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(chemistry) a series of 15 radioactive elements with increasing atomic numbers from actinium to lawrencium actinide series
radioactive element of the alkali-metal group discovered as a disintegration product of actinium atomic number 87 , Fr , francium
short-lived radioactive metallic element formed from uranium and disintegrating into actinium and then into lead atomic number 91 , Pa , protactinium , protoactinium
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