from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A rare, extremely toxic, radioactive element chemically similar to uranium, having 13 known isotopes, the most common of which is protactinium 231 with a half-life of 32,480 years. Atomic number 91; melting point 1,230°C; specific gravity 15.37; valence 4, 5. See Table at element.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A chemical element (symbol Pa) with atomic number 91.
from the GNU version of the Collaborative International Dictionary of English
- The radioactive chemical element, of atomic number 91; Atomic symbol Pa; at. wt. of longest-lived isotope, 231 (T1/2 = 32,500 yrs.) Also called brevium, Uranium X2 and UX2.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a short-lived radioactive metallic element formed from uranium and disintegrating into actinium and then into lead
prot(o)- + actinium (so called because it decays into actinium).(American Heritage® Dictionary of the English Language, Fourth Edition)
proto- + actinium. (Wiktionary)