Zirconium

Zirconium ([Kr]4d2 5s2) sits below titanium in group 4 and, like titanium, uses all four valence electrons, forming compounds mainly in the +4 state, such as zirconium dioxide. It also shares titanium's trick of protecting itself with a tough oxide film, giving it outstanding resistance to acids, alkalis and seawater. Zirconium's closest relative, though, is hafnium, the element below it: the lanthanide contraction makes their atoms and ions almost exactly the same size, so they have nearly identical chemistry, always occur together and are very hard to separate.

Zirconium barely absorbs slow neutrons, so zirconium alloys called Zircaloy are used to clad the uranium fuel in nuclear reactors. Hafnium absorbs neutrons strongly, so it must be removed first. Above about 1200 °C zirconium reacts with steam to release hydrogen, which contributed to the explosions at Fukushima in 2011. Most zirconium never becomes metal: zircon sand is used in ceramics and foundry molds, and zirconia is made into dental crowns, oxygen sensors and cubic zirconia gems.

Discovery

Gems of the mineral zircon have been known since antiquity. In 1789 Martin Heinrich Klaproth analyzed zircon from Sri Lanka and found a new oxide, zirconia. Jöns Jacob Berzelius isolated impure zirconium metal in 1824 by heating potassium hexafluorozirconate with potassium. Pure, ductile zirconium came only in 1925 from the van Arkel–de Boer crystal-bar process. The name comes from zircon, probably from the Persian zargun, gold-colored.