Of the elements with atomic numbers 1 to 92, most can be found in nature, having stable isotopes (such as oxygen) or very long-lived radioisotopes (such as uranium), or existing as common decay products of the decay of uranium and thorium (such as radon). The exceptions are elements technetium, promethium, astatine, and francium ; all four occur in nature, but only in very minor branches of the uranium and thorium decay chains, and thus all save francium were first discovered by synthesis in the laboratory rather than in nature.