UK:*UK and possibly other pronunciationsUK and possibly other pronunciations/pluːˈtəʊniəm/US:USA pronunciation: IPA and respellingUSA pronuncation: IPA/pluˈtoʊniəm/ ,USA pronunciation: respelling(plo̅o̅ tōnē əm)

WordReference Random House Learner's Dictionary of American English © 2020
plu•to•ni•um /pluˈtoʊniəm/USA pronunciation   n. [uncountable]
  1. Chemistry, Physics, Physicsa radioactive, metallic element that can be produced from certain uranium.

WordReference Random House Unabridged Dictionary of American English © 2020
plu•to•ni•um  (plo̅o̅ tōnē əm),USA pronunciation n. [Chem., Physics.]
  1. Chemistry, Physics, Physicsa transuranic element with a fissile isotope of mass number 239(plutonium 239) that can be produced from non-fissile uranium 238, as in a breeder reactor. Symbol: Pu;
    at. no.: 94.
  • Greek Ploútōn Pluto + -ium
  • 1940–45

Collins Concise English Dictionary © HarperCollins Publishers::
plutonium /pluːˈtəʊnɪəm/ n
  1. a highly toxic metallic transuranic element. It occurs in trace amounts in uranium ores and is produced in a nuclear reactor by neutron bombardment of uranium-238. The most stable and important isotope, plutonium-239, readily undergoes fission and is used as a reactor fuel in nuclear power stations and in nuclear weapons. Symbol: Pu; atomic no: 94; half-life of 239Pu: 24 360 years; valency: 3, 4, 5, or 6; relative density (alpha modification): 19.84; melting pt: 640°C; boiling pt: 3230°C
Etymology: 20th Century: named after the dwarf planet Pluto because Pluto lies beyond Neptune and plutonium was discovered soon after neptunium
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