francium

noun

1. (uncountable, usually)The chemical element (symbol Fr) with an atomic number of 87. It is an intensely radioactive alkali metal that is not found in nature.

   “The most elusive element of all, however, appears to be francium, which is so rare that it is thought that our entire planet may contain, at any given moment, fewer than twenty francium atoms.”

2. (countable, usually)A single atom of this element.

   “Notice that franciums are balanced without even thinking in moles, atoms, or whatever.”

   “(a) Radons and poloniums are compared, (b) poloniums and astatines are compared and (c) radons and franciums are compared.”