Lanthanum is a rare earth metal element with the chemical symbol La, atomic number 57, and atomic weight 138.90547. The element name comes from Greek and originally means "hidden". Silver gray luster, soft texture, density of 6.162g/cm3, melting point of 920 ℃, boiling point of 3464 ℃ (atmospheric pressure), active chemical properties, exposed to air quickly lose metallic luster and form a layer of blue oxide film, but it cannot protect the metal and further oxidize to form white oxide powder. It can react slowly with cold water, is soluble in acids, and can react with various non-metallic substances. Lanthanum metal is generally preserved in mineral oil or rare gases. The content of lanthanum in the crust is 0.00183%, second only to cerium in rare earth elements. Lanthanum has two natural isotopes: lanthanum-139 and radioactive lanthanum-138.