Protactinium (Pa). Diagram of the nuclear composition, electron configuration, and valence (outer electron) orbitals of an atom of protactinium-231 (atomic number: 91), the most stable isotope of this element. The nucleus consists of 91 protons (blue) and 140 neutrons (red). Protactinium is an actinide in period 7, and the f-block of the periodic table. Actinides are soft radioactive metals. Protactinium-231, rare in nature and usually extracted from spent nuclear fuel, has a half-life of 32, 760 years. Its unstable nucleus decays by emitting an alpha particle to form actinium-227. | |
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Credit: | Science Photo Library / Clarivan, Carlos |
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