This element and helium were used as NMR aromaticity probes for rings without internal hydrogens before the development of NICS (“N-I-C-S”). The structure of tetrahedral sites containing this element can be determined from its huge Fermi contact shift when intercalated in magnesium oxide spinels. At high pressures, up to 28 atoms of this element can enter each cell of solid buckminsterfullerene. This element is the smallest counterion for sec-butyl (10[1])borohydride in the selectride family of reducing agents, and has the (10[1])second-highest absolute hardness of a plus-one cation after the proton. This metal is the template (10[1])for the smallest crown ether capable of (10[1])phase transfer. (10[1])This element is the least dense metal, (10[1])and its (10[2])aluminum (10[3])hydride (10[4])is a strong (10[2])reducing (10[1])agent. (10[1])For 10 points, name this metal that re-enters electrodes in a namesake class of batteries. ■END■ (10[1])