Sentence:  The most elusive element of all, however, appears to be <WSD>francium</WSD>, which is so rare that it is thought that our entire planet may contain, at any given moment, fewer than twenty <WSD>francium</WSD> atoms. 
Meaning of the word marked with <WSD> tags:  The chemical element ("symbol" Fr) with an atomic number of 87. It is an intensely radioactive alkali metal that is not found in nature.