ytterbium

Soft, lustrous, silvery, malleable, and ductile metallic element of the lanthanide series, symbol Yb, atomic number 70, relative atomic mass 173.04. It occurs with (and resembles) yttrium in gadolinite and other minerals, and is used in making steel and other alloys.

In 1878 Swiss chemist Jean-Charles de Marignac gave the name ytterbium (after the Swedish town of Ytterby, near where it was found) to what he believed to be a new element. French chemist Georges Urbain (1872-1938) discovered in 1907 that this was in fact a mixture of two elements: ytterbium and lutetium.