(So named in 1843 by Carl Gustaf Mosander, Swedish chemist after Ytterby, a village in Sweden where the rare earth minerals were first found) A grayish silver trivalent chemical element, one of the rare-earth metals. It is an important constituent of signal repeaters in long-distance telephone cables.

Symbol: Er
Atomic number: 68
Atomic weight: 167.26
Density (at room temperature and pressure): 9.066 g/cc
Melting point: 1,529°C
Boiling point: 2,868°C
Valence: +3
Ground state electron configuration: [Xe]4f126s2