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Uranium (U). Diagram of the nuclear composition and electron configuration of an atom of uranium-238 (atomic number: 92), the most stable isotope of this element. The nucleus consists of 92 protons (red) and 146 neutrons (blue). Uranium is an actinide in period 7, and the f-block of the periodic table. Actinides are soft radioactive metals. Uranium-238, used as a nuclear fuel, contributes to the Earth's internal heat. It has a half-life of 4.5 billion years. Its nucleus decays by spontaneous fission or emitting an alpha particle to form thorium-234.
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