francium

frankium A metallic chemical element (symbol Fr) with an atomic number of 87.
2003, (w), A Short History of Nearly Everything, BCA, p. 221:

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.

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.