Radium seventh of a gram of radium. One kilogram of the Earth's crust contains about 900 picograms of radium, and one liter of sea water contains about 89 femtograms of radium. Radium was discovered by Marie Sklodowska-Curie and her husband Pierre Curie on 21 December 1898, in a uraninite (pitchblende) sample. While studying the mineral earlier, the Curies removed uranium from it and found that the remaining material was still radioactive. They separated out an element similar to bismuth from pitchblende in July 1898, that turned out to be polonium. They then separated out a radioactive mixture consisting mostly of two
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