5. Francium
An ounce of this element can be found throughout the Earth’s crust because the 223- isotope of francium continually forms and decays. The largest amount of Francium that was produced in the laboratory was a cluster of more than 300,000 atoms.
4. Hydrogen
3. Cesium
2. Methane
1. Uranium
There are 27 known isotopes of uranium and most of them are used in nuclear chain reaction and produce the fissile material for nuclear weapons. The isotope uranium-239 decays by beta emission to neptunium-239 that decays in its turn, within a few days into plutonium-239. 239-Pu was used as fissile material in the first atomic bomb detonated in the “Trinity test” on 15 July 1945 in New Mexico.