Titanium

Titanium has four valence electrons ([Ar]3d2 4s2), and its most stable compounds use all of them, as in titanium dioxide (TiO2) and titanium tetrachloride (TiCl4). On paper titanium is a reactive metal that should corrode easily. In reality it is one of the most corrosion-resistant metals known, because its surface instantly forms a dense, tightly bonded oxide film only a few nanometers thick. If the surface is scratched, the film re-forms in milliseconds, which is why titanium survives seawater, chlorine and body fluids.

Titanium is the ninth most abundant element in Earth's crust, yet the metal is expensive. Its affinity for oxygen and nitrogen means it cannot be smelted with carbon like iron; instead it is made by the slow, batch-wise Kroll process. With a density of 4.5 g/cm3, about 57 percent that of steel, and strength comparable to many steels, it is prized in aircraft, implants and chemical plants. Even so, about 95 percent of mined titanium never becomes metal: it is turned into TiO2, the brilliant white pigment in paint, paper, plastics and toothpaste.

Discovery

In 1791 William Gregor, a clergyman and amateur mineralogist in Cornwall, found an unknown metal oxide in black sand from the Manaccan valley. In 1795 the German chemist Martin Heinrich Klaproth independently found the same element in rutile and named it after the Titans of Greek mythology. Pure titanium metal was not made until 1910, when Matthew Hunter reduced TiCl4 with sodium; William Kroll's magnesium process of the 1940s made it commercially practical.