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Thorium (Th). Diagram of the nuclear composition and electron configuration of an atom of thorium-232 (atomic number: 90), the most stable isotope of this element. The nucleus consists of 90 protons (red) and 142 neutrons (blue). Thorium is an actinide in period 7, and the f-block of the periodic table. Actinides are soft radioactive metals. Thorium-232, which is used as a nuclear fuel, contributes a large part of the Earth's internal heat. It has a half-life of 14 billion years. Its nucleus decays by emitting an alpha particle to form radium-228.
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