The lanthanide or lanthanoid series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through the lutetium.
The series always includes the 14 elements with atomic numbers 58 through 71, which are (in order of increasing atomic number) cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.
Lanthanides share the following common properties: Silvery-white metals that tarnish when exposed to air, forming their oxides. Relatively soft metals. Hardness increases somewhat with a higher atomic number. When contaminated with nonmetals, such as oxygen or nitrogen, the lanthanides become brittle. They will also corrode more easily if contaminated with other metals, such as calcium.