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
Polonium hydride selenol and tellurol). However, no salts from polonium hydride are known. An example of a polonide is lead polonide (PbPo), which occurs naturally as lead is formed in the alpha decay of polonium. Polonium hydride is difficult to work with due to the extreme radioactivity of polonium and its compounds and has only been prepared in very dilute tracer quantities. As a result, its physical properties are not definitely known. It is also unknown if polonium hydride forms an acidic solution in water like its lighter homologues, or if it behaves more like a metal hydride (see also hydrogen astatide).