An atom of any particular element can have several different weights, which we call isotopes. For example, a hydrogen atom could be composed of a single proton, or a proton plus a neutron, or a proton plus two neutrons. The first two of these are stable, but the third one is radioactive. Some elements, like plutonium, have no stable isotopes. All the atoms eventually break apart and form different elements. You can make plutonium in certain kinds of nuclear reactors, but it will begin to decay right away. The decay may take days or thousands of years, but this is still a short amount of time compared to the life of Earth, so there's essentially no natural plutonium left on Earth.

The heaviest element that is found in nature is uranium. A number of elements have been synthesized in the laboratory, most of them heavier than uranium. There is no "magical" property that these synthetic elements have; indeed, they probably do form in nature under extremely rare circumstances, but their radioactive half-lives are so short that they never make it from the supernovae in which they are made to the Earth. Uranium, by contrast, is stable enough that it can sit around for billions of years, and therefore it and its decay products can be found on Earth.

The heaviest element that is not radioactive is lead - however, bismuth is stable enough that it might as well not be radioactive, even though it technically is not completely stable.