Lanthanide

A lanthanide is an element in the series from lanthanum, atomic number 57, through lutetium, atomic number 71. The progression is associated with filling the 4f shell and the systematic size change called the lanthanide contraction.

Many lanthanides favour trivalent ions with similar coordination chemistry. Their similarity makes separation demanding, while differences in ionic size and accessible oxidation states permit selectivity.

Ytterbium is the chemical element with atomic number 70 and symbol $\mathrm{Yb}$. It is a lanthanide. It has important $+2$ and $+3$ chemistry. Its narrow optical transitions are useful in precision atomic experiments, showing how a chemical element can also provide a highly reproducible spectroscopic reference.^[ref]

Europium is the chemical element with atomic number 63 and symbol $\mathrm{Eu}$. It is a lanthanide. Its $+2$ state is unusually accessible compared with many neighboring lanthanides. Europium ions are used in luminescent materials, where changes in electronic energy are emitted as characteristic colors.^[ref]