Technetium (Tc) and promethium (Pm), two radioactive elements no longer naturally present on Earth, along with trace radioactive elements (namely Po, At, Rn, Fr, Ra, Ac, Pa) and minor actinides (Np, Pu, Am, Cm and further short-lived artificial elements) are shown as synthetic, because their occurrence in nature is fleeting and is entirely dependent on their long-lived parents Th and U, and they are not very mobile. For instance, polonium's chemistry would predict it to be a chalcophile, but in actuality it tends to occur instead as a lithophile along with its parent uranium; even radon, which is a gas, does not usually have time to travel very far from the original uranium source before decaying. When needed, these elements are typically produced synthetically in nuclear reactors instead of using the tedious and laborious process of extraction from uranium ores.