Polonium the Curies while they were investigating the cause of pitchblende radioactivity. Pitchblende, after removal of the radioactive elements uranium and thorium, was more radioactive than the uranium and thorium combined. This spurred the Curies to search for additional radioactive elements. They first separated out polonium from pitchblende in July 1898, and five months later, also isolated radium. German scientist Willy Marckwald successfully isolated 3 milligrams of polonium in 1902, though at the time he believed it was a new element, which he dubbed "radio-tellurium", and it was not until 1905 that it was demonstrated to be the same as polonium.
Polonium trioxide that polonium trioxide may be formed by heating polonium dioxide and chromium trioxide together in air. It is very difficult to oxidize polonium beyond Po(IV); for example, the only hexahalide of polonium is the hexafluoride, PoF, and fluorine is already the most electronegative element (though polonium hexaiodide was once reportedly formed in the vapour phase, it immediately decomposed). However, the difficulty in obtaining polonium trioxide and polonates (containing the anion, analogous to sulfate, selenate, and tellurate) by direct oxidation of Po(IV) compounds may be due to the fact that polonium-210, while the most easily available isotope of polonium, is strongly