From Citizendium, the Citizens' Compendium
Lanthanum is a chemical element, having the chemical symbol La. Its atomic number (the number of protons) is 57. It has a standard atomic weight of 138.9055 g•mol −1 and is a solid in its elemental form.
A white, soft element, lanthanum is classified in a number of different "categories" of elements. In addition to being a transition metal, it belongs to the "rare earth metals", (which include scandium, yttrium, and actinium), and is also the namesake of the "lanthanides", a group of elements in the periodic table having atomic numbers from 57 (lanthinide itself) to 71 (lutetium).