Lanthanum

Lanthanum is a soft, silvery-white metal that gives its name to the lanthanides, the fifteen elements from lanthanum to lutetium. Its configuration is [Xe] 5d1 6s2: the 4f subshell that the following elements fill is still empty, so chemically lanthanum behaves like scandium and yttrium in group 3. It loses all three outer electrons to form La^3+, its only important oxidation state, and with no f electrons this ion is colourless and non-magnetic. Lanthanum is reactive, tarnishing in air within hours and slowly releasing hydrogen from cold water.

Among the rare earth elements, lanthanum is relatively plentiful, more abundant in the crust than lead. Its largest use is in the zeolite catalysts that crack heavy petroleum into petrol. Lanthanum oxide raises the refractive index of glass while keeping dispersion low, so it appears in high-quality camera and telescope lenses. Alloys based on LaNi5 absorb and release hydrogen reversibly, which made them the negative electrodes of nickel-metal hydride batteries in early hybrid cars. Lanthanum hexaboride emits electrons readily when heated and serves as the cathode in electron microscopes, and lanthanum carbonate is a medicine that binds phosphate in kidney patients.

Discovery

In 1839 Carl Gustaf Mosander, a student of Berzelius in Stockholm, partly decomposed cerium nitrate by heating it and found that dilute nitric acid extracted a new oxide. Because the element had been hidden inside cerium minerals, Berzelius suggested the name lanthana, from the Greek lanthanein, 'to lie hidden'. Lanthanum metal was not prepared in reasonably pure form until 1923.