Lanthanum begins the lanthanide series in the commonly used 15-element classification.
Lanthanum has atomic number 57 and the symbol La. In many periodic-table conventions, the lanthanides run from lanthanum through lutetium, covering atomic numbers 57 to 71. Some tables instead place lutetium as the first element of the f-block and describe the series as beginning with cerium, which is why the classification can cause confusion. For standard trivia usage, lanthanum is commonly identified as the first lanthanide.
The element’s name comes from the Greek lanthanein, meaning “to lie hidden,” because lanthanum was concealed in cerium minerals. Carl Gustaf Mosander discovered it in 1838. Lanthanum is a soft, silvery metal that tarnishes in air and is used in some optical glasses, catalysts, and rechargeable-battery materials. It is not the first actinide: actinium begins the actinide series.