Short answers, pulled from the story.
Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly in air and is the first and prototype of the lanthanide series.
Lanthanum was first found by the Swedish chemist Carl Gustaf Mosander in 1839 as an impurity in cerium nitrate. Relatively pure lanthanum metal was not isolated until 1923.
Lanthanum takes its name from the ancient Greek lanthanein, meaning 'to lie hidden', because it lay concealed within cerium salts. Its properties differed only slightly from cerium and occurred along with it, so it stayed undetected.
Lanthanum is traditionally counted among the rare-earth elements, but it is the 28th most abundant element in the Earth's crust, almost three times as abundant as lead. It makes up 39 milligrams per kilogram of the crust, behind neodymium and cerium.
Lanthanum is used in nickel-metal hydride batteries such as those in the Toyota Prius, lighter flint mischmetal, carbon arc lamps for studio lighting, electron cathodes, scintillators, ZBLAN fiber-optic glass, and gas tungsten arc welding electrodes. Lanthanum carbonate, sold as Fosrenol, treats high blood phosphate in kidney failure.
Lanthanum has no known biological role in humans and is poorly absorbed, but it is used by some bacteria. It is a cofactor for the methanol dehydrogenase of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV.