Radium bromide Institute had four grams of radium total, which at the time was more than half the world supply. Countries all over the world set out to make as much radium as possible, a time consuming and expensive task. It was reported in "Science" magazine in 1919 that the United States had produced approximately 55 grams of radium since 1913, which was also more than half the radium produced in the world at the time. A principal source for radium is pitchblende, which holds a total of 257 mg of radium per ton of UO. Obviously with so little product recovered
Radium of 5.5 g/cm, higher than that of barium, again confirming periodic trends; the radium-barium density ratio is comparable to the radium-barium atomic mass ratio, due to the two elements' similar crystal structures. Radium has 33 known isotopes, with mass numbers from 202 to 234: all of them are radioactive. Four of these – Ra (half-life 11.4 days), Ra (3.64 days), Ra (1600 years), and Ra (5.75 years) – occur naturally in the decay chains of primordial thorium-232, uranium-235, and uranium-238 (Ra from uranium-235, Ra from uranium-238, and the other two from thorium-232). These isotopes nevertheless still have half-lives too short