Lanthanum

Lanthanum was discovered in 1839 by the Swedish chemist Carl Gustaf Mosander, who found it concealed within a sample of what had been taken for pure cerium. Its name comes from the Greek lanthanein, to lie hidden, aptly chosen for an element that had hidden inside another. Lanthanum is a soft, silvery, highly reactive metal that heads the lanthanide series, and it is used in camera lenses, hydrogen-storage alloys and catalysts. Among the more abundant of the rare earths despite its name, lanthanum goes into the mischmetal of lighter flints, into high-refractive-index optical glass, and, historically, into the nickel-metal-hydride batteries of early hybrid cars.

Lanthanum halide crystals, particularly lanthanum bromide doped with cerium, are used in some of the most sensitive modern scintillation detectors for gamma radiation, valued for their unusually sharp energy resolution, and such detectors are now deployed in radiation-portal monitors at ports and border crossings screening cargo for smuggled nuclear or radiological material, a distinctly modern security role for an element first noticed as a stubborn contaminant in someone else's cerium. Lanthanum oxide is also blended into the glass of high-refractive-index camera and telescope lenses, since it raises the glass's bending power without the excessive colour dispersion that heavier lead-based optical glasses introduce.

Mosander's name was a knowing joke at the expense of the rare earths, which seemed endlessly to conceal one element within another. Working patiently through cerium salts, he uncovered first lanthanum and then didymium, and the pattern of hidden elements emerging from apparently pure samples would continue for decades as chemists teased the lanthanides apart. Lanthanum, the hidden one, gave its name to the whole series that follows it, the lanthanides, fifteen chemically similar metals whose separation was one of the great labours of nineteenth and twentieth-century chemistry. The element has no ancient or symbolic tradition, being a product of that painstaking work; its very name is a comment on the difficulty of the task that revealed it.

Chemists still debate, even now, exactly which two elements belong in group three beneath scandium and yttrium: lanthanum and actinium were the traditional choice for over a century, but a growing body of physical and chemical evidence has led many modern textbooks and even some IUPAC-linked recommendations to favour lutetium and lawrencium instead, a live argument about the periodic table's structure that traces directly back to lanthanum's own contested position at the head of its series. The dispute remains unresolved by any binding international ruling, so lanthanum's textbook position continues to vary from one country and curriculum to the next.