismuth-209 is notable as the only naturally occurring isotope of an element long considered stable. A further 10 nuclides, platinum-190, samarium-147, lanthanum-138, rubidium-87, rhenium-187, lutetium-176, thorium-232, uranium-238, potassium-40, and uranium-235 have half-lives between 700 million and 1 trillion years, which means they have experienced significant depletion since the formation of the solar system about 4.6 billion years ago, but still exist on earth in significant quantities.