Hafnium

The lanthanide contraction leaves hafnium and zirconium with almost exactly the same ionic radius, so they behave identically in every mineral and every separation scheme; hafnium was not discovered until 1923, one of the last stable elements found, and Coster and Hevesy located it in Copenhagen by X-ray spectroscopy exactly where Bohr's atomic theory said it should be. Nuclear engineering is what forces the two apart: hafnium's thermal neutron cross-section is roughly 600 times zirconium's, so reactor cladding must be hafnium-free zirconium while the control rods are made of the hafnium stripped out of it. Hafnium oxide's high dielectric constant also replaced silicon dioxide as the gate insulator in CPUs from the 45 nm generation onward, which is a large part of why Moore's law survived past 2007.