| Definition/Meaning | Word/Phrase |
|---|---|
| (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 |