Found in the following dictionaries
OTCWL2014 (NASPA Word List 2014)
"Roentgenium" Definition
noun
An artificially produced radioactive element with atomic number 111 and a mass number of 280, whose most stable known isotope has a half-life of 3.6 seconds. See Table at element.
Source: American Heritage Dictionary
A short-lived radioactive element produced artificially .
Source: Merriam-Webster
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