RICHARD WAREHAM FOTOGRAFIE / SCIENCE PHOTO LIBRARY RICHARD WAREHAM FOTOGRAFIE / SCIENCE PHOTO LIBRARY
Protactinium. Chemical symbol and values for the radioactive element protactinium (Pa), an actinide metal. Rare in nature, it is usually extracted from spent nuclear fuel. It has a half-life of 32,760 years. The nucleus of its most stable isotope (protactinium-231) decays by emitting an alpha particle to form actinium-227. Shown here are this chemical element's atomic number (91) and its standard atomic weight (231.036).
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