Radium has 33 known isotopes, with mass numbers from 202 to 234: all of them are radioactive. Four of these – 223Ra (half-life 11.4 days), 224Ra (3.64 days), 226Ra (1600 years), and 228Ra (5.75 years) – occur naturally in the decay chains of primordial thorium-232, uranium-235, and uranium-238 (223Ra from uranium-235, 226Ra from uranium-238, and the other two from thorium-232). These isotopes nevertheless still have half-lives too short to be primordial radionuclides and only exist in nature from these decay chains. Together with the mostly artificial 225Ra (15 d), which occurs in nature only as a decay product of minute traces of 237Np, these are the five most stable isotopes of radium. All other known radium isotopes have half-lives under two hours, and the majority have half-lives under a minute. At least 12 nuclear isomers have been reported; the most stable of them is radium-205m, with a half-life between 130 and 230 milliseconds; this is still shorter than twenty-four ground-state radium isotopes.