Ytterbium (Yb, 60, $4d^1$)

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]