More abundant than better known metals such as silver and gold, hafnium (from the Latin Hafnia, a name for Copenhagen) was not discovered until 1923 by Coster and de Hevesy. The reason is the similarity of hafnium to zirconium. Mendeleev had predicted the existence of element 72 but had wrongly suggested it might be found along with titanium ores. Instead it lay hidden with "pure" samples of zirconium. Later Niels Bohr predicted the arrangement of outer electrons for element 72 and it was using X-ray techniques to study this very thing that led to the identification of hafnium as a separate element (thus the connection to Copenhagen in the name for the metal: Bohr's hometown).
Hafnium metal is used in the manufacture of control rods for nuclear reactors because of its ability to absorb neutrons.
Stephen R. Marsden (ChemTopics)